up. 1!»! . :l 5.1.23.1! v E»: . .. S. ... i... «use: a" ‘ “yusv‘x—vvu V-“tvyv: ‘1'?ij . .. 5.! {3.32:} 7.3.7: .Iitrtr-ln..r¢.l nun... ’3 {Kill 4.... I . l1:.lll(fltlchl.(fr1(f vrfrrl. {I y r a . Ir)..Iff..II! - 4 ., an. -.I .. . . . .. . .. .. . A. . . . . .. ..._ 313.4494? . . . . . . .11.... :15... . 1.11.1 . 1. .32.... V. i.f. ii I %6%54%00 l||||l||lllll||||l|||Hl|||llllllllllllllllll|l||llll|||ll|||| new.” 3 1293 00611 9725 Mid‘ligan State University This is to certify that the dissertation entitled Modeling Freeway Interchange Accidents presented by 1 Tae Gon Kim \ has been accepted towards fulfillment of the requirements for Ph.D. degree in Civil Engineering [Li/KW,” é’ . 42% .41 Major professor/ Date April 14, 1989 MSU is an Affirmative Action/Equal Opportunity Institution 0-12771 PLACE IN RETURN BOX to remove this checkout from your record. TO AVOID FINES return on or before date due. DATE DUE DATE DUE DATE DUE .‘1 is ' 235. ’4 MSU Is An Affirmative Action/Equal Opportunity Institution MODELING FREEWAY INTERCHANGE ACCIDENTS BY Taegon Kim A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Civil and Environmental Engineering 1989 56/79 2w ABSTRACT MODELING FREEWAY INTERCHANGE ACCIDENTS BY Taegon Kim The accident frequency and rate for the various components of freeway interchanges were identified and compared using accident data from the State of Michigan for the years 1982 through 1984. The accident rates at various types of interchanges were compared, and accident predictive algorithms for freeway interchange elements were developed. A master data file which was composed of geometric, accident and traffic data was constructed by merging existing data bases. The data were then classified into 3 area types (urban, rural and fringe) and further classified into different types of interchanges based.on the interchange type. The interchange types were grouped into 12 homogeneous groups based on the accident rate and variance. Accident predictive linear regression models were constructed and tested against data not used in calibrating the models. An analysis was made of those interchange groups exhibiting a value greater than 0.7 in multiple R coefficient. Based upon the results of this study, the average accident rates on the ramp units are greater than those on the mainline and crossroad units; the interchange is a very important variable in predicting accidents; the average daily traffic (ADT) has the greatest explanatory power in predicting the accident frequency; and interchange lighting, freeway over or under the crossroad, and ramp control demonstrate the potential of being important variables if sufficient data are available to make additional stratifications. To my mother, mother-in-law, brothers and wife ii ACKNOWLEDGEMENTS I am grateful to Dr. William C. Taylor, my major professor, for his advice and support throughout my graduate studies. His intimate guidance and aid throughout this research.progranlis gratefully'appreciated” I am also grateful to Dr. Thomas L. Maleck for his interest, advice and support for this research. Thanks are extended to Dr. Kunwar Rajendra and Joseph C. Gardiner of my advisory committee for their advice and suggestions. I would like to thank Mr. Jack D. Benac and Mr. Al Dewey for their assistance in collecting the data from the Michigan Department of'Transportation. Special thanks are also due Fred Coleman for his advice and help throughout my graduate studies. Finally, I wish to express my deepest gratitude to Meesook, my wife for her understanding, patience and sacrifices, and Myunggon, my brother for his advice and support throughout the years of my graduate studies. iii TABLES OF CONTENTS Inge LIST OF TABLES ........................................ Vi LIST OF CHARTS ........................................ xi LIST OF GRAPHS ........................................ xii CHAPTER I. INTRODUCTION ............................. 1 Statement of the Problem ......................... 2 CHAPTER II. LITERATURE REVIEW ........................ 4 CHAPTER III. DATA ACQUISITION ......................... l7 III-1. Data Needed .............................. 17 III-2. Sample Size ..... ......................... 20 III-3. Unit of Analysis ......................... 24 III-4. Mathematical Inspection .................. 26 CHAPTER IV. PROCEDURE ................................ 30 Models constructed before Data Stratification .... 31 Data Stratification ..... ...... . .................. 33 IV-1. Data File Format ......................... 33 IV—Z. Summary of Results by Groups ............. 43 IV-3. Summary of Results ....................... 58 CHAPTER V. MODELS ................................... 115 Models constructed for the Mainline Unit ......... 115 Models constructed for the Crossroad Unit ........ 122 Models constructed for the On-ramp Unit .......... 126 Models constructed for the Off-ramp Unit ......... 127 Summary of Results ........ ..... ............ ...... 129 CHAPTER VI. CALIBRATION . ............................. 135 Models calibrated for the Mainline Unit .......... 135 Models calibrated for the Crossroad Unit ......... 137 Models calibrated for the On-ramp Unit ........... 140 Models calibrated for the Off-ramp Unit .......... 140 Summary of Results ............................... 140 iv TABLES OF CONTENTS Ikmp CHAPTER VII. SUMMARY and CONCLUSIONS ........... . ...... 179 Summary .......................................... 179 Conclusions ...................................... 184 BIBLIOGRAPHY ........................................ .. 187 IV.2 IV.3 IV.4 IV.5 IV.6a IV.6b IV.6c IV.6d IV.6e IV.7 LIST OF TABLES Accident rate by proximity to interchange ahead or behind (1)...... ...... .... ....... . ....... .... Accident rate by interchange unit and area type (_1_) ............................................ Accident rate by type of freeway ramp (1) ...... Comparison of factors related to accidents on freeway interchanges .... ....................... Mean accident rate and variance by interchange type ....................................... .... Mean accident rate and variance by interchange group ................................ ... ....... Classification of significance between groups based on t-test ................................ Total number of accidents by uncollapsed interchange elements ........................... Total number of accidents by collapsed interchange elements ........................... Total number of accidents by each type accidents and interchange elements ....................... Total number of accidents by each type accidents and interchange elements ....................... Total number of accidents by each type accidents and interchange elements ....................... Total number of accidents by each type accidents and interchange elements ....................... Total number of accidents by each type accidents and interchange elements ................. ...... Accident types by Collapsed interchange elements OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO vi BEE 13 14 15 16 6O 61 62 63 64 65 66 67 68 69 7O Table IV.9 IV.10 IV.11 IV.12 IV.13 IV.14 IV.15 IV.16 IV.17 IV.18 IV.19 IV.20 IV.21 IV.22 Accident of urban Accident of urban Accident of urban Accident of urban Accident of urban Accident of urban Accident of urban Accident of urban Accident of urban Accident of urban Accident of urban Accident of urban Accident of rural Accident of rural Accident of rural LIST OF TABLES types group 1 types group 2 types group 3 types group 4 types group 5 types group 6 types group 7 types group 8 types group 9 types group types group types group types group 1 types group 2 types by group 3 Collapsed interchange elements ............................... vii BEE 71 72 73 74 75 76 77 78 79 80 81 '82 83 84 85 LIST OF TABLES Table Egg IV.23 Accident types by Collapsed interchange elements of rural group 4 ............................ . 86 IV.24 Accident types by Collapsed interchange elements of rural group 5 ............................... 87 IV.25 Accident types by Collapsed interchange elements of rural group 6 ............................... 88 IV.26 Accident types by Collapsed interchange elements of rural group 7 ............................... 89 IV.27 Accident types by Collapsed interchange elements of rural group 8 ............................... 9O IV.28 Accident types by Collapsed interchange elements of rural group 9 ............................... 91 IV.29 Accident types by Collapsed interchange elements of rural group 10 .............................. 92 IV.3O Accident types by Collapsed interchange elements of fringe group 1 ............................... 93 IV.31 Accident types by Collapsed interchange elements of fringe group 2 ............................... 94 IV.32 Accident types by Collapsed interchange elements of fringe group 3 ............................... 95 IV.33 Accident types by Collapsed interchange elements of fringe group 4 ............................... 96 IV.34 Accident types by Collapsed interchange elements of fringe group 5 ............................... 97 IV.35 Accident types by Collapsed interchange elements of fringe group 6 ............................... 98 IV.36 Accident types by Collapsed interchange elements of fringe group 7 ................ . .............. 99 IV.3? Accident types by Collapsed interchange elements of fringe group 8 ............................... 100 viii Table IV. IV. IV. IV. IV. IV. IV. IV. IV. IV. VI. 38 39 4O 41 42 43 44 45 46 47 V1.2 VI.3 LIST OF TABLES BEE Accident types by Collapsed interchange elements of fringe group 9 ........ . ...................... 101 Accident types by Collapsed interchange elements of fringe group 10 ...... .. ...................... 102 Accident types by urban groups .................. 103 Accident types by rural groups .................. 104 Accident types by fringe groups ................. 105 The average number of accidents per interchange by units used ...................................... 106 Sample Data File ................................ 107 Comparison of the analysis units by groups based on the highest and lowest accident rates ........ 108 The average number of accidents per interchange by sample units .................................... 109 The average accident rate per interchange by sample units .................................... 110 Classification of signs of variable terms used in in the models constructed ....................... 132 Number of interchanges for each group of analysis unit ............................................ 133 Multiple R Coefficient for each group of analysis unit ............................................ 134 Comparison of Actual and Predicted values of Total accident frequency in'UM-Group 3 ... ....... 143 Comparison of Actual and Predicted values of Total accident frequency in UM-Group 5 .......... 145 Comparison of Actual and Predicted values of Total accident frequency in RM-Group 1 .......... 147 ix LIST OF TABLES Table Egg V1.4 Comparison of Actual and Predicted values of Total accident frequency in.RM-Group 3 .......... 149 V1.5 Comparison of Actual and Predicted values of Total accident frequency in.RM-Group 4 .......... 151 V1.6 Comparison of Actual and Predicted values of Total accident frequency in FM-Group 2 .......... 153 V1.7 Comparison of Actual and Predicted values of Total accident frequency in FM-Group 8 .......... 155 V1.8 Comparison of Actual and Predicted values of Total accident frequency in FM-Group 9 .......... 157 V1.9 Comparison of Actual and Predicted values of Total accident frequency in UC-Group 3 .......... 159 V1.10 Comparison of Actual and Predicted values of Total accident frequency in‘UC—Group 4 .......... 161 V1.11 Comparison of Actual and Predicted values of Total accident frequency in RC-Group 1 .......... 163 V1.12 Comparison of Actual and Predicted values of Total accident frequency in.RC-Group 3 .......... 165 V1.13 Comparison of Actual and Predicted values of Total accident frequency in RC-Group 6 .......... 167 V1.14 Comparison of Actual and Predicted values of Total accident frequency in RC-Group 7 .......... 169 V1.15 Comparison of Actual and Predicted values of Total accident frequency in RC-Group 9 .......... 171 V1.16 Comparison of Actual and Predicted values of Total accident frequency in FC-Group 2 .......... 173 V1.17 Comparison of Actual and Predicted values of Total accident frequency in‘UON-Group 2 ......... 175 V1.18 Comparison of Actual and Predicted values of Total accident frequency in'UOF-Group 2 ......... 177 ME Chart 4.1 Chart 4.2 Chart 4.3 Chart 4.4 Data file Data file Data file Data file LIST OF CHARTS format chart ..................... set by urban groups .............. set by rural groups .............. set by fringe groups ............. xi 113 114 we Graph Graph Graph Graph Graph Graph Graph Graph Graph Graph Graph Graph Graph Graph Graph Graph Graph Graph LIST OF GRAPHS P_ag§_ 6 l ............................................. 144 6 2 ............................................. 146 6.3 ............................................. 148 6.4 ............................................. 150 6.5 ............................................. 152 6.6 ............................................. 154 6.7 ............................................. 156 6.8 ............................................. 158 6.9 ............................................. 160 6.10 ............................................ 162 6.11 ............................................ 164 6.12 ............................................ 