ANALYSIS OF A. DUA.L~DUAL FREEWAY

Thesis for the Degree of M. S.
MICHIGAN STATE UNIVERSITY
BERNARD DONALD. ALKIRE
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ABSTRACT

ANALYSIS OF A DUAL-DUAL FREEWAY

by Bernard Donald Alkire

A capacity and level of service analysis is per—
formed on a portion of a dual-dual freeway. This is done
to determine if the design of a dual-dual freeway will
satisfy requirements for a high volume corridor with
fewer than four lanes of traffic on any roadway, and a
level of service for each roadway of at least D.

A procedure for the analysis of each of the ele-
ments of the dual-dual freeway is develOped and is used
for obtaining Spot volumes at critical locations on the
freeway. These volumes are then compared to known stand-
ards to obtain the level of service for each section.

The elements analyzed in this fashion are: the freeway;
weaving areas; two-lane entrance and exit ramps; one—lane
ramps; and left—hand entrance and exit ramps.

The results indicate the dual-dual freeway is.

a good method for moving high volumes of traffic and

l

Bernard Donald Alkire

still maintain only four lanes per roadway. Of all the
elements analyzed, the design of the two-lane ramps at
major interchanges is the most critical. The use of
parallel acceleration or deceleration lanes is analyzed

in View of overcoming problems in these areas.

ANALYSIS OF A DUAL—DUAL FREEWAY

BY

Bernard Donald Alkire

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Civil Engineering

1969

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ACKNOWLEDGEMENTS

I would like to extend special thanks to Dr. Gail
Blomquist, my advisor, for his suggestions and assistance
in the completion of this thesis.

To the Michigan Department of State Highways for
the use of their base maps and traffic assignment figures;
and to Mr. Ernest Morey my squad leader at the Michigan
State Highway Department.

Particular thanks to my wife Patricia for her
help and understanding during the preparation of this

thesis.

ii

TABLE OF CONTENTS

ACKNOWLEDGEMENTS. . . . . . . . . . .

LIST OF TABLES. . . . . . . . . . . .

LIST OF FIGURES . . . . . . . . . . .

LIST OF APPENDICES. . . . . . . . . .

CHAPTER

I. PROBLEM AND DEFINITION OF TERMS

The Problem . . . . . . . .

Statement of the Problem.

Justification of the Dual—Dual Freeway.

Definition of Terms Used. .
Level of Service. . . . .
Capacity. . . . . . - . .

Lane Designations . . . .

Organization of Remainder of the Thesis

II. GENERAL DESIGN CHARACTERISTICS.

Geometric Standards . . . .

Traffic Standards . . . . .

iii

Page

ii

vi

viii

ix

Table of Contents.--Cont.

CHAPTER

Safety Standards. . . . . . .

Factors Affecting Level of Service.

Roadway Factors . . . . .
Shoulders . . . . . .
Lateral Clearance . .
Alignment . . . . .
Grades. . . . . . . . . .

Traffic Factors . . . . . .

Trucks. . . . . . . . . .
Peak-hour Factor. . . . .

Method of Numerical Analysis.

III. ANALYSIS AND RESULTS OF THE DESIGN ELEMENTS

Freeway Lanes . . . . . . . .
Analysis. . . . . . . .
Results . . . . . . . . . .

Weaving Section . . . . . .
Analysis. . . . . . . . . .

Results . . . . . . . . .

Two—Lane Entrance and Exit Ramps.

Analysis. . . . . . . . . .

Two-Lane Exit Ramps . . .
Two-Lane Entrance Ramps .

iv

Page

11
ll
11
12
12
l3
l3

13
14

15

17

l7

17

23

26

26

29

3O

3O

3O
35

Table of Contents.--Cont.