166 6.13 ............................................ 168 6.14 ............................................ 170 6.15 ............................................ 172 6.16 ............................................ 174 6 17 ............................................ 176 6.18 ............................................ 178 CHAPTER I INTRODUCTION Fundamentally, an interchange simply provides an opportunity for traffic to transfer from one road to another. As used ixtaatmodern freeway system, an interchange plays a very important role in providing greater capacity, maintaining higher operating speeds, and reducing the probability of vehicular conflicts during this transfer (1). As the Interstate Highway Systemmwas constructed, a large number of freeway interchanges were designed and constructed. The design standards for these freeway interchanges, however, were not derived from an analysis of past experience, since there was very little past experience upon which the design engineer could draw. Instead, most designs were modifications of existing freeway interchanges, since substantive knowledge for redesign of freeway interchanges was rather limited in regard to the performance expected from individual elements in terms of efficient traffic movement and adequate safety (2). With many freeway systems approaching their design life, the data are now available to evaluate the safety attributes of various freeway configurations and various freeway elements under traffic conditions. Some evaluation of the geometric design and operational characteristics of interchanges has been done using the rate of traffic accidents as an evaluation parameter (1). However, these safety analyses were not sufficiently detailed to allow freeway designers to select an optimal interchange design. STATEMENT OF THE PROBLEM In Michigan, freeway interchanges have been in use since the Davidson Freeway and the Ford Freeway were open to traffic in the early 1940's. There are approximately 700 interchanges in the state of Michigan. Of those interchanges, there are few, if any, exactly alike. The geometric interchange designs vary from the simple rural diamond interchange with at—grade intersections of the ramp with the crossroad to the urban multi-level directional freeway-to—freeway interchange with lane drops, multi-lane turning roadways, weaving lanes, and freeflow merges. Research. has indicated that accident rates are not uniform for the various interchange types or interchange elements. One study showed that the accident rate (346/100 mvm) for a rural exit ramp is more than twice that (161/100 mvm) of a rural entrance ramp (1). Interestingly enough, the reverse is true for urban interchanges with entrance ramps having an accident rate (718/100 mvm) that is twice that (378/100 mvm) of the exit ramp (1). While this study identified accident rates for various elements of a freeway interchange and compared those accident rates with each other, it did not compare the accident rate at various types of interchanges, nor did it establish predictive algorithms based on design features of the freeway interchange elements. The Michigan Department of Transportation recently completed a geometric inventory of its freeway interchanges, and has assigned accidents to elements. Thus, the geometric data, accident data, and traffic data (such as traffic volumes, population, no. of ramp lanes) could now be used to develop standards based upon the minimization of accidents, if models were available that expressed the relationship among these variables. The purpose of this analysis of interchanges in Michigan is to: compare the accident rates in Michigan with those from the Interstate System Accident Research Study, Interim Report 11 by Cirillo, J. A. (1); identify accident rates as they relate to parameters of the interchange elements; and, finally, to establish interchange accident predictive models based upon accident rates on the elements which comprise the interchange. The results of this study will provide guidance as design decisions are made during the reconstruction of the freeway system in Michigan. I'm... CHAPTER II LITERATURE REVIEW, In an analysis of the effect of location on accident rates on the interstate highway system, Julie A. Cirillo (1) used data collected by 20 State Highway Departments to compare accident rates on various roadway elements. The initial categorization was between-interchange units and at- interchange units respectively. These were then further divided into urban and rural sections. Each mainline unit was described by its proximity to an interchange. Units which were located at. the same distance from two interchanges were divided equally between the two study units (1). From the results of the between-interchange accident rate analysis, the results shown in Table 11.1 were reported. Some of the important conclusions from this study were: . The accident rate increased on urban sections as the study unit was positioned closer to an exit ramp with the highest rate occurring in those sections located less than 0.2 mile from the exit ramp. Also, as a study unit was stationed closer to the entrance ramp area, the accident rate increased, although not uniformly. . On rural sections, the change in accident rates was not significantly altered as a unit was positioned closer to the interchange and in the exit direction it remained constant The results of the at-interchange accident rates as shown in Table 11.2, indicated that: . The accident rate for urban interchanges was substantially higher than for rural interchanges, as these areas carried more traffic, making merging and diverging maneuvers more difficult. . The exceptionally high accident rate on entrance ramps in urban areas might be caused by inadequate acceleration lanes, or lack of them, on many sections, necessitating vehicles to stop at the bottom of the ramp before moving into the traffic stream. . The accident rate on the mainline within the interchange area decreased after the deceleration lane had been passed (1) - The general conclusions of this study were that: sections in proximity of interchanges experienced a higher accident rate than other sections; ramps have much higher accident rates than speed-change lanes; and these, in turn, have generally higher rates than the other portions of the main roadways (1). In a study of the relationship between interchange design features and traffic safety, Joseph C. Oppenlander and Robert F. Dawson (1) found that: . Relatively safer designs were produced when the mainline freeway passed over the minor facility and when the ramp terminals were at least 750 ft. from the structure. . On-ramps became high accident locations in‘ urban areas, while in rural areas the off-ramps represented the greatest accident rate locations. . Entrance terminals were improved with geometric designs that provided auxiliary lanes or deceleration lanes of 800 ft. or more. This eliminated traffic friction on the through lanes which resulted in reduced accident rates. . Adequate sight distances were essential at entrance and exit terminals. . Geometric designs for weaving maneuvers should provide weaving sections that are at least 800 ft. in length. In another study of accidents and design features at interchanges, R. L. Fisher (1) found that: . There were no accidents that could be ascribed to the curvature on loops which had radii of over 100 ft. . Speed-change lanes of adequate length together with careful treatment of the terminals practically eliminated accidents at interchanges. . All of the left-hand entrances and exits had a poor accident record. In a comparative freeway study concerning alignment and accidents at interchanges, John Vostrez and Richard A. Lundy (1) classified the ramp alignment into 6 types. These alignments, in order of low to high accident rates were straight level, straight upgrade, straight downgrade, curved level, curved.upgrade, and curved.downgrade respectively. They found that: . With heavy truck traffic the straight upgrade was more detrimental than the straight downgrade while all of the curved classifications were the same. . Fixed objects were involved in about 28 percent of all freeway interchange accidents. Piers, abutments and bmidge rails were apparently the most vulnerable, with signs, guardrails, and light standards following in that order. . Ramps associated with diamond-type interchanges were the safest type, and on-ramps generally had better accident rates than off-ramps. The downhill on-ramp was the safest type of on-ramp and the uphill off-ramp was the safest type off-ramp. Left-hand ramps (enters or leaves the freeway at high speed lane) had a higher accident rate than any other class. In an analysis with regard to lighting of interchanges, M. S. Janoff, M. Freedman, and Decina, L. E. (5) reported that: . Complete Interchange Lighting (CIL) systems perform better than Partial Interchange Lighting (PIL) systems consisting of one, two, or four luminaries. . Either CIL or PIL normally perform better than no lighting. . PIL systems with fewer luminaries (one or two) frequently perform better than PIL systems with a greater number of luminaries (four). . There is a trade-off between cost and traffic operations and safety factors in the design of freeway interchange lighting systems. . Existing CIL systems should not be reduced to PIL systems if safety and traffic flow are important considerations. In an investigation of factors affecting the design and location of freeway ramps from an operational VieWpoint, William E. Tipton and Charles Pinnel (6) indicated that: . Standard interchange designs cannot always fulfill the various desired movements at 800 ft.), and thus do not provide data on accidents related to continuous variables which would allow an analyst to develop predictive algorithms based upon design features of the freeway interchange elements. For example, it may not be possible to extend a speed- change lane from 700 ft. to 800 ft., but it would still be desirable to know the effect of lengthening it to 750 ft. The past studies would not assist in this analysis, since both 700 ft. and 750 ft. are less than the dividing point used in the preceding analysis. The same is true for other design features (adequate sight distance versus inadequate sight distance, ramps with a radius greater than 100 ft. versus those with a radius less than 100 ft., greater than 750 ft. in distances between ramp terminals and structures versus less than 750 £12.). 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Data Needed The objectives of this research were to: 1) compare the accident rates in Michigan with those from the Interstate System Accident Research Study II, Interim Report II by J. A. Cirillo (l) and 2) develop and calibrate accident predictive models based upon accident rates on the elements which comprise a freeway interchange. In the state of Michigan, there are approximately 700 freeway interchanges. The Michigan Department of Transportation recently completed a geometric inventory of its freeway interchanges, and merged the accident file to the geometric file to identify the number of accidents associated with each interchange element. Thus, the three types of data needed for this research were available from this file. Geometric data: Data describing the elements of the freeway interchanges geometrically. The geometric data were collected from. the IMichigan Department of 'Transportation's Highway Accident Master Data file. The geometric data used in this study are: 1). Interchange Number 17 2). Interchange Element Code 3). Control Section 4). Milepoint 5). Prime Road (PR) Number 6). Beginning PR Milepoint 7). Ending PR Milepoint 8). Beginning Ramp Terminal Milepoint 9). Ending Ramp Terminal Milepoint 10). Geometric and Laneage Code 11). Ramp Terminal or Intersection Code 12). Ramp Terminal Lane Usage Code 13). Interchange Light Code 14). Interchange Type 15). Activity Density 16). Junction Type Code Traffic data: Data describing the level of use of the freeway interchange elements. The traffic data are available from the Michigan Department of Transportation's TVM (Trunkline Vehicle Miles) Master Data file and Traffic FlOW'Map. The traffic data needed for this study are: 1). ADT (Average Daily Traffic) on Mainline 2). ADT (Average Daily Traffic) on Crossroad 3). ADT (Average Daily Traffic) on Ramp 4). Population of the county in which the interchange is 18 located Accident data: interchanges in Michigan. from. the IMichigan Department of ‘Transportation's Highway Accident Master Data file. study are: 1). 2). 3). 4). 5). 6). 7). 8). 10). 11). 12). 13). 14). 15). 16). 17). 18). Miscellaneous single vehicle accident Overturn accident Hit train accident Hit parked vehicle accident Backing accident Parking accident Pedestrian accident Fixed object accident Other object accident Animal accident Bicycle accident Head-on accident Angle straight accident Rear-end accident Angle turn accident Side swipe same direction accident Rear-end left-turn accident Rear-end right-turn accident 19 Data on accidents that occurred on the freeway The accident data were collected The accident data needed for this 19). Other drive way related accident 20). Angle drive way related accident 21). Rear-end drive way related accident 22). Side swipe opposite direction accident 23). Head-on left—turn accident 24). Dual left-turn accident 25). Dual right-turn accident The accident data used for this study were data from 1982 to 1984. The geometric data base used for this study was completed by 1984, but many parts of the data base had been partly created before 1984. The geometric data base completed in 1984 was used since the geometry had not changed significantly. The traffic volume data for 1983 was used since traffic volumes are updated with a traffic growth factor every 3 years. The population data based on this same year was used. The master data file comprises the geometric data and the accident data. Merging the traffic data into the master data file and analyzing those data was done by computer programs (that is, FORTRAN and SPSS). III-g. Sample size The purpose of sampling is to gain information about the nature or distribution of elements in a particular population 20 without studying the entire population. In determining a sample size, there are two major considerations: First, assumptions must be made about the underlying distribution of these elements when selecting a sample size. One common assumption is that the population is normally distributed. Under the normal distribution the same proportion of observations will always lie between the mean and a specified number of standard deviations below or above the mean. For example, 68.26 percent of the area under the normal distribution will be within one standard deviation of the mean, and 95.46 percent, within two standard deviations for any normally distributed variable. Second, some.decision must.be made about the acceptable limits of error for the sample. This is usually done by specifying that the sample mean for a data item should be within some value d of the true average for a certain percentage of samples which is called the level of confidence. This level of confidence is denoted as 100(1 - a), where a is the fraction of the area under the normal distribution falling outside the confidence limits. Thus, in the case that d = 2 and a = 0.05, :2 units around the true value will include the estimated value 95 percent of the time (8). There are two} types of equations considered for determining a sample size: sampling with replacement and sampling without replacement. Sampling with replacement assumes that the sample n is small relative to the total 21 population size, but sampling without replacement does not. Under sampling with replacement, the equation for determining a sample size to achieve a precision of d units with 100(1 - a) percent confidence is ZZ1-(1/2)a02 n = d2 where n = sample size d = tolerable margin of error of mean value a = standard deviation of population distribution a = fraction of area under normal curve representing events not within confidence level (thus, 1 - a is desired level of confidence) Z = standard normal statistic corresponding to 1 - 1'(1/2)a a confidence level If the standard deviation of the population distribution is unknown, s(standard deviation of sample distribution) might be used instead as follows: ZZ1-(1/2)azSZ n 2 d2 where s = standard deviation of sample distribution 22 Under sampling without replacement, the equation considered for determining a sample size is n In = 1 + n/N where n = number of sample observations with replacement In = adjusted number of observations N = total population In the state of Michigan, there are approximately 700 freeway interchanges. Suppose that there are an average of 4 ramps per freeway interchange. It is desired to know the number of accidents per ramp within d (tolerable margin of error) = 2, 32 (sample variance in accidents per ramp) = 100, and Z = 1.96, assuming 95 % confidence level. Then, (l.96)2(lOO) (2)2 96.04 96.04 n1: 1 + 96.04/2800 = 92.855 Thus, under sampling with replacement, the sample size is at least 97 ramps, and under sampling without replacement, the 23 sample size is at least 93 ramps. The actual sample size to be used in this study will be determined following sufficient analysis to determine the probable value of the variance. III-g. Unit of Analysis In the Interstate System Accident Research Study II, Interim Report II by Cirillo, J. A. (_1_) , the following analysis units of freeway interchange were included in the analysis: . Deceleration lanes including taper . Acceleration lanes including taper . Exit ramps . Entrance ramps . Mainline units between speed—change lanes . Combined acceleration-deceleration lanes Each analysis unit was analyzed based on whether the analysis unit.was within an urban or rural area. The accidents occurring between interchanges were coded as a distance from either the interchange ahead or behind based on the distances to the exit-ramp nose and the entrance-ramp nose, respectively. However, in this study all exit ramps were combined, regardless of length, ADT, number of lanes, type of interchange, etc. The same was true for entrance ramps and 24 mainline sections. No analysis was made of accidents occurring on the cross roads. Thus, it is not possible to predict the total accident frequency or rate for an interchange. The analysis units for this study will be further classified based on ADT, the interchange type and the length of the various elements. The population of the county in which the interchange exists will be used.as a surrogate measure for activity density. The elements of the freeway interchanges will be considered in detail, and the following base analysis units will be included in this research: . Mainline unit . Crossroad unit . On-ramp unit . Off-ramp unit The base units of analysis to be considered were defined as the following: . Mainline units start at a point 500 ft. before the deceleration lane for the off-ramp and end at a point 500 ft. after the acceleration lane for the on-ramp; . Crossroad units start at a point 250 ft. before the intersection of the on-ramp and the crossroad and end at a point 250 ft. after the intersection of the off-ramp and the crossroad; 25 . On-ramp units start at the point they meet the cross-road and end at the end of the acceleration lane; . Off-ramp units start at the beginning of the deceleration lane on the freeway and end at the point the ramp meets the crossroad. III-4. Mathematical Inspection using the identified data, sample size, and units of analysis, models were constructed based upon the following analysis: Linear model: The data were first analyzed using linear models. Suppose that interest lies in a certain (response) variable p, which is thought to be dependent on the functionally independent variables Z1, 22' ..., ZS, that is, p = £(z1, ..., ZS). Then, it is said that p obeys a linear model if u = f(Zv , 29 k =.Z ,6ij(21, ..., ZS) j=l where Xj== functions of the Zj only 8,, , Bk = unknown parameters which enter into the above (2). 26 Regression Analysis: As a statistical tool that utilizes the relation between two or more quantitative variables, regression analysis is used to predict one variable from the other or others. Regression analysis is based on a linear regression model which fits the scattered observations on a straight line by the least square method. There are two types of linear regression models: simple linear regression models and multiple linear regression models. A linear regression model which contains only one independent variable is called a simple linear regression model, while the linear regression model which contains a number of independent variables is called a multiple linear regression model. Thus, the simple linear regression model can be stated as follows: Y'==‘% + 399 + 5i where Y. = the value of the response variable in the ith trial BO and E1 = parameters Xi = a known constant, namely, the value of the independent variable in the ith trial 6. = a random error term with mean E(efl == 0 and variance 02(6fl = 02, eiand ejare uncorrelated so that the covariance 0(ew 6]) for all i, j; i is not equal to j i = 1’ O O O I n 27 Method of Least Sggare: In order to find good estimators of the regression parameters (i.e., 30 and fig), the method of least squares was employed. Suppose that there is a sample observation (Xi, TL). Then the method of least squares 1 considers the deviation of Yi from its expected value: Y} ’ (30'+ 39%) In particular, the method of least squares requires that the sum of the n squared deviations is considered. This criterion is denoted by Q: Q = Yi - (pa + fi1xi) }2 IIML'S . { l 1 Thus, the estimators of 50 and 61 are those values b0.and b1, respectively, that minimize the criterion Q for the given sample observations (xi, Yi) (_1_Q). Regression Procedure: In selecting a regression procedure, there are three possible regression procedures which require the fitting of every possible regression equation. The backward elimination procedure which determines the "best" regression using all variables and then determines the best equation for each step in which the number of variables in the equation is reduced. The forward selection procedure inserts 28 variables in turn until the regression equation is satisfactory. The stepwise regression procedure which is an improved version of forward selection procedure which examines the variables incorporated into the model at every stage of the regression, provides a judgement on the contribution made by each. variable, and removes any 'variable ‘which. has a nonsignificant contribution at a later stage even if it may have been the best single variable at the early stage (1;). The stepwise regression procedure was used for this study. Using the geometric, traffic and accident data, the regression models were constructed for each unit of analysis. In the linear regression models, accident rates based on the different types of accident (i.e., total accident rate, injury accident rate, etc.) were the dependent variable, and the geometric and traffic data were the independent variables. In those instances where the relationship did not appear to be linear, the intrinsically linear regression model by transformation was used. 29 CHAPTER IV PROCEDURE This study concerned itself with the development of linear regression models for predicting accidents occurring on freeway interchanges. One of the questions investigated in this research was a determination of whether stratified data or nonstratified data would result in better accident prediction models. The units of analysis for constructing the accident predictive models for the total freeway interchange were based on individual predictive models for the following elements: 0 Mainline unit 0 Crossroad unit 0 Ramp unit Mainline units include the freeway lanes from a point 500 ft. before the deceleration lane for the off-ramp to a point 500 ft. after the acceleration lane of the on-ramp. Crossroad units include the roadway from a point 250 ft. before the intersection of the on-ramp and the crossroad to a point 250 2ft. after the intersection of the off-ramp and the crossroad. Ramp units include the on-ramp units from the intersection of the cross-road to the end of the acceleration lane and the off-ramp units from the beginning of the deceleration lane on 30 the freeway to the intersection with the crossroad. Models constructed before data stratification Based upon the above units of analysis, a linear regression model was constructed using the total data based on the following formula: Accidents = f(ADT, Population, Lane mileage, # of Ramps) The best linear regression models of accident prediction on each unit of analysis was as follows: Model for Mainline Unit Y = -8.0362 + 0.00021658x1 + 0.05523x2 + 0.000008697X3 + 3.39985X4 - 1.86537XS where Y = Total number of accidents per unit )6 = Average Daily Traffic (ADT) X2== Lane mileage X3== Population )6 = Number of Off-ramps >< Ln ll Number of On-ramps 31 For this model, the multiple regression coefficient (R) was 0.5624. Model for Crossroad Unit Y = 5.9372 + 0.00019387X1-+ 0.01979X2-+ 0.000004297X3 where Y = Total number of accidents per unit Average Daily Traffic (ADT) AX ll )9 = Lane mileage X3== Population For this model, the multiple regression coefficient (R) was 0.386. Model for Ramp Unit Y = -0.8302 + 0.00001575X1-+ 0.02555X2-+ 0.000000671X3-+ 0.26683X4 where Y = Total number of accidents per unit )9 = Average Daily Traffic (ADT) X2== Ramp lane mileage )g = Population X 5 ll Number of Off-ramps 32 For this model, the multiple regression coefficient (R) was 0.323. It was obvious that there was more variance in the accident data than that which could be satisfactorily explained by these linear regression models. Thus, a systematic method of stratifying the interchanges to reduce the variance, and increase the explanatory power of the models was undertaken. As a simple test to determine whether stratification might be useful, a model of nighttime accidents on Cloverleaf freeway interchanges was developed as follows: Y = -2.4229 + 0.00004974X1-+ 0.05896X2 where Y = Dark accidents )6 = Average Daily Traffic (ADT) X2== Lane mileage For this model, the multiple regression coefficient (R) was 0.724. Thus, it appears that stratifying the data can lead to improved model reliability. The remaining question is whether the stratification procedure will result in useful data upon which design decisions can be made. Data Stratification Iv-l. Data File Format 33 As described in the preceding section, the objectives of this research are to: :1) identify accident rates as they relate to parameters of the interchange elements; 2) compare the accident rates in Michigan with those from J. A. Cirillo; and 3) develop and calibrate interchange accident predictive models based on the accident rates on elements which comprise the interchange. With the above objectives, the data needed were obtained from the databases of the Michigan Department of Transportation. After data were obtained, they were merged