CHAPTER Page

Results . . . . . . . . . . . . . . . . 37

Two—Lane Exit Ramps . . . . . . . . . 38
Two—Lane Entrance Ramps . . . . . . . 39

Single-Lane Ramps . . . . . . . . . . . . 40
Analysis. . . . . . . . . . . . . . . . 40

Results . . . . . . . . . . . . . . . . 44
Left-Hand Entrance and Exit Ramps . . . . 45
Analysis. . . . . . . . . . . . . . . . 47

Results . . . . . . . . . . . . . . . . 49

IV. CONCLUSIONS AND RECOMMENDATIONS . . . . . . 50
Conclusions . . . . . . . . . . . . . . . 50

Recommendations . . . . . . . . . . . . . 53

BIBLIOGRAPHY. . . . . . . . . . . . . . . . . . . . 55

APPENDIX. . . . . . . . . . . . . . . . . . . . . . 59

LIST OF TABLES
Table Page
I. LEVEL OF SERVICE FOR UNINTERRUPTED FLOW

SERVICE VOLUME/CAPACITY (PHF 0.91). . . . 66

II. LANE DISTRIBUTION LANE 1 (NO RAMP MOVEMENT
WITHIN 4000 FEET) . . . . . . . . . . . . 67

III. PERCENTAGE OF RAMP TRAFFIC IN LANE l. . . . 68

IV. PERCENTAGE OF EXIT TRAFFIC UPSTREAM OF
RAMP WITH AUXILIARY LANE. . . . . . . . . 69

V. PERCENTAGE OF ENTRANCE TRAFFIC DOWNSTREAM

OF RAMP WITH AUXILIARY LANE . . . . . . . 70
VI. ANALYSIS OF FREEWAY SECTION . . . . . . . . 72
VII. ANALYSIS OF WEAVING AREAS . . . . . . . . . 73

VIII. VOLUME DISTRIBUTION RAMP 2 TWO-LANE EXIT
RAMP SPOT VOLUME. . . . . . . . . . . . . 74

IX. VOLUME DISTRIBUTION RAMP 5 TWO-LANE
ENTRANCE SPOT VOLUME. . . . . . . . . . . 75

X. VOLUME DISTRIBUTION RAMP l7 TWO-LANE
ENTRANCE RAMP SPOT VOLUME . . . . . . . . 76

XI. VOLUME DISTRIBUTION RAMP 20 TWO-LANE EXIT
RAMP SPOT VOLUME. . . . . . . . . . . . . 77

XII. VOLUME DISTRIBUTION RAMP 23 TWO-LANE EXIT
RAMP SPOT VOLUME. . . . . . . . . . . . . 78

vi

List of Tables.—-Cont.

Table

XIII.

XIV.

XVII.

XVIII.

XIX.

VOLUME DISTRIBUTION RAMP 25 TWO-LANE
ENTRANCE RAMP SPOT VOLUME . . . . . . . .

VOLUME DISTRIBUTION RAMP 27 TWO-LANE
ENTRANCE RAMP SPOT VOLUME . . . . . . . .

VOLUME DISTRIBUTION RAMP 28 TWO—LANE EXIT
RAMP SPOT VOLUME. . . . . . . . . . . .

VOLUME DISTRIBUTION RAMP 7 ONE-LANE EXIT
RAMP SPOT VOLUME. . . . . . . . . . . . .

VOLUME DISTRIBUTION RAMPS 8 & 9 ONE—LANE
ENTRANCE RAMP SPOT VOLUME . . . . . . . .

VOLUME DISTRIBUTION RAMP 12 ONE-LANE
ENTRANCE RAMP SPOT VOLUME . . . . . . . .

VOLUME DISTRIBUTION RAMPS l3 & 14 ONE—LANE
EXIT RAMPS SPOT VOLUME. . . . . . . . .

VOLUME DISTRIBUTION RAMPS 3 & 4 ONE-LANE
LEFT-HAND ENTRANCE AND EXIT RAMP SPOT
VOLUMES . . . . . . . . . . . . . . . . .