into the master data file using Fortran programs. The master data file contains elements from the geometric data file, the accident data file and the traffic data file. This master data file also includes a category code. Thus, the master data file can be sorted into specific data file sets as needed for research. From the master data file, the categories used to format the data file sets needed for this research were: . Activity Density . Interchange Type . Interchange Element 1). Data file set by Activity Density For' the activity' density, the 'master' data file ‘was 34 classified into 3 area types of activity density: urban; rural; and fringe. The number of interchanges and the percentage of each area type are: . Urban: 28.07 % (185 out of 659) . Rural: 49.47 % (326 out of 659) \ . Fringe: 22.46 % (148 out of 659) 2). Data file set pv Interchange Type For the freeway interchange type, the master data file was divided into 30 interchange types with the percentage and the total number of interchanges for each interchange type: \ . Diamond: 19.12 % (126 out of 659) \ . Tight diamond: 11.68 % (77 out of 659) . Modified diamond: 4.10 % (27 out of 659) . Modified tight diamond: 4.10 % (27 out of 659) . Partial diamond: 2.73 % (18 out of 659) . Partial tight diamond: 4.70 % (31 out of 659) . Split diamond: .2.28 % (15 out of 659) o\° . Diamond plus 1 loop: 2.88 (19 out of 659) . Parclo A: 2.73 % (18 out of 659) \ . Parclo A 4 quad: 6.98 % (46 out of 659) . Parclo B: 3.03 % (20 out of 659) . Parclo B 4 quad: 1.67 % (11 out of 659) 35 Parclo AB: 4.10 % (27 out of 659) Parclo AB 4 quad: 1.82 % (12 out of 659) Cloverleaf: 1.37 % (9 out of 659) Cloverleaf with CD roads: 1.21 % (8 out of 659) Cloverleaf minus 1 loop: 0.46 % (3 out of 659) o\° Trumpet A: 1.37 (9 out of 659) o\° Trumpet B: 1.37 (9 out of 659) Full directional: 1.52 % (10 out of 659) Partial directional: 2.12 % (14 out of 659) Directional Y: 1.21 % (8 out of 659) General Directional: 0.76 % (5 out of 659) Partial directional Y: 4.40 % (29 out of 659) Directional with loops: 1.37 % (9 out of 659) General: 1.52 % (10 out of 659) Urban diamond: 6.68 % (44 out of 659) o\° SRI-A: 0.15 (1 out of 659) SRI-B: 0.45 o\° (3 out 659) Other: 2.12 % (14 out of 659) These freeway interchange 'types represent 'the interchanges in Michigan. Based upon the above created, Table IV.1. interchange types, data files for each interchange type were number of accidents for each data file was found as shown in these data files were collapsed into a smaller number of 36 and the total number of accidents and the average Since many of the sample sizes were too small, groups based upon a similar mean accident rate and variance. These groups are shown in Table IV.2; The interchange types comprising each group are as follows: . Group 1: Diamond . Group 2: Tight diamond, Urban diamond . Group 3: Modified diamond, Modified tight diamond, Parclo A 4 quad . Group 4: Partial diamond, Partial tight diamond, Trumpet A, Partial Directional Y . Group 5: Split diamond, General directional, Other . Group 6: Diamond plus 1 loop, Parclo B 4 quad, Trumpet B, Directional Y . Group 7: Parclo AB, Partial directional . Group 8: Cloverleaf, Cloverleaf with CD roads, Cloverleaf minus 1 loop, Directional with loops . Group 9: Parclo A, Parclo B, Parclo AB 4 quad . Group 10: Full directional, General . Group 11: SRI-A . Group 12: SRI—B A "t" test was run to determine if these groups were statistically significantly different. While not all groups were different from all other groups, all groups were different from at least one other group. The result of this analysis is shown in Table IV.3. 37 3). Data file set by Interchange Element For the freeway interchange element, the master data file was classified into 33 elements of the freeway interchange as follows: . NB mainline . SB mainline . EB mainline . WB mainline . Crossroad . Spread on ramp from crossroad to freeway . Spread off ramp from freeway to crossroad . Tight on ramp from crossroad to freeway . Tight off ramp from freeway to crossroad . Loop on ramp from crossroad to freeway . Loop off ramp from freeway to crossroad . Collector distributor . On ramp from service road to freeway . Off ramp from freeway to service road . Service road from off ramp to crossroad . Service road from crossroad to on ramp . On ramp from crossroad to CD . Off ramp from CD . Ramp from CD to CD . Off ramp from CD to service road . On ramp from service road to CD' 38 . Directional loop ramp . Directional ramp . Loop ramp from CD to CD . Loop ramp from CD to crossroad . Loop ramp from crossroad to CD . Off ramp from freeway to CD . On ramp from CD to freeway . Turning roadway . Loop ramp from freeway to CD . Ramp from service road to service road . Service road . Other These freeway interchange elements represent the freeway interchange in total. For this research the freeway interchange elements were collapsed into 4 analysis units based on the role on the freeway interchange: . Mainline unit - NB mainline SB mainline EB mainline WB mainline . Crossroad unit - Crossroad . On-ramp unit - Spread on ramp from crossroad to freeway Tight on ramp from crossroad to freeway Loop on ramp from crossroad to freeway 39 On ramp from service road to freeway On ramp from CD to freeway . Off-ramp unit - Spread off ramp from freeway to crossroad Tight off ramp from freeway to crossroad Loop off ramp from freeway to crossroad Off ramp from freeway to service road Off ramp from freeway to CD Loop ramp from freeway to CD The total number of accidents by each element and each group of elements is shown in Table IV.4 and IV.5 respectively. 4). Da a file set by Agcidgnt Tvpg With the above-described data file set, the types of accidents available on the accident data file were: . Miscellaneous single vehicle . Overturn . Hit train . Hit parked vehicle . Backing . Parking . Pedestrian . Fixed object . Other object 40 . Animal . Bicycle . Head-on . Angle straight . Rear-end . Angle turn . Side swipe same . Rear-end left turn . Rear-end right turn . Other drive . Angle drive . Rear-end drive . Side swipe opposite . Head-on left turn . Dual left turn . Dual right turn The number of accidents by type for each interchange element is shown in Table IV.6a through IV.6e. The number of accidents by collapsed interchange elements is shown in Table IV.7. 5) . Data file set by Interchange groups with Activity density Based upon the above interchange types and activity densities, the following combined data file sets were created: 41 Urban Groups Urban Urban Urban Urban Urban Urban Urban Urban Urban Urban Urban Urban group group group group group group group group group group group group Rural Groups Rural Rural Rural Rural Rural Rural Rural Rural Rural Rural group group group group group group group group group group 10 ll 12 10 42 Fringe Groups . Fringe . Fringe . Fringe . Fringe . Fringe . Fringe . Fringe . Fringe . Fringe . Fringe group group group group group group group group group group _.m’_. ,1. I, ___L_.~ ...; .... 10 These data file sets represent the total data file by freeway interchange type and the analysis units considered. The total number of accidents in each cell based upon these categories was found, and the accident rate (accidents per interchange) for each analysis unit was also determined as shown in Table IV.8 through IV.38. IV-2. Summaryeof Reeults by Groups 1). Summary of accident typee bv collapsed interchange elements Based upon the accident types and the interchange elements described, the total number of accidents for each 43 accident type by each interchange element is shown in Table IV.39 through IV.41. From the results shown in these tables, the average number of accidents that occurred on the analysis units is shown in Table IV.42. Mainline unit: 4464 accidents out of the total 9534 accidents that occurred on those analysis units occurred on the mainline unit (46.82 percent). The major types of accidents and the percentage of each major accident type were: \ . Rear-end: 39.83 % (1778 out of 4464) . Fixed object: 27.76 % (1239 out of 4464) . Animal: 12.25 ° (547 out of 4464) o\ . Overturn: 8.2 % (364 out of 4464) . Miscellaneous single vehicle: 3.3 % (148 out of 4464) These major types of accidents represent 91.31 percent of the total accidents on the mainline unit. Crossroad unit: 2536 accidents out of the total 9534 that occurred on those analysis units occurred on the crossroad unit (26.60 percent). The major types of accidents and the percentage of each major accident type were: \ . Rear-end: 29.77 % (755 out of 2536) . Fixed object: 16.56 % (420 out of 2536) . Angle straight: 9.62 % (244 out of 2536) . Angle turn: 8.16 % (207 out of 2536) 44 . Rear-end drive: 4.53 % (115 out of 2536) . Head-on left turn: 3.94 % (100 out of 2536) These major types of accidents represent 72.59 percent of all accidents on the crossroad unit. On-ramp unit: 919 accidents out of the total 9534 accidents that occurred on those analysis units occurred on the on-ramp unit (9.64 percent). The major types of accidents and the percentage of each accident type were: \ . Fixed object: 36.02 % (331 out of 919) . Rear-end: 33.51 % (308 out of 919) . Overturn: 16.32 % (150 out of 919) These major types of accidents represent 85.85 percent of the total accidents on the on-ramp unit. Off-ramp unit: 1615 accidents out of the total 9534 accidents that occurred on those analysis units occurred on the off-ramp unit (16.94 percent). The major types of accidents and the percentage of each accident type were: \ . Rear-end: 41.24 % (666 out of 1615) . Fixed object: 30.4 % (491 out of 1615) \ . Overturn: 10.46 % (169 out of 1615) These major types of accidents represent 82.11 percent of all 45 accidents on the off-ramp unit. 2 . Summary of Urpan Groups based on Analysis Units Following stratification, some of the groups were too small to be modeled. An analysis of the sample size for each group is discussed below: Urban Group 1: As shown in Table IV.8, the total number of accidents was small and the number of interchanges was 5. This group was excluded from further analysis. Urban Group 2: As shown in Table IV.9, the number of interchanges was 50. The accident rate for the mainline unit was 5.86 accidents per interchange, and the major types of accidents were rear-end and fixed object. The accident rate for the crossroad unit was 3.66, and the major types of accidents were rear-end, angle straight, angle turn, fixed object, other drive, and angle drive. The accident rate for the on-ramp unit was 2.26, and the major types of accidents were rear-end and fixed object. For the off-ramp unit the accident rate was 2.04, and the major types of accident were rear-end and fixed object. Urban Group 3: As shown in Table IV.10, the number of interchanges was 17. The accident rate for the mainline unit was 9.53 accidents per interchange, and the major types of accidents were rear—end and fixed object. The accident rate 46 for the crossroad unit was 6.18, and the major types of accidents Were rear-end and angle turn. The accident rate for the on-ramp unit was 3.41, and the major types of accidents were fixed object and rear-end. For the off-ramp unit the accident rate was 4.82, and the major types of accident were rear-end and fixed object. Urban Group 4: As shown in Table IV.11, the number of interchanges was 38. The accident rate for the mainline unit was 5.24 accidents per interchange (the lowest value among the urban groups), and the major types of accidents were rear-end and fixed object. The accident rate for the crossroad unit was 3.37 (the lowest value among the urban groups), and the major types of accidents were angle straight, rear-end and hit parked vehicle. The accident rate for the on-ramp unit was 1 (the lowest value among the urban groups), and the major types of accidents were rear-end and fixed object. For the off-ramp unit the accident rate was 1.63, and the major types of accident were rear-end and fixed object (the off-ramp value is also the lowest among the urban groups). Urban Group 5: As shown in Table IV.12, the number of interchanges was 21. The accident rate for the mainline unit was 8.90 accidents per interchange, and the major types of accidents were rear-end and fixed object. The accident rate for the crossroad unit was 5.52, and the major types of accidents were reareend, angle straight, fixed object, and rear-end drive. The accident rate for the on-ramp unit was 47 2.14, and the major types of accidents were rear-end and fixed object. For the off-ramp unit the accident rate was 2.71, and the major types of accident were rear-end and fixed object. Urban Group 6: As shown in Table IV.13, the number of interchanges was 10. The accident rate for the mainline unit was 11 accidents per interchange, and the major types of accidents were rear-end and fixed object. The accident rate for the crossroad unit was 6, and the major type of accident was rear-end. The accident rate for the on-ramp unit was 3.5 (the highest value among the urban groups), and the major types of accidents were rear-end and fixed object. For the off-ramp unit the accident rate was 6, and the major types of accident were rear-end and fixed object (the off-ramp value is the highest among the urban groups). Urban Group 7: As shown in Table IV.14, the number of interchanges was 12. The accident rate for the mainline unit was 7.75 accidents per interchange, and the major types of accidents were rear-end and fixed object. The accident rate for the crossroad unit was 6.5, and the major type of accident was rear-end. The accident rate for the on-ramp unit was 1.83, and the major type of accident was fixed object. For the off- ramp unit the accident rate was 2.5, and the major type of accident was rear-end. Urban Group 8: As shown in Table IV.15, the number of interchanges was 7. This group was excluded from further analysis. 