VOLUME DISTRIBUTION RAMPS 18 & 19 ONEFLANE

LEFT-HAND ENTRANCE AND EXIT RAMPS SPOT
VOLUMES . . . . . . . . . . . . . . . . .

vii

Page

79

80

81

82

83

84

85

86

87

L IST OF F IGURES

Figure Page

1. Dual-Dual Freeway Plan . . . . . . . . . . . 60
2. Typical Half Section Dual-Dual Freeway . . . 61
3. Typical Section Intersecting Freeway . . . . 62
4. Traffic Assignment Plan. . . . . . . . . . . 63

5. Treatment of Inner Roadway at Section 7—8
& 12-13. . . . . . . . . . . . . . . . . . 64

viii

LIST OF APPENDICES
Appendix Page
A. GENERAL PLAN OF DUAL-DUAL FREEWAY AND
TYPICAL CROSS SECTION. . . . . . . . . . . 59

B0 VOLUME DISTRIBUTION TABLES . . . . . . . . . 65

C. VOLUME DISTRIBUTION RESULTS. . . . . . . . . 71

ix

CHAPTER I

PROBLEM AND DEFINITION OF TERMS USED

It is a well-known fact that urban freeway travel
has increased to a point where the rush hour traffic jam
has become a permanent part of urban life. This condition
has produced a public awareness of the inadequacy of our
freeway systems and a clamor for additional freeways. It
is the function of the highway engineer to design these
new facilities and to provide safety, economy, and con-
venience for the freeway user. This thesis presents an
analysis of one of the recent innovations in highway de—

sign that is being used to realize these goals.

The Problem

Statement of the Problem

As the need for more freeways increased, it be-
came apparent that it would be necessary to design a free-
way that would provide: (1) high volumes of traffic

1

without having a roadway with more than four lanes of
traffic in one direction, (2) separate roadways with
minimal friction for high speed through traffic, (3)
latest in safety features, and (4) a level of service
of at least D for the various elements that make up the

freeway.

Justification of the
Dual-Dual Freeway

 

 

A dual-dual freeway, as the name indicates, has
two positively divided roadways for each direction. This
type of freeway is designed to reduce hazardous maneuvers
associated with very wide paved areas. It has been shown
that the maximum number of lanes that can be safely util-
ized in one roadway is four [1]. Above this number, the
increased weaving which takes place within a freeway sec-
tion will reduce the capacity and increase the accident
rate.

Further justification for the selection of the
dual—dual freeway is the fact that this type of freeway
will provide two types of service on one right of way.

First, the outer roadways of the freeway provide access

and egress to interchanges and carry the majority of short
trip drivers. These outer roadways are characterized by
high traffic volumes and a large number of weaving moVe-
ments. As a result of the weaving and the short trip na-
ture of the driving it should be expected that the speeds
and capacity on the outer roadways will be somewhat below
that of the inner roadways. In contrast to this, the
inner roadways provide high speed and noninterrupted

flow with a minimum of friction from weaving. Since there
is only one point of exit and entrance into the inner
roadways, it is expected that the inner roadway should
provide the express route that is needed to get the long
distance commuter from the central business district to

his home with the least possible interruption.

Definition of Terms Used

 

Level of Service

'Level of service is a term which broadly
interpreted denotes any one of an infinite
number of differing combinations of operating
conditions that may occur on a given road.

In practice, selected levels are defined in

terms of particular limiting values, such as
speed, volume per lane, weaving volumes, di-
verge volumes, and merge volumes [2]°

Level of Service is indicated by letter A, B, C, D, E,

or F.

Capacity

Capacity is the maximum number of vehicles
which has a reasonable expectation of passing
over a given section of a lane or a roadway
during a given period of time under prevailing
roadway and traffic conditions [2].

Lane Designations

 

In the analysis of a section of freeway it is ne-
cessary to designate, by number, each lane on the freeway
and two-lane ramps. For this thesis the lanes of the
freeway are numbered right to left in the direction of
traffic. Thus, for a six lane freeway the lanes for
either direction are l for the right—hand lane, 2 for the
middle lane, and 3 for the lane adjacent to the median.
On a two-lane ramp the lane on the right is designated

lane B and the left lane is lane A.

Organization of Remainder of the Thesis

In order to analyze the different elements of the
freeway the remainder of the thesis is divided into two
parts. The first part is a general discussion of the
factors affecting level of service and how they affect
the design of the dual-dual section of freeway. The
second part of the analysis is quantitative in nature
and will be concerned with the actual volume that can be
carried by a section of the freeway and the level of
service for that section. In the second part of the
analysis the critical sections of the freeway are iso—
lated into four groups of similar characteristics: (1)
the freeway sections; (2) the right-hand entrances and
exits (including both one— and two—lane ramps); (3) the
left-hand entrances and exits; and (4) the weaving areas.
Each of these groups has a different method of numerical
analysis and is treated separately.

In order to locate the critical sections, Figure 1,
Appendix A, may be used. In this figure each section of
freeway and ramp is given a number or letter designation.
Discussion of the critical sections are identified by use

of the number or letter only in the remainder of the thesis.

CHAPTER II

GENERAL DESIGN CHARACTERISTICS

The proposed facility is being designed for a
highly urbanized area with a metrOpolitan population of
approximately three million people. The need for the
freeway has been established by an origin-destination
study made for the purpose of planning the metropolitan
freeway system. Corridor locations and traffic assign—
ments were made as part of this study.

The freeway is generally of the depressed type
and is at grade only for the portions necessary to design
the interchanges. The types of interchanges have been
determined and, for the section being analyzed, a four
level directional interchange (Maltese Cross) has been
selected. A major existing freeway intersects the pro-
posed facility and must be integrated into the design of
the interchange. Alterations to the existing freeway
must be kept to a minimum and should not interfere with

traffic on this route. Thus, the low levels of service

provided on the intersecting freeway are a result of the

existing conditions and are not subject to improvements.

Geometric Standards

 

The design standards for the freeway and inter-
change sections generally conform to the standards set
forth in A Policy on Geometric Design of Rural Highways [3].

The main items are as follows:

A. Design speed.
l. Freeway--6O mph.
2. Ramps and turning roadways——35 mph.
B. Gradient.
l. Freeways-—equal to or less than 3 per cent.
20 Ramps and turning roadways——less than 5 per
cent desirable for positive grades--6 per
cent maximum.
C. Traffic lane width.
1. Freeway——12 feet.
2. Ramps and turning roadways—-12.5 feet, with
additional width provided on curves of short

radius as provided for in the Policy on Geo-
metric Design of Rural Highways [3].

D. Horizontal curvature.
l. Freeway--3 degrees or less.

2. Ramps and turning roadways--as per Policy
on Geometric Design of Rural Highways [3].

 

E. Shoulders.

1. Continuous full width refuge shoulders pro—
vided at both right and left side of four
lane freeways and at the right side of free-
ways with less than four lanes, including
the turning roadway and ramps.

F. Median width.

1. Minimum 26 feet between opposing roadways
with positive median barrier and 26 feet
between inner and outer roadways in the
dual—dual section. The intersecting free-
way has 12 foot medians with a positive
median barrier.

G. Length of acceleration and deceleration lanes
and lane drOps are Michigan standards and are
considered adequate [4].
A typical section of the dual—dual freeway and the inter-

secting freeway are included in the Appendix A as Figures 2

and 3.

Traffic Standards

 

In addition to the geometric standards it is nec-

essary to establish standards for evaluating the ability

of the section to carry the assigned traffic. The stand—

ards for this category are as follows:

Level of service D--this is the lowest level of
stable flow with traffic approaching instability.
At this level of service, drivers have little
freedom to maneuver and comfort and convenience
are low. Under ideal conditions an average of
1,800 passenger cars per lane per hour can be
carried in any section [2]. This is the level
of service that is considered to be the minimum
design standard for the freeway sections.

Percentage of trucks--5 per cent on all roadways
except the inner dual roadways for which 3 per-
cent is used for design and analysis.