48 Urban Group 9: As shown in Table IV.16, the number of interchanges was 8. This group was excluded from further analysis. Urban Group 10: As shown in Table IV.17, the number of interchanges was 11. The accident rate for the mainline unit was 15.0 accidents per interchange (the highest value for any urban group), and the major types of accidents were rear-end and fixed object. The accident rate for the crossroad unit was 7.09 (the highest value for any urban group), and the major types of accidents were rear-end, fixed object and angle straight. The accident rate for the on—ramp unit was 1.27, and the major type of accident was rear-end. For the off-ramp unit the accident rate was 2.82, and the major types of accidents were rear-end and fixed object. Urban Group 11: As shown in Table IV.18, the number of interchanges was 1. This group was excluded from further analysis. Urban Group 12: As shown in Table IV.19, the number of interchanges was 3. This group was excluded from further analysis. The highest and lowest accident rates for each analysis unit in the urban groups are: . Group 10 had the highest mainline accident rate (13.75) . Group 4 had the lowest mainline accident rate (5.24) . Group 10 had the highest crossroad accident rate (6.5) 49 . Group 4 had the lowest crossroad accident rate (3.37) . Group 6 had the highest on-ramp accident rate (3.5) . Group 4 had the lowest on-ramp accident rate (1) . Group 6 had the highest off-ramp accident rate (6.0) . Group 4 had the lowest off-ramp accident rate (1.63) 3). Summary of Rural Groups based upon Analysis Units Rural Group 1: As shown in Table IV.20, the total number of interchanges was 106. The accident rate of the mainline unit was 5.03, and the major types of accidents were animal, fixed object, rear-end, and overturn. The accident rate for the crossroad unit was 2.25, and the major types of accidents were fixed object, animal, rear-end, angle straight, angle turn, and head-on. The accident rate for the on—ramp unit was 0.4 (the lowest value among the rural groups), and the major types of accidents were fixed object and overturn. For the off-ramp the accident rate was 1.18, and the major types of accidents were fixed object, rear-end, overturn, and animal. Rural Group 2: As shown in Table IV.21, the number of interchanges was 47. The accident rate for the mainline unit was 5.57 accidents per interchange, and the major types of accidents were animal, fixed object and rear-end. The accident rate for the crossroad unit was 2.74, and the major types of accidents were fixed object, rear-end, angle.straight, animal, angle turn. The accident rate for the on-ramp unit was 0.60, 50 and the major type of accident was fixed object. For the off- ramp unit the accident rate was 1.57, and the major types of accident were rear-end and fixed object. Rural Group 3: As shown in Table IV.22, the number of interchanges was 43. The accident rate for the mainline unit was 5.93 accidents per interchange, and the major types of accidents were rear-end, fixed object, animal, and overturn. The accident rate for the crossroad unit was 4.19, and the major types of accidents were rear-end and fixed object. The accident rate for the on-ramp unit was 1.07, and the major types of accidents were fixed object, overturn and rear-end. For the off-ramp unit the accident rate was 2.47, and the major types of accident were rear-end and fixed object. Rural Group 4: As shown in Table IV.23, the number of interchanges was 36. The accident rate for the mainline unit was 4.97 accidents per interchange (the lowest value among the rural groups), and the major types of accidents were rear-end, fixed object, animal and overturn. The accident rate for the crossroad unit was 1.06 (the lowest value among the rural groups), and the major types of accidents were fixed object and rear-end. The accident rate for the on-ramp unit.was 0.44, and the major type of accident was fixed object. For the off- ramp unit the accident rate was 0.56 (the lowest value among the rural groups), and the major type of accident was fixed object. Rural Group 5: As shown in Table IV.24, the number of 51 interchanges was 7. This group was excluded from further analysis. Rural Group 6: As shown in Table IV.25, the number of interchanges was 27. The accident rate for the mainline unit was 7.41 accidents per interchange (the highest value among the rural groups), and the major types of accidents were rear- end, fixed object, animal and overturn. The accident rate for the crossroad unit was 2.96, and the major types of accidents were rear-end and fixed object. The accident rate for the on- ramp unit was 0.74, and the major types of accidents were overturn and fixed object. For the off-ramp unit the accident rate was 1.74, and the major types of accidents were rear-end and fixed object. Rural Group 7: As shown in Table IV.26, the number of interchanges was 17. The accident rate for the mainline unit was 6.94 accidents per interchange, and the major types of accidents were fixed object, rear—end, animal and overturn. The accident rate for the crossroad unit was 4.24, and the major types of accidents were rear-end and fixed object. The accident rate for the on-ramp unit was 1.18 (the highest value among the rural groups), and the major types of accidents were overturn and fixed object. For the off-ramp unit the accident rate was 2.53 (the highest value among the rural groups), and the major types of accidents were rear-end and fixed object. Rural Group 8: As shown in Table IV.27, the number of interchanges was 8. This group was excluded from further 52 analysis. Rural Group 9: As shown in Table IV.28, the number of interchanges was 29. The accident rate for the mainline unit was 6 accidents per interchange, and the major types of accidents were fixed object, rear-end, animal and overturn. The accident rate for the crossroad unit was 4.34 (the highest value among the rural groups), and the major types of accidents were fixed object and rear-end. The accident rate for the on-ramp unit. was 0.90, and the 'major ‘types of accidents were fixed object and overturn. For the off-ramp unit the accident rate was 2.31, and the major types of accidents were fixed object, rear-end, and overturn. Rural Group 10: .As shown in Table 1V329, the number of interchanges was 6. This group was excluded from further analysis. The highest and lowest accident rates for each analysis unit in the rural groups are: . Group 6 had the highest mainline accident rate (7.41) . Group 4 had the lowest mainline accident rate (4.84) . Group 9 had the highest crossroad accident rate (4.5) . Group 4 had the lowest crossroad accident rate (1.03) . Group 7 had the highest onwramp accident rate (1.18) . Group 1 had the lowest on—ramp accident rate (0.39) . Group 7 had the highest off-ramp accident rate (2.53) 53 . Group 4 had the lowest off-ramp accident rate (0.54) 4). Summary of Fringe Groups based on Analysis Units Fringe Group 1: As shown in Table IV.30, the total number of interchanges was 13. The accident rate of the mainline unit was 6, and the major types of accidents were animal, fixed object and rear-end. The accident rate for the crossroad unit was 3.85, and the major types of accidents were fixed object, rear-end, angle straight and angle turn. The accident rate for the on-ramp unit was 1.38, and the major types of accident were fixed object and overturn. For the off-ramp unit the accident rate was 3.23, and the major types of accidents were rear-end, fixed object and overturn. Fringe Group 2: As shown in Table IV.31, the number of interchanges was 25.The accident rate for the mainline unit was 6.6 accidents per interchange, and the major types of accidents were rear-end, fixed object, overturn and animal. The accident rate for the crossroad unit was 3.6, and the major types of accidents were rear-end, fixed object, angle straight and angle turn. The accident rate for the on-ramp unit was 1.24 (the lowest value among the fringe groups), and the major types of accidents were fixed object and rear-end. For the off-ramp unit the accident rate was 3.36, and the major types of accident were rear—end and fixed object. Fringe Group 3: As shown in Table IV.32, the number of 54 interchanges was 40. The accident rate for the mainline unit was 7.48 accidents per interchange, and the major types of accidents were rear-end, fixed object, overturn and animal. The accident rate for the crossroad unit.was 5.98 (the highest value among the fringe groups), and the major types of accidents were rear-end, fixed object, angle turn, angle straight and rear-end drive. The accident rate for the on-ramp unit was 2.18, and the major types of accidents were fixed object, rear-end and overturn. For the off-ramp unit the accident rate was 3.75, and the major types of accident were rear-end, fixed object and overturn. Fringe Group 4: . As shown in Table IV.33, the number of interchanges was 12. The accident rate for the mainline unit was 5.5 accidents per interchange (the lowest value among the fringe groups), and the major types of accidents were rear- end and fixed object. The accident rate for the crossroad unit was 1.5 (the lowest value among the fringe groups), and the major type of accident was nothing to be considered. The accident rate for the on-ramp unit was 1.25, and the major types of accidents were rear-end and fixed object. For the off-ramp unit the accident rate was 1.25 (the lowest value among the fringe groups), and the major types of accident were rear-end and fixed object. Fringe Group 5: As shown in Table IV.34, the number of interchanges was 6. This group was excluded from further analysis. 55 Fringe Group 6: As shown in Table IV.35, the number of interchanges was 11. The accident rate for the mainline unit was 7.27 accidents per interchange, and the major types of accidents were fixed object and rear-end. The accident rate for the crossroad unit was 4.82, and the major types of accidents were rear-end and fixed object. The accident rate for the on-ramp unit was 1.73, and the major type of accident was fixed object. For the off-ramp unit the accident rate was 4.36, and the major types of accidents were rear-end, fixed object and overturn. Fringe Group 7: As shown in Table IV.36, the number of interchanges was 12. The accident rate for the mainline unit was 5.92 accidents per interchange, and the major types of accidents were rear-end and fixed object. The accident rate for the crossroad unit was 3.67, and the major type of accident was fixed object. The accident rate for the on-ramp unit was 1.25, and the major type of accident was fixed object. For the off-ramp unit the accident rate was 2.5, and the major types of accidents were fixed object and rear-end. Fringe Group 8: As shown in Table IV.37, the number of interchanges was 14. The accident rate for the mainline unit was 13.71 accidents per interchange (the highest value among the fringe groups), and the major types of accidents were rear-end and fixed object. The accident rate for the crossroad unit was 5.71, and the major types of accidents were rear-end and fixed object. The accident rate for the on-ramp unit was 56 2.93 (the highest value among the fringe groups), and the major types of accidents were rear-end and fixed object. For the off-ramp unit the accident rate was 3.79, and the major types of accidents were rear-end and fixed object. Fringe Group 9: As shown in Table IV.38, the number of interchanges was 12. The accident rate for the mainline unit was 6.92 accidents per interchange, and the major types of accidents were rear-end, fixed object and.animal. The accident rate for the crossroad unit was 4.83, and the major type of accident was rear-end. The accident rate for the on-ramp unit was 1.92, and the major type of accident was fixed object. For the off-ramp unit the accident rate was 4.58 (the highest value among' the fringe «groups), and. the :major' types of accidents were fixed object and rear-end. Fringe Group 10: As shown in Table IV.39, the number of interchanges was 2. This group was excluded from further analysis. The highest and lowest accident rates for each analysis unit in the fringe groups are: . Group 8 had the highest mainline accident rate (12.8) . Group 4 had the lowest mainline accident rate (5.5) . Group 3 had the highest crossroad accident rate (5.98) . Group 4 had the lowest crossroad accident rate (1.5) . Group 8 had the highest on-ramp accident rate (2.73) 57 . Group 4 had the lowest on-ramp accident rate (1.29) . Group 6 had the highest off-ramp accident rate (4.8) . Group 4 had the lowest off-ramp accident rate (1.25) 5). Comparison of the accident rates From the results of the urban groups, group 4 had the lowest accident rates for any analysis units, whereas group 6 had the highest accident rates for the ramp units and group 10 had the highest accident rates for the. mainline and crossroad units. In the rural groups, group 4 again had the lowest accident rates for any analysis units excluding the on- ramp unit for which group 1.1nni the lowest accident rate. Group 6, group 7 and group 9 had the highest accident rates for the mainline unit, the ramp units and the crossroad unit, respectively. For the fringe groups, group 4 once again had the lowest accident rates for the same analysis units as the rural groups excluding the on-ramp unit for which group 2 had the lowest accident rate. Group 3, group 8 and group 9 had the highest accident rates for the crossroad unit, the mainline and on-ramp units, and the off—ramp unit, respectively. These groups by the analysis units can be compared as shown in Table IV.45. Iv-3. Summary of Regulte 58 Based upon the results from each group of some sample interchanges, the average number of accidents on the mainline unit were generally higher than those on the other units as shown in Table IV.46. However, the accident rates on the ramp units were higher than those on the mainline unit as shown in Table IV.47 when the traffic volumes were considered with those average accidents. The data file set to be used in modelling will be selected based on the Charts 4.1 through 4.4. The groups with less than 10 interchanges were excluded from the analysis. In the charts, the groups included for further analysis were marked by **. 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F—zn a2<¢a;;0 F—ZD &2<¢-20 FFz: O<0¢mw0¢u FFZD wz_42_<2 mma>F FszFUU< 7 1 a macho cash: no mucoaoam mmaanuumuafl oommmHHoo an mmmfiu uqmuwoofl m.>H manna «O.N oo.N 00.M OO.M mmOz—¢O Ozw-¢_¢O mauz< vvv M u>_¢o ¢wzF0 cvv v M M 2¢DF on~¢ Ozm-¢OFm OOO N 4<2Fz< OOO F v Fumwmo ¢w=>0 ONO 0M MN NF 0M Fumamo Ome; OOO M F c F z OOX¢0 OFO M F O mgu_zw> maoz_w m00mz<44m0m~2 OOO FFzD a2<¢-;;0 FFZO a2<¢-20 FFZD O<0¢mm0¢0 FFz: wz_az_<2 mwa>F szOFOO< 72 N macho sank: no muzaaoao nonmaoumuaw omeMHaoo ha mama» pamowoom a .>H QHQdB NO.v Fv.M OF.o MM.0 mmozF¢O Ozm-¢F¢O m4oz< ¢¢¢ m>F¢O ¢sz0 O¢¢ M z¢DF F:O_¢ Ozw-¢u_m OOO o 0<2Fz< OOO N N Fumamo ¢wz>0 ONO ON NN o O¢ Fumwmo OMXF; OOO F z Om¥¢0 OFO F F N w4031m> w4oz_m w00w2<44w0m_2 OOO FFzO a2<¢-;;0 F32: a2<¢-20 FFZD O<0¢mm0¢0 FFzO szazF<2 ww¢>F FszFOO< 73 m macho dunno no mpnmEoHo omnmnououaw oommmaaoo ha momma unmowoo< 0H .>H ”Hana. mo.F F Fm.m «N.m mmuz<=umszF ;o u \ szNQFQU< ;o a an n mmoz<=uaszF mo Numzaz No mm NNF ooF szmaFUU< mo «mmzaz 4Fao azm-¢FNo m4oz< «cc v m>Fmo awzFo oqq m m zmaF quFN ozw-¢ au¥¢o oFo F F o NONFzm> mdosz maomz<44mum_: coo FFz: a2<¢-;;0 F_z: a2<¢-20 FFZD O<0¢mm0¢0 F—23 wz~azF<2 mma>F szOFOO< 711 v macho Guano no munmaoao wounnouounw vomamaaoo kn mama» unmuwoom «H.>H manna 0M.N MO.N NN.M OM.O mwozF¢O O2m-¢~¢o mauz< qvc N w>_¢O ¢wzF0 Ovv F 2¢DF F:O_¢ Ozw-¢u_m OOO F 4<2_z< OOO F Fumwmo ¢sz0 ONO FF O MF «M Fumwmo owXF; OOO N z Ow¥¢0 OFO F M m40F2w> mJOsz m30mz<44wOMF2 OOO FFZD a2<¢-;;0 FFzO a2<¢-20 FFz: O<0¢wm0¢0 FFZO wz_az_<2 wwa>F szo~00< 75 m msoum manna mo mucoaoao ooausouoqu vommmHHoo an mama» nzmvwoom Nd .>H QHQMB o M.M o FF mmOz<=0¢wFZF ;0 a \ szmOFuu< ;0 % OF u mmuz<=u¢mF2F ;0 ¢mm232 O0 MM O0 OFF MF2OOFOO< ;0 ¢mm232 4_¢o ozm-¢_¢O w4oz< «v9 F w>F¢O ¢sz0 qu F F z¢OF F:OF¢ ozm-¢OFO OOO M 4<2~z< OOO F F F Fumwmo ¢m=>0 ONO MF NF O FM Fumwmo Ome; OOO F N F z wa¢0 OFO o OOO—zm> maosz w00mz<44mum~2 OOO F — FFZO a2<¢-;;0 FFZO a2<¢-zO FFZD O<0¢mm0¢0 FFZO mz_42_<2 wwa>F FszFOO< 76 o macho away: no muaoaoao omnmnouousw umeMHHoo an mama» uaoowoo¢ Md .>H GHQMB m.N MO.F m.o mF.F OOO2<=OOOF2F OO O \ szmOFOO< OO O NF u mmozFOO Ozm-mONO OOO2< OOO m m>FOO OOOFO OOO N m N szF FOOFO Ozw-m Om¥¢<¢ FOO OMO zF<¢F FF: ONO m N F N ZOOFOO>O OFO F F q OOOF=O> OOOsz OOOmzH 0HQMB FN.N M¢.M 0N.o M.OF mwoz<=0¢sz_ ;0 n \ MFszFOO< ;0 u N u mmuz<=u¢mF2_ ;0 ¢mm232 OF «N Mo NN szmOFUU< ;0 ¢mm20z 4F¢O Ozm-¢_¢o maoz< qu m>~¢O ¢m:FO 04v F F F z¢OF on~¢ Ozm-¢0~m OOO M O<2_z< OOO F Fumamo ¢meO ONO M FF 0F FN Fuwamo Ome; OOO 2 Om¥¢O OFO F M OOO—zw> NJOZFO m00m2<44m0m_2 OOO = a FFz: ;2<¢-;;0 FFZD ;2<¢-20 FFzD O<0¢mw0¢0 FFz: wz_42_<2 mwa>F Fzmo_00< 78 a macho away: no munoamam mocngouounw cmeMHHoo an mwmmu unmvwoo¢ m." .>H 0HDMB mF.m mF.O F mF.O mmOz<=O¢mF2_ “O a F szwOFOO< OO O O u mmozF¢O Ozm-OFOO OOO2< OOO m w>FOO OOOFO OOO N ZOOF FOOFO Ozm-O OOgOO OFO n F OOOFIO> OOOsz OOOmzH GHAUB OM.N NF.F OM.o MN.MF mmoz<=0¢w>z~ ;0 fi \ MFzmoFuO< ;0 n NF n mszF¢O ozw-¢F¢O w402< OOO F m>_¢O ¢m=>0 OOO F z¢OF F:0_¢ Ozm-¢OFO OOO O<2Fz< OOO N Fumwmo ¢wzF0 ONO OF N MF 0M Fuwwmo waF; OOO F z Om2¢0 OFO F q MOO—:w> waoz_m m30wz<44mumF2 OOO . a‘ F FFz: a2<¢.;;0 FFzO ;2<¢-20 FFzD O<0¢wm0¢0 FFz: wz_az_<2 mm¢>F szOFOO< 80 ca macho Guano no munoaoam omnusououafi commaduoo ha momhu unmofioo< NH .>H OHQflB M F O O mmozF¢O ozm-¢~¢o wduz< OOO m>~¢a ¢m=FO OOO F:O_¢ ozw-¢OFO OOO O<2Fz< OOO Fumnmo ¢sz0 ONO Fumwmo Ome; OOO z Ow¥¢0 OFO F MOO—:w> NOOZFO m00mz<44wOOF2 OOO FFz: ¢2<¢-;;0 FFZO ;2<¢-20 FFZO O<0¢wm0¢0 FFzD wz~gz_<2 mma>F szOFOO< | Ad macaw gonna no magmaoam omamnouounw commmaaoo an mama» uamcfloofl OH .>H ”Anna. 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O F O 98.5 :23. O F asogu FmO3¢ « « OF QJoOO COOL: O O O anoOO emnOO FF FF F. a O QJOLU cont: F F F F F 1F F _ F0 FmozoF co amazoF Ou amazoF c: mezoF Fo FOOLOFI co FOOLOFI Lu mecsz c: FOmOOFI OQDOOO | moumu unmoflooa unused can amonoOz on» no woman mmnouv an mafia: mwmhamam on» no nomaummaoo mv.>H manna 108 109 box.“ v m h©.oa N mbomwm m N mfiv o m mDOme m O m NH m macme ¢ m m Ha m mDome m.v hm.H m.m o m mDOmGD mamnlmuo :0 mfimuuco co . Unogmmonu :0 mcflacflmz :0 .wcH\mychHOU< .u:H\muchHoo¢ .unH\mu:mUwoo4 .ucH\muchfl00¢ QDOHU | mafia: oamadm ha ooamsoumvnfi Hon muamcfiooa no Hwnasc monum>¢ was ov.>H manna 110 ham.o OHN.O mma.o wHH.o m abomwm hmm.o mom.o >HH.o moa.o m abomOD firm.w Hmm.o ovm.o moa.o m abomwb mmm.o bom.o mba.o hNH.o m abome mmv.o Hwa.o mmo.o wmo.o N mbomwb mamnlmuo :0 mamulco :0 Umoummouo co mewacflmz :0 muwm ucmcfioo< mumm ucmcflood mumm ucmcfloo< mumm ucmnfloo« macho mafia: mamaam >3 mmamsoumunw Ham ouch unmUwoou oonum>m was hv.>H OHQUB HHMSO HMEHOh mafia MUMQ H.v HHMAU _ _ w umm umm umm mvmn mumo mpma mmcflum amusm CMQHD _ _ maflm mumo Hmpmwz _ OF mHHh mama omemue mHHm mung quUHOOd maflm mumo ofluumfiomw 111 Urban Data Set -— Group 1 —+ Group 2 ** —% Group 3 ** —~ Group 4 ** —a Group 5 ** —4 Group 6 ** —fi Group 7 ** —u Group 8 —— Group 9 ~—4 Group 10**1 HA Group 11 La Group 12 L L L L L L L L L L L . 1 Mainline Unit Crossroad Unit On-ramp Unit Off-ramp Unit Mainline Unit Crossroad Unit On-ramp Unit Off-ramp Unit Mainline Unit Crossroad Unit On-ramp Unit Off-ramp Unit Mainline Unit Crossroad Unit On-ramp Unit Off-ramp Unit Mainline Unit Crossroad Unit On°ramp Unit Off-ramp Unit Mainline Unit Crossroad Unit On—ramp Unit Off-ramp Unit Mainline Unit Crossroad Unit On-ramp Unit Off-ramp Unit Mainline Unit Crossroad Unit On-ramp Unit Off-ramp Unit Mainline Unit Crossroad Unit On-ramp Unit Off-ramp Unit Mainline Unit Crossroad Unit On-ramp Unit Off-ramp Unit Mainline Unit Crossroad Unit On-ramp Unit Off-ramp Unit Mainline Unit Crossroad Unit On-ramp Unit Off-ramp Unit Chart 4.2 Data file set by Urban Groups 112 Rural Data Group Group Group 3** Group Group Set Chart 4.3 Group 6** Group 7** Group Group git Group Data file set by Rural Groups Mainline Unit Crossroad Unit On-ramp Unit Off-ramp Unit Mainline Unit Crossroad Unit On-ramp Unit Off-ramp Unit Mainline Unit Crossroad Unit On-ramp Unit Off-ramp Unit Mainline Unit Crossroad Unit On-ramp Unit Off-ramp Unit Mainline Unit Crossroad Unit On-ramp Unit Off-ramp Unit Mainline Unit Crossroad Unit On-ramp Unit Off-ramp Unit Mainline Unit Crossroad Unit On-ramp Unit Off-ramp Unit Mainline Unit Crossroad Unit On-ramp Unit Off-ramp Unit Mainline Unit Crossroad Unit On-ramp Unit Off-ramp Unit Mainline Unit Crossroad Unit On-ramp Unit Off-ramp Unit 113 Fringe Data Group ** Group ** Group it Group ** Group Set Group ** Chart 4.4 Group ** Group _fi L L L L L L L 1' Group 9 Group 10 L L Data file set by Fringe Groups Mainline Unit Crossroad Unit On-ramp Unit Off-ramp Unit Mainline Unit Crossroad Unit On-ramp Unit Off-ramp Unit Mainline Unit Crossroad Unit On-ramp Unit Off-ramp Unit Mainline Unit Crossroad Unit On-ramp Unit Off-ramp Unit Mainline Unit Crossroad Unit On-ramp Unit Off-ramp Unit Mainline Unit Crossroad Unit On-ramp Unit Off-ramp Unit Mainline Unit Crossroad Unit On-ramp Unit Off-ramp Unit Mainline Unit Crossroad Unit On-ramp Unit Off-ramp Unit Mainline Unit Crossroad Unit On-ramp Unit Off-ramp Unit Mainline Unit Crossroad Unit On-ramp Unit Off-ramp Unit 114 CHAPTER V MODELS Having determined that models constructed on the total data base did.not produce results sufficiently reliable to use in selecting alternative design parameters, the stratified data set was used for further analyses. As described in chapter IV, the data had been stratified by activity density (urban, rural and fringe) and by interchange design groups. The data records were divided into cells representing this two way classification, and models were constructed for each of the analysis units within a cell. Only those cells with at least 10 interchanges were modeled. These models were based upon the following formula: Y = f(X1, X2, x3, x,, X5) where Y = Number of accidents on road segment (i) __>< ll Population (in 1000's) of the county )9 = Lane mileage of the analysis unit (in 0.01 mile units) )5 = Number of on-ramps )Q = Number of off-ramps )g = Average Daily Traffic (ADT) v.1 Models constructed on the Mainline Unit 115 Using stepwise linear regression, the following models - provided the highest (R2) value for each group. A. Models constructed based on the total accidents on the mainline units 1. Model of urban group 2 Y = -l9.9lO + 0.000816X5 where Y = Total number of Accidents on road segment (i) X5:= Average Daily Traffic (ADT) From the above linear regression model, the multiple regression coefficient (R) was 0.5441, based on 19 interchanges (38 road segments). 2. Model of urban group 3 Y = -l4.551 + 0.00115X5 where Y = Total number of accidents on road segment (i) )g = Average Daily Traffic (ADT) From the above linear regression model, the multiple regression coefficient (R) was 0.9025, based on 11 interchanges (22 road segments). 116 3. Model of urban group 5 Y = -42.267 - 0.038X1-+ 0.00215X5 where Y = Total number of accidents on road segment (i) )9 = Population (in 1000's) of the county )8 = Average Daily Traffic (ADT) From the above linear regression model, the multiple regression coefficient (R) was 0.7839, based on 11 interchanges (28 road segments). 4. Model of rural group 1 Y = 0.937 - 0.00880X1-+ 0.000657XS where Y = Total number of accidents on road segment (i) X1 Population (in 1000's) of the county )g = Average Daily Traffic (ADT) From the above linear regression model, the multiple regression coefficient (R) was 0.7336, based on 50 interchanges (99 road segments). 5. Model of rural group 2 117 Y = 4.650 + 0.000258X5 where Y = Total number of accidents on road segment (i) X5== Average Daily Traffic (ADT) From the above linear regression model, the multiple regression coefficient (R) was 0.3547, based on 27 interchanges (54 road segments). 6. Model of rural group 3 Y = -5.007 + 0.03OX2 + 0.000354X5 where Y = Total number of accidents on road segment (i) X2== Lane mileage )g = Average Daily Traffic (ADT) From the above linear regression model, the multiple regression coefficient (R) was 0.7150, based on 25 interchanges (49 road segments). 7. Model of rural group 4 Y = 3.435 + 0.06ox2 - 8.534X3-+ 0.000289xS where Y = Total number of accidents on road segment (i) 118 )9 = Lane mileage )9 = Number of on-ramps )g = Average Daily Traffic (ADT) From the above linear regression model, the multiple regression coefficient (R) was 0.8262, based on 23 interchanges (55 road segments). 8. Model of rural group 6 Y = 0.182 + 0.000410XS where Y = Total number of accidents on road segment (i) )g = Average Daily Traffic (ADT) From the above linear regression model, the multiple regression coefficient (R) was 0.4087, based on 18 interchanges (48 road segments). 9. Model of rural group 7 Y = -6.247 + 0.069X2-+ 0.000316XS where Y = Total number of accidents on road segment (i) X2== Lane mileage )g Average Daily Traffic (ADT) 119 From the above linear regression model, the multiple regression coefficient (R) was 0.6497, based on 12 interchanges (26 road segments). 9. Model of rural group 9 Y = -27.865 + 11.561X3-+ 0.000585X5 where Y = Total number of accidents on road segment (i) )g = Number of on-ramps )g = Average Daily Traffic (ADT) From the above linear regression model, the multiple regression coefficient (R) was 0. 6290, based on 19 interchanges (38 road segments). 10. Model of fringe group 2 Y = 7.932 - 0.067X2-+ 0.000685X5 where Y = Total number of accidents on road segment (i) X2:= Lane mileage )% = Average Daily Traffic (ADT) From the above linear regression model, the multiple regression coefficient (R) was 0.8200, based on 14 120 interchanges (28 road segments). 11. Model of fringe group 3 Y = -9.340 - 0.00629X1 + 0.062X2 + 0.000566X5 where Y = Total number of accidents on road segment (i) X1== Population (in 1000's) of the county )9 = Lane mileage )g = Average Daily Traffic (ADT) From the above linear regression model, the multiple regression coefficient (R) was 0.6872, based on 26 interchanges (52 road segments). 12. Model of fringe group 8 Y = —7.127 + 0.149x2 where Y = Total number of accidents on road segment (i) X2==-Lane mileage From the above linear regression model, the multiple regression coefficient (R) was 0.7452, based (Ni 10 interchanges (26 road segments). 121 13. Model of fringe group 9 Y = 5.488 + 0.077x2 where Y = Total number of accidents on road segment (i) X2== Lane mileage From the above linear regression model, the multiple regression coefficient (R) was 0.7162, based on 10 interchanges (20 road segments). v.2 Models conetructed on the crossroed unit Using stepwise linear regression, the following models provided the highest (R?)'value for each group. A. Models constructed based on the total accidents on the crossroad units 1. Model of urban group 3 Y = -57.966 + 0.553x2-+ 0.00257xS where Y = Total number of accidents on road segment (i) )9 = Lane mileage XS Average Daily Traffic (ADT) 122 From the above linear regression model, the multiple regression coefficient (R) was 0.8590, based on 11 interchanges (11 road segments). 