Average daily traffic and design hourly volume—-
this information has been provided by the Michigan
Department of State Highways [5]. (See Figure 4,
Appendix A.)

Safety Standards

 

Early in 1968, a report entitled, Highway Design

and Operational Practices Related to Highway Safety [6],

was made available to designers of highway facilities.

10

This report made recommendations on practices that should
be followed if maximum driver safety is to be assured.
Many of the recommendations of this report are incorpor—
ated into the design of the dual-dual freeway. .Particular
attention is focused on the design of the median barriers.
The safety report notes that a concrete barrier with a
curved face is very effective on roadways with narrow
medians. Since the cross section of the dual-dual free-
way shows that the medians are only 26 feet, it was de-
cided to adopt this type of barrier. Other design fea-
tures used to improve the safety of the freeway section
are increased distance to fixed objects adjacent to the
freeway and provisions for flatter side slopes in areas
of embankment and cuts. The objective of these design
improvements is to reduce the number of objects being
struck by vehicles leaving the freeway out of control.

In the design of the dual—dual section of free—
way, 30 feet is considered to be the minimum distance to
any fixed object except the median barrier or a retain-
ing wall, in which 14 feet is used as the minimum side

clearance.

11
Factors Affecting Level of Service

Prior to making the numerical analysis of the
design some of the factors that affect the value of ca—
pacity and level of service should be considered. These
factors are divided into two general groups, the roadway
and traffic factors. The roadway factors are those phys-
ical items of design that could be restrictive to the
flow of traffic if adequate design standards are not
maintained. The traffic factors are the same in the
sense that they may be restrictive to the flow of traf-
fic, though these factors are determined by the composi-
tion of the traffic and the habits of the driver. Both
types of factors, if inadequate, tend to reduce the ca-
pacity of the roadway by introducing conditions that are
not considered to be normal as far as calculation of ca-

pacity is concerned.

Roadway Factors

Shoulders.-—As stated in the section on design

 

standards, continuous full width refuge shoulders are to

be provided on both sides of the four lane roadways and

12

on the right side of roadways of less than four lanes.
Turning roadways and ramps will also have a full width
right shoulder and a left shoulder of at least six feet.
Bridge sections have the right shoulder carried through
and the left shoulders as recommended in the Poligy on
Arterial Highways in Urban Areas [7]. In no case is
insufficient shoulder width a problem in the analysis
of the interchange and no capacity adjustments are re-
quired for this factor.

Lateral Clearance.--As a result of the major re-

 

appraisal of safety standards in the design of expressways
brought forth with the issuance of the Yellow Book [6],
the lateral clearance is set at a 14 foot minimum for re-
taining walls and bridge parapets; and 30 feet for piers
and columns in the outside shoulders. The inner roadways
have half of the 26 foot medians as side clearance. While
there may be some doubts as to the sufficiency of the side
clearance from the point of safety, it is unlikely that
the clearance will affect the capacity.

Alignment.--Considering that the alignment ele-

 

ments (vertical and horizontal curves) are controlled by

the design speed of 60 miles per hour, there will be no

13

adjustment necessary to compensate for inadequate align-
ment.

Grades.--One of the factors that has very large
influence on the capacity of any section is the grade of
that section. On a long grade the right-hand lane will
be occupied almost exclusively by slow moving vehicles
which will reduce the number of freeway lanes available
for other vehicles. In order to carry the same capacity
as on a level section it is necessary to add a lane to
the freeway. It is assumed that freeway grades of less
than 2 per cent and one half mile length will have little
effect on capacity [2]. As the previous statement sug-
gests, the level of service on any given section of grade
is a function of the number of slow vehicles, the rate of
grade, and the length of the grade. A discussion of the
effect of grades will be included in the analysis of the

section that is being affected by grade.

Traffic Factors

Trucks.--It is apparent from the discussion on
grades that the percentage of trucks on a roadway affect

the capacity of the roadway. Trucks, by their physical

14

dimensions are equivalent to two or more passenger cars.
However, the main affect on capacity is not caused by
truck size, but rather by truck performance character—
istics [8]. It may be observed from the performance char-
acteristics that as length and steepness of grade in—
crease, speed is reduced and platooning becomes more pro-
nounced. In the extreme case where trucks cause a sub-
stantial reduction in Speed, passenger cars avoid the
right-hand lane altogether; and an additional lane is
required to retain the original level of service. In

the analysis of the dual-dual freeway an overall factor

of 0.95 is used to adjust for the 5 per cent trucks that
are expected on the freeway. This factor will apply to
the outer roadways of the dual—dual freeway, all turning
roadways, and ramps. At this point it should be mentioned
that it is standard procedure for all volumes used in this
analysis be in terms of mixed traffic rather than the more
artificial term of equivalent passenger-car volumes.

Peak-hour factor.—-Variations in the demand

 

throughout any given hour may produce rates of flow for
short periods which substantially exceed the average rate.

To compensate for this possibility a peak-hour factor is

15

used to modify the design volumes for level of service

C and D. This modification is called the peak-hour fac-
tor and is related to the population of the metropolitan
area for which it is used [9]. For the evaluation of
the design being carried out in this paper, a peak-hour

factor of 0.91 is used for all roadways.

Method of Numerical Analysis

 

The procedure used in the analysis of the design

will follow that recommended by the Highway Capacity Man—

 

.32; [2]. However, there are numerous occasions that arise
that do not fit the criteria used in that manual. In
these cases, research literature is used to supplement
the manual's methods of analysis. The steps in the numer-
ical analysis are as follows:

A. Subdivide the roadway under consideration into

sections having reasonably uniform conditions.
B. Analyze all freeway sections for capacity.

C. Isolate and identify all critical sections (en-
trance and exit ramps, weaving areas, etc.).

16
Group all critical sections into the following
categories:
1. Freeways
2. Turning roadways
3. Right-hand entrance and exits
4. Two-lane entrance and exit ramps
5. Left-hand entrances and exits
6. Weaving areas

Make the numerical analysis of each of the cate—
gories and determine the level of service.

Make recommendations and conclusions.

CHAPTER III

ANALYSIS AND RESULTS OF THE

DESIGN ELEMENTS

Freeway Lanes

 

Analysis

Analyzing the freeway lane capacity within the
interchange area is a major part of the numerical analysis
for level of service. Generally, it is standard practice
to determine the level of service only at critical loca-
tions on the freeway. However, in order to obtain a bal—
anced design, it is necessary to evaluate the level of
service at all changes of traffic flow conditions. The

Highway Capacity Manual states that the variations in

 

level of service between adjacent sections should not be
more than one level of service if a reasonable balance of
design is to be obtained [2]. This is one of the condi-

tions that should be met as the analysis proceeds and is

17

18

a major consideration in the recommendation for improve-
ment.

As noted in Chapter II, where the factors affect-
ing level of service were discussed, one adjustment
factor for capacity that can be expected on a high volume
freeway is the adjustment factor for long grades. In the
design of the dual-dual freeway all grades are less than
3 per cent and as stated in a study by Newman and Mosko-
witz, "Grades between 2 and 3 per cent will form queues
but will move fast enough to maintain high rates of flow"
[9]. Thus, the capacity on the west bound roadways should
not be reduced. On the east bound roadways there is a
3 per cent downgrade, but again no correction is applied
because flat and intermediate downgrades are considered
to be the same as level [2].

The only other factor that might reduce the ca-
pacity of the freeways is the presence of trucks on the
roadways. In the Traffic Standards section of this thesis
it is stated that traffic will include 5 per cent trucks
on the outer roadways and 3 per cent on the inner road-
ways. Therefore, an appropriate factor is used for ad-
justing the capacity values used in determining the level

of service for all sections.