2. Model of urban group 4 Y = -21.355 + 0.425x2-+ 0.00167xS where Y = Total number of accidents on road segment (i) X2== Lane mileage X5== Average Daily Traffic (ADT) From the above linear regression model, the multiple regression coefficient (R) was 0.8656, based on 11 interchanges (11 road Segments). 3. Model of rural group 1 Y = -3.654 + 0.00348xS where Y = Total number of accidents on road segment (i) X5== Average Daily Traffic (ADT) From the above linear regression model, the multiple regression coefficient (R) was 0.7063, based on 50 123 interchanges (50 road segments). 4. Model of rural group 2 Y = 1.959 + 0.00246x5 where Y = Total number of accidents on road segment (i) )g = Average Daily Traffic (ADT) From the above linear regression model, the multiple regression coefficient (R) was 0.6711, based on 27 interchanges (27 road segments). 5. Model of rural group 3 Y = 0.257 + 0.00235XS where Y = Total number of accidents on road segment (i) )g = Average Daily Traffic (ADT) From the above linear regression model, the multiple regression coefficient (R) was 0.8034, based on 25 interchanges (25 road segments). 6. Model of rural group 6 Y = ~175.947 + 0.373X2-+ 48.619X4 124 where Y = Total number of accidents on road segment (i) )9 = Lane mileage )9 = Number of off-ramps From the above linear regression model, the multiple regression coefficient (R) was 0.8795, based on 12 interchanges (12 road segments). 7. Model of rural group 7 Y = 4.597 + 0.00144X5 where Y = Total number of accidents on road segment (i) )9 = Average Daily Traffic (ADT) From the above linear regression model, the multiple regression coefficient (R) was 0.8525, based on 10 interchanges (10 road segments). 8. Model of rural group 9 Y = -O.514 + 0.00384X5 where Y = Total number of accidents on road segment (i) )9 = Average Daily Traffic (ADT) 125 ms- fi*_‘——;i From the above linear regression model, the multiple regression coefficient (R) was 0.8356, based on 19 interchanges (19 road segments). 9. Model of fringe group 2 Y = 4.077 + 0.00283x5 where Y = Total number of accidents on road segment (i) )9 = Average Daily Traffic (ADT) From the above linear regression model, the multiple regression coefficient (R) was 0.7534, based on 14 interchanges (14 road segments). v.3 Models constructed on the on-ramp units Using stepwise linear regression, the following models provided the highest (R2) value for each group. A. Model constructed based on the total accidents on the on- ramp units 1. Model of urban group 2 126 Y = 4.723 - 0.00118XS where Y = Total number of accidents on road segment (i) )9 = Average Daily Traffic (ADT) From the above linear regression model, the multiple regression coefficient (R) was 0.5051, based on 17 interchanges (18 on-ramps). B. Models conetructed based on the fixed object accidents on the on-ramp units 1. Model of urban group 2 Y = 0.883 + 0.039X2 - 0.000605XS where Y = Total number of accidents on road segment (i) X2== Lane mileage )9 = Average Daily Traffic (ADT) From the above linear regression model, the multiple regression coefficient (R) was 0.7388, based on 17 interchanges (18 on-ramps). v.4 Models conetructed on the off-ramp unit Using stepwise linear regression, the following models 127 provided the highest (R2) value for each group. A. Model constructed based on the total accident rate on the off-ramp unite 1. Model of urban group 2 Y = 8.236 - 0.00236X1 + 0.271X2 - 0.000565X5 where Y = Total number of accidents on road segment (i) _?< H Population (in 1000's) of the county )9 = Lane mileage >< 1| 5 Average Daily Traffic (ADT) From the above linear regression model, the multiple regression coefficient (R) was 0.9091, based on 17 interchanges (l9 off-ramps). B. Models constructed based on the fixed obiect accidents on the off-ramp units 1. Model of urban group 2 Y = 3.226 - 0.00102X1 where Y = Total number of accidents on road segment (i) X1== Population (in 1000's) of the county 128 From the above linear regression model, the multiple regression coefficient (R) was 0.5034, based on 1] interchanges (19 off-ramps). C. Models conetructed based on the rear-end accidents on the off-ramp units 1. Model of urban group 2 Y = -O.267 + 0.152X2 where Y = Total number of accidents on road segment (i) X2== Lane mileage From the above linear regression model, the multiple regression coefficient (R) was 0.6195, based on 17 interchanges (l9 off-ramps). Summary of Reeulte Based upon the above models, the sign of each variable term was recorded for each group as shown in Table V.1. Some general observations that resulted from a review of these models were as follows: 129 1. On the mainline unit of urban freeway interchanges, all the models have a positive sign in the average daily traffic term as would be expected. 2. On the mainline unit of rural freeway interchanges, all the models again have a positive sign in the average daily traffic term. Also, all the models have a positive sign in the lane mileage term. This indicates that the number of accidents increases with the length of the road segment. Most of the models have a positive sign in the number of on-ramps term, indicating that the number of accidents on the rural mainline unit increases where there are more on-ramps. 3. On the mainline unit of fringe freeway interchanges, all the models again have a positive sign in the average daily traffic term. This is consistent with the results in the urban and rural areas. Also, most of the models have a positive sign in the lane mileage term, indicating that the number of accidents is greater for the longer fringe mainline units. 4. On the crossroad unit of urban freeway interchanges, all the models have a pmsitive sign in the lane mileage and average daily traffic terms, indicating that the number of accidents increases with the longer crossroad units and increased traffic. 5. On the crossroad unit of rural and fringe freeway interchanges, all the models have a positive sign in the average daily traffic term. 6. There were an insufficient number of models of the on-ramp 130 and off-ramp accident frequency to draw any general conclusions. 7. Attempts to model specific accident types (fixed object accidents, rear-end accidents) did not improve the model accuracy (R2 value). Therefore, models built on predicting total accidents were retained for further testing. 8. Relatively high values of R were achieved for group 3 interchange mainline and crossroad units in all three categories (urban, rural and fringe). Group 3 interchanges include Modified Diamond, Modified Tight Diamond and Parclo A 4 Quad. Thus, it may be possible to accurately predict the accidents to be expected at these interchanges. 9. Group 2 interchanges (Tight Diamond and Urban Diamond) were not easily modeled, with low values of R resulting from the urban and rural mainline models. The fringe area model fit the data better, with an R value of 0.8200. 10. In the rural area, group 4 interchange (Partial Diamond, Partial Tight Diamond, Trumpet A and Partial Directional Y) models showed a good fit for both the crossroad and the mainline segments. There was an insignificant sample of this interchange group in the urban and fringe areas, so no models were constructed for this interchange groups. 131 .deme-+wo memos ago .mQEmL-co mcmoe mzo .pmoammoeu mmc_ee mcmoe .pmoemmoeu datum mcmoe .pmotmmOLU cane: mcmoe .oc_dc_mz omc_uu mcmoe .oc_dc_mz data“ mcmme .mc_dc_mz cont: mcmoe u; on u: x; 2m 2: Oeuwmue waemo .m>< mmemuimwo mo umnfidz mmamnlso wo HOQEDZ mmmeHE mama :oflDMHsmom mmo mZO Um UM UD 2m 2m SD com: moanmflhm> 132 couonuunsoo maocoa on» :w can: mayo» manoeuu> no human Ho sowumowuwmmoao H.> manna OF 6 1:0«u mozF¢1 oF oF w enema musz1 6N 6N m 1:0xu musz1 93 OF N aaoao wuzFe1 mm 6F 6F 6 aaoeu F OHQMB NoF~.o o asoeu 162Fe1 oFew.o NmeN.o a 1:6«6 wozF¢1 NFmo.o m aaoxu musz1 manna CHAPTER VI CALIBRATION In order to test the above models, all models which had a multiple R coefficient greater than 0.7 and were based on more than 10 interchanges for the mainline and crossroad units were considered. However, all the groups for the ramp units were considered.during the calibration procedure since samples of ramp data collected were so small. The linear regression models were constructed for each group of mainline, crossroad and ramp units based on population (1000's) of the county (X1), lane mileage (in 0.01 mile unit) of the analysis unit (X2) , number of on-ramps (X3), number of off-ramps (X4), average daily traffic (ADT) (X5) and the total number of accidents. VI. 1 Modele for the Meinline Unit 1. Model of urban group 3 Y = -14.551 + 0.00115xS From the above linear regression model, the predicted values for each interchange not used in constructing the model for this group were found, and these were compared with the actual values as shown in Table V1.1. 135 2. Model of urban group 5 Y = -42.267 - 0.038X1-+ 0.00215XS From the above linear regression model, the predicted values and the actual values are shown in Table VI.2. 3. Model of rural group 1 Y = 0.937 - 0.00880X1-+ 0.000657XS From the above linear regression model, the predicted values and the actual values are shown in Table VI.3. 4. Model of rural group 3 Y = -5.007 + 0.030X2-+ 0.000354X5 From the above linear regression model, the predicted values and the actual values are shown in Table V1.4. 5. Model of rural group 4 Y = 3.435 + 0.06OX2 - 8.534x3 + 0.000289XS From the above linear regression model, the predicted values 136 and the actual values are shown in Table VI.5. 6. Model of fringe group 2 Y = 7.932 - 0.067X2-+ 0.000685XS From the above linear regression model, the predicted values and the actual values are shown in Table VI.6. 7. Model of fringe group 8 Y = -7.127 + 0.149X2 From the above linear regression model, the predicted values and the actual values are shown in Table VI.7. 8. Model of fringe group 9 Y = 5.488 + 0.077X2 From the aboVe linear regression model, the predicted values and the actual values are shown in Table V1.8. v.2 Modele for the croeeroed unit 1. Model of urban group 3 137 Y = -57.966 + 0.553x2-+ 0.00257xS From the above linear regression model, the predicted values and the actual values are shown in Table VI.9. 2. Model of urban group 4 Y = -21.355 + 0.425x2-+ 0.00167xS From the above linear regression model, the predicted values and the actual values are shown in Table V1.10. 3. Model of rural group 1 Y = -3.654 + 0.00348XS From the above linear regression model, the predicted values and the actual values are shown in Table VI.11. 4. Model of rural group 3 Y = 0.257 + 0.00235X5 From the above linear regression model, the predicted values and the actual values are shown in Table V1.12. 138 5. Model of rural group 6 Y = -175.947 + 0.373X2-+ 48.619X4 From the above linear regression model, the predicted values and the actual values are shown in Table V1.13. 6. Model of rural group 7 Y = 4.597 + 0.00144XS From the above linear regression model, the predicted values and the actual values are shown in Table VI.14. 7. Model of rural group 9 Y = -0.514 + 0.00384XS From the above linear regression model, the predicted values and the actual values are shown in Table VI.15. 8. Model of fringe group 2 Y = 4.077 + 0.00283x5 From the above linear regression model, the predicted values 139 and the actual values are shown in Table V1.16. v.3 Models for the on-ramp unit 1. Model of urban group 2 Y = 4.723 - 0.00118X1 From the above linear regression model, the predicted values and the actual values are shown in Table VI.17. v.4 Modele for the off-ramp unit 1. Model of urban group 2 Y = 8.236 - 0.00236x1 + 0.271x2 - 0.000565x.5 From the above linear regression model, the predicted values and the actual values are shown in Table VI.18. Summary of Results Based upon the results of model calibration, the following conclusions were drawn: 1. Out of the urban and rural mainline groups, group 3 which 140 comprises the interchange types of Modified Diamond, Modified Tight Diamond and Parclo A 4 Quad predicts the observed values well as shown in Graphs 1 and 4. These models indicate that the number of accidents on the urban freeway interchanges of Modified Diamond, Modified Tight Diamond and Parclo A 4 Quad types depends primarily on the average.daily traffic (ADT) and increases with increased traffic. However, models to predict the number of accidents on the rural freeway interchanges of the same types also include the lane mileage variable with the accident frequency increasing with the length of the road segment. 2. Out of the fringe mainline groups, group 2 and group 8 demonstrated good prediction capability as shown in Graphs 6 and 7. Group 2 comprises Tight Diamond and Urban Diamond types. Groupi8 comprises Cloverleaf, Cloverleaf'with.CD roads, Cloverleaf minus 1 loop and Directional with loops types. In group 2, the models indicate that the number of accidents on these types of interchanges depend on the lane mileage and average daily traffic (ADT). However, in group 8, the number of accidents on these types of interchanges depends only on the lane mileage and increases with the length of road segment. 3. Out of the rural crossroad groups, group 3 and group 7 predict the observed values well as shown in Graphs 12 and 14. Group 3tcomprises Modified Diamond, Modified Tight Diamond and Parclo A 4 Quad. Group 7 comprises Parclo AB and Partial 141 Directional. The number of accidents on these types of interchanges depends on the average daily traffic (ADT) and increases with the increased traffic. This is consistent with the results of other groups. 4. Out of the urban off-ramp groups, group 2 which comprises Modified Diamond, Modified Tight Diamond and Parclo A 4 Quad types has a good prediction for the observed values as shown in Graph 18. The number of accidents on these types of interchanges depends on the population, lane mileage and average daily traffic (ADT), and increases with less population, longer length and reduced traffic. 5. Out of the remaining groups, 3 groups had at least one negative predicted value and most of the remaining groups gave poor predictions for the observed values. This might.be result of the existence of outliers within the data. If the outliers are removed, better predictions for the remaining groups could be expected. 142 Table VI.1. Comparison.of.Actual and Predicted values of Total accident frequency in UM-Group 3 _ Actual Values Rank Predicted Values Rank 100 3 118.32 3 167 2 215.62 1 173 1 129.36 2 89 4 68.88 4 143 OON F 921me 1| 653001“. .6 mm:_m> .6398. one OOF om o L F 1 6:02.661“. Fo mo:_m> 8.06011 2852 :38 m 9.010123 F0 _mUo_>_ F.6 guano om 00—. of. OON 0mm 144 Table VI.2 Comparison of Actual and Predicted values of total accident frequency in UM-Group 5 — Actual Values Rank Predicted Values Rank 72 2 154.60 2 229 1 280.08 1 15 3 -24.34 4 8 4 71.80 3 145 0mm OON F 621mm l1 16:039.... .6 mo:_m> .mBoFF. omw OOF on o _ a 1 18.16231“. Fo wo:_m> “0.90605 E8591 .88 m 95.5123 ..0 EUOE N.» guano om OOF omr OON com com 146 Table VI.3 Comparison of Actual and Predicted values of Total accident frequency in RM-Group 1 Actual Values Rank Predicted Values Rank 23 3 16.36 7 O 17 54.40 1 21 5 16.32 8 11 10 15.50 9 3 15 7.34 17 45 1 35.28 2 l3 9 26.84 3 7 13 15.14 10 18 8 26.06 4 11 10 9.60 14 22 4 17.00 6 27 2 20.32 5 8 12 9.44 15 3 15 12.32 13 7 13 9.38 16 20 6 12.84 12 19 7 14.28 11 147 on F motow 1 65:02.1 Fo wo:_m> .mFFFog. m.6 genus ov om ON OF 0 F e _ fl 16530011 .6 wo:_m> 620605 2852 .98 F 990.3 .5 .822 OF ON om 0v om om 148 Table‘VI.4 icomparison.of Actual and Predicted values of Total accident frequency in RM-Group 3 Actual Values Rank Predicted Values Rank 52 l 29.68 2 39 2 24.33 3 19 5 10.58 6 26 3 12.94 5 11 6 7.78 7 11 6 6.74 9 10 8 7.02 8 21 4 75.96 1 10 8 21.34 4 149 v.6 gmmuw F wmtmw 1 18:03.02“. .6 wo:_m> .6395. CO om CV 00 ON O_. O _ _ F F 1 0 Ya \.\\. om 6:969“. Fo wo:_w> 8860111 E8591 :28 m 92.0.53 .5 .822 150 Table V1.5 Comparison of Actual and Predicted values of Total accident frequency in RM-Group 4 — Actual Values Rank Predicted Values Rank 45 2 14.64 3 5 6 9.30 5 4 7 14.52 4 20 3 19.54 2 6 5 37.70 1 3 8 3.60 8 12 4 4.70 7 69 l 7.80 6 151 185301“. Fo mo:_m> .6394 on 00 om m.m genus F motow |.| 0v om ON 9 O F F 111I11I11r1It1I11I111111111111111111111111. . 1FUF F F F F 0 1//1+] F4 QJOFGISE .5 _mUo_>_ ov 18.103001“. Fo mm:_m> 886011 28.8... .88 152 Table v1.6 Comparison of Actual and Predicted values of Total accident frequency in FM-Group 2 Actual Values Rank Predicted Values Rank 23 2 45.23 2 17 4 33.17 3 19 3 27.92 4 14 5 21.45 5 60 1 75.59 1 153 on F motow 1| 552311 .6 mm:_m> FmsFo< 6.6 gauge 00 om OFF. on ON OF 0 F F F F F F 55:09.1 Fo mm:_m> meoFFFanF chEoo< .901. N 99.0-5.1 ..6 .822 ON CV 00 ow 154 W.fmw- Table V1.7 Comparison of Actual and Predicted values of Total accident frequency in FM-Group 8 _ Actual Values Rank Predicted Values Rank 167 1 153.56 1 25 4 77.38 3 124 2 79.67 2 40 3 49.86 4 155 OON F motow 1.1 5:862“. Fo mo:_a> FaBo< 09 00.. om o F F F >ocozco1u. Fo mo:_m> coFoFUoE €8.62 .98 m QJOFOIFZHF .5 FmFqu>F F.o guano 0 ON OFN om ow OOF ONF OFF 00—. 156 Table V1.8 Comparison of Actual and Predicted values of Total accident frequency in FM-Group 9 — Actual Values Rank Predicted Values Rank 39 2 38.24 1 42 1 35.69 3 21 3 31.46 4 16 4 37.61 2 157 1“ Film F wotow 1| 55581 10 mo:_a> FaBo< a.o guano om OFa om ON OF 0 F F F F I. 55:08". Fo oo:_m> quoGoE E852 .98 a 9.01612“. .6 Foco_>_ OF ON om 0v 158 Table V1.9 Comparison of Actual and Predicted values of Total accident frequency in UC-Group 3 Actual Values Rank Predicted Values Rank 87 2 141.48 1 43 ‘ 3 65.04 3 30 4 -25.20 4 161 1 98.96 2 159 OON F wotow 1! 165308“. .6 oo:_a> FaBo< omF 00.. on o F F F 5:858”. Fo mo:_m> poFoGoi chEoo< :32. o 820-0: .5 .865. m.o guano om OOF omF. 160 Table V1.10 Comparison of Actual and Predicted values of'Total accident frequency in UC-Group 4 — Actual Values Rank Predicted Values Rank 46 1 35.85 3 11 3 49.46 1 28 2 45.87 2 4 4 4.22 4 161 om on.o guano F motow |.| 5:268... .Fo mo:_a> _m:Fo< OFN om ON OF 0 F F F J 5:858“. Fo mo:_a> noFoFUoFu E8.8< .88 F4 9510-03 .5 FoUoSF OF. ON om OF... on 00 162 Table V1.11 Comparison of Actual and Predicted values of Total accident frequency in RC-Group 1 — Actual Values Rank Predicted Values Rank 3 10 2.26 16 10 4 16.08 6 4 8 17.23 5 2 12 10.61 7 1 13 2.61 14 12 3 2.37 15 14 2 9.99 8 0 17 5.05 12 37 1 49.94 1 3 10 5.74 11 1 13 6.79 10 4 8 18.62 4 l 13 0.87 17 8 5 22.45 2 1 13 9.57 9 7 6 3.65 13 6 7 22.45 2 163 0% F motom |.| 55:08“. .6 mo:_a> FaSo< om ON 9 an.o guano F T . F om 16:869.“. Fo oo:_a> 62068.1 28.8,... .98 F 865-91 .5 .822 164 -45” Table V1.12 Comparison.of.Actual and‘Predicted values.of Total accident frequency in RC-Group 3 — Actual Values Rank Predicted Values Rank 54 2 24.20 2 6 5 9.42 6 11 3 10.83 4 1 9 1.27 9 7 4 1.29 8 2 7 9.66 5 5 6 4.09 7 67 1 61.12 1 2 7 13.68 3 165 Fun F wotow 5:858“. Fo wo:_a> Fa30< 00 on O? on ON OF Nn.o guano F F F F F OF. ON om 0v. om 1 om on 5:868“. ..o wo:_a> 83:55 2852 .98 o 865-8. .5 .86.). 166 Table VI.13 Comparison of Actual and Predicted values of Total accident frequency in RC-Group 6 — Actual Values Rank Predicted Values Rank 44 1 29.59 2 1 3 -66.03 4 1 3 -60.81 3 29 2 82.68 1 167 mn.o guano . F wotom 1.1 16:33.... .5 mo:_a> FazFo< om 0v om ON OF 0 F F F F owl ”loo: 10¢: low: 0 ..oN now low low 00.. 6:868“. 10 mo:_a> 69068.1 chEood. :38. a 820-01 .5 .86.). W8 Table‘VI.14 Comparison.of.Actual and.Predicted values of'Total accident frequency in RC-Group 7 Actual Values Rank Predicted Values Rank 2 4 9.08 4 11 3 19.72 2 21 2 13.96 3 49 1 30.76 1 169 om «N.o guano F wotow l1 35:08“. .6 mo:_a> _a:Fo< 0v om ON OF 0 F F F F 5:11:31“. .6 mo:_a> ooFQFpoFu chEoo< :32. F 82.6-8. .5 .80.). OF mF ON mm on mm 170 Table'VI.15 Comparison of Actual and Predicted values of Total accident frequency in RC-Group 9 — Actual Values . Rank Predicted Values Rank 16 1 19.72 2 16 1 17.92 3 2 6 16.57 4 2 6 2.44 6 3 5 9.47 5 5 3 22.53 1 4 4 2.17 7 171 ’r‘.'-rp ’ ON mn.o guano F wotom 1.1 55581 Fo moFFFa> FmFFFo< mF OF m o F F F 0 OF. mF. ON mm 5:868“. Fo wo:_a> quoFFFanF FcoEoo< FchF. 0 QFFOFGIONF .5 FocoSF 172 Table‘VI.16 Comparison.of.Actual and Predicted values of Total accident frequency in FC-Group 2 Actual Values Rank Predicted Values Rank 46 3 32.52 3 56 2 13.30 5 18 4 21.06 4 13 5 50.21 2 75 1 55.44 1 173 om on.o guano F wotow I1 .6558“. F0 ooFFFa> FaFFFo< 00 OF» ON 0 F F F 16.5.58“. Fo oo:_a> poFoFUoFu chEoo< FmFoF. N QDOFGIOHF .5 EDGE 0.. ON on 0v cm 00 174 Table‘VI.17 Comparison.of Actual and Predicted values of Total accident frequency in UON-Group 2 Actual Values Rank Predicted Values Rank 4 1 2.07 3 1 3 2.07 3 2 2 3.91 1 1 3 2.07 3 1 3 2.07 3 1 3 3.91 1 175 an.o guano F wotow l1 1653311 .6 woFFFm> FaFFFo< . F4 m N F o F F F F 55:08”. Fo mo:_a> poFoFUoFu 1.202006. FmFoF. N 820-20: .0 .805. m 176 Table V1.18 Comparison.of Actual and Predicted values of Total accident frequency in OOF-Group 2 Actual Values Rank Predicted Values Rank 2 4 5.99 3 2 4 4.61 5 15 l 17.30 1 9 2 1.07 6 1 6 5.15 4 9 2 15.43 2 177 ,carw” an.o guano F motow |.1 5:868“. Fo mo:_a> .8394 m: 3 NF 9 a o F. N o F F F F F F a O ON >ocozco1u .Fo ooFFFa> 6220an. 285$ .88 N 90.0.19. .5 .822 178 CHAPTER VI I SUMMARY and CONCLUS IONS Summary Freeway interchanges play' a 'very important role in reducing the probability of vehicular conflicts during transfer frem one road to another. However, the number of accidents on freeway interchanges is increasing with increased traffic on the freeway. The purpose of this study was to identify the type of accidents that occurred on the elements of the interchanges, compare the accident rates with the results from .1. In Cirillo's study, and finally construct freeway interchange accident predictive models based on the elements which comprise an interchange. The first step iniconstructing these models was to obtain geometric data, accident data and traffic data for interchanges located in the State of Michigan. The data obtained were: 0 Geometric data describing the elements of the freeway interchange geometry were obtained from the Michigan Department of Transportation's Highway Accident Master Data file. o Accident data from 1982 to 1984 were obtained from the 179 Michigan Department of Transportation's Highway Accident Master Data file. 0 Traffic Data describing the level of use of the freeway interchange elements were available from the Michigan Department of Transportation's TVM (Trunkline Vehicle Miles) Master Data file and Traffic Flow map. These data files were merged to produce a master data file composed of geometric data, accident data and traffic data. This file was then stratified into 3 analysis units for constructing freeway interchange accident predictive models for each element of an interchange. These analysis units were: o mainline unit 0 crossroad unit 0 ramp unit During the study period 4464 out of 9534 accidents occurred on the mainline unit, 2536 accidents occurred on the crossroad unit and 2534 accidents occurred on the ramp unit, respectively. Based upon the total number of accidents for each analysis unit, the first attempt was made to construct a linear regression model using all interchanges in one model. The multiple regression R coefficient for the mainline unit was 0.5624, indicating that less than 30 % of the variance in 180 accident frequency was explained by the independent variables used. The master data file was then stratified into 3 area types of activity density (urban, rural and fringe). The master' data file ‘was further classified into groups of interchange configurations with similar accident rates. The interchange type with the lowest average accident rate (accidents per interchange per 3 years) was 0.91 for the Partial Diamond type interchanges and the highest value of the average accident rate was 14.29 for the Full Directional type interchanges. Interchange types that were similar to each other in the average accident rate and variance were grouped for further analysis. The interchanges were classified into 12 groups for each analysis unit: . Group 1 - Diamond . Group 2 - Tight Diamond, Urban Diamond . Group 3 - Modified Diamond, Modified Tight Diamond, Parclo A 4 Quad . Group 4 - Partial Diamond, Partial Tight Diamond, Trumpet A, Partial Directional Y . Group 5 - Split Diamond, General Directional, Other . Group 6 - Diamond plus 1 loop, Parclo B 4 Quad, Trumpet B, Directional Y . Group 7 - Parclo AB, Partial Directional . Group 8 - Cloverleaf, Cloverleaf with CD Roads, Cloverleaf 181 minus 1 loop, Directional with loops . Group 9 - Parclo A, Parclo B, Parclo AB 4 Quad . Group 10 - Full Directional, General . Group 11 - SRI-A . Group 12 - SRI-B For the classified groups, group 4 had the lowest value of 1.25 acc./int./3 yrs. and group 10 had the highest value of 10.27 acc./int./3 yrs. The most common accident types occurring on the urban interchanges were fixed object, rear-end and angle straight accidents. Groupiz had.the highest percentage of fixed object, rear-end and angle straight accidents. The accident types on the rural interchanges were fixed object, rear-end and animal accidents. Group 1 had the highest percentage of fixed object and animal accidents, and group 3 had the highest percentage of rear-end accidents. The predominant accident types on the fringe interchanges were fixed object, rear-end and animal accidents. Group 3 had the highest percentage of fixed object and rear-end accidents, and groupil had the highest.percentage of animal accidents. Based on the total number of accidents per interchange for each analysis unit (considering only groups with more than 10 interchanges), rural group 4 and urban group 10 had the lowest value of 4.84 acc./int. and highest value of 13.75 acc./int., respectively on the mainline unit. On the crossroad 182 unit rural group 4 had the lowest value of 1.03 acc./int., and urban groups 7 and 10 had the highest value of 6.5 acc./int. On the on-ramp unit rural group 2 had the lowest value of 0.60 acc./int., and urban group 6 had the highest value of 3.50 acc./int. On the off-ramp unit rural group 4 had the lowest value of 0.54 acc./int., and urban group 3 had the highest value of 4.82 acc./int. From the classified groups, only groups with more than 10 interchanges were selected for constructing linear regression models. Three fourth.of the stratified data in each cell were used to construct the model and the remaining 25 % of the data were used for calibrating the model. Variables used to construct the models were population (in 1000's) of the county, lane mileage (0.01 mile units) of the analysis unit, number of on-ramps, number of off-ramps, average daily traffic (ADT) for the independent variables and the total number of accidents for the dependent variable. Models with greater than 0.7 in multiple R