19

Also included in this section is a determination
of freeway lane capacity for the intersecting freeway.
The intersecting freeway is an existing facility with
standards lower than the dual-dual freeway. The level
of service that has to be accepted for this roadway is
level E. It is anticipated that a possible source of
congestion on this roadway will be the areas adjacent
and downstream from the two-lane entrance ramps. A de-
tailed discussion of the design of ramps is included in
the section covering the two-lane entrance and two-lane
exit ramps.

The method of computing the level of service for
the freeway sections was adopted from the Highwgy Capacity
Manual [2], but is modified to suit the data available.
As is the case with many engineering problems a trial and
error solution is required. To make the process as func-
tional as possible, the following procedure is used for
all freeway sections:

A. List all known factors; such as, demand volume,
number of lanes, alignment conditions, lateral
restrictions, and percentage of trucks.

B. Calculate or find the necessary correction fac-

tors. TL, the truck correction factor, is cal-
culated using an assumed level of service.

20

C. Solve the equation SV = 2000 - N . V/C . W - TL
for V/C ratio [2]. In this equation SV equals

the demand volume; N equals number of lanes; W

equals the truck factor at the assumed level of

service; and V/C = Volume Capacity ratio.
D. Convert the V/C ratio into basic V/C. Basic

V/C = V/C % PHF. If the results of C above are

in the range of values for level of service A

and B, then Basic V/C = V/C.

E. From Appendix B, Table I, determine the level of

service by comparing the calculated V/C ratio x

PHF to the table of limiting values of V/C ratio

x PHF. (For level of service A and B the peak

hour factor is neglected.)

F. If the assumed level of service is correct, accept
the level of service as determined, if not, assume

a new level of service and recompute.

The results of the freeway capacity cheek indicate
several features about the design that might not be appar-
ent if the capacity determination were made only at crit-
ical locations. As stated previously, one of the major
reasons for determining the level of service is to observe
the balance of the design. Table VI, Appendix C, illus-
trates the fact that generally the balance for the outer
roadways is good, but the inner roadways are out of bal-
ance at the interchange. One of the criteria for balanced

design is the requirement that the level of service be—

tween adjacent sections should not vary more than one

21

level of service. This principle is violated in three
locations on the inner roadways, twice on the eastbound
inner roadway, and once on the westbound inner roadway.
In the outer roadways there are two different levels of
service on the entire roadway and on the eastbound road—
way level B is maintained for four out of five of the
sections.

The balance is not as good for the inner roadway
because the level of service is greatly affected by the
ramps that allow traffic to exit from or enter the inner
roadways. From Table VI, Appendix C it is possible to
interpret the results of crossing over from inner to outer
roadway. For section 6A-7 of the dual-dual freeway the
level of service remains constant at level D. However, as
traffic exits from the inner roadway to the outer roadway
the volume of traffic remaining on the inner roadway is
reduced and the level of service for the inner roadway
raises to level B (Section 7—8). Further downstream two
entrance ramps add traffic to the inner roadway and the
level of service drops to level C (Section 8-9) at the
first ramp and to level E (Section 9-10) at the second

ramp. The level of service E is carried through the

22

remaining portion of the inner roadway until it ends
10,000 feet downstream.

From the above discussion it is apparent that
the design of the two sections on the inner roadways
must be altered to meet the requirements of balanced
design. The alterations should be: (1) the level of
service for Section 7—8 should be lowered to level C,
and (2) the level of service for Section 9-10 should be
raised to level D. If this is done all sections on the
inner roadway will be level of service C or D; with the
majority of the inner roadways at a level D.

The westbound inner roadway has a similar problem
at Section 12—13, except in this case the level of serv-
ice is raised from level D to level C where an exit ramp
removes traffic from the inner roadway. In this section
the level of service should be lowered to level D to
eliminate the unbalance.

The intersecting freeway is a much more compli—
cated problem as far as obtaining balance design because
of the impossibility of altering the existing facilities.
The level of service E for Sections 22—23, 24—25, 25—27,

and 28-29 can not be improved without adding lanes or

23

metering the flow. The only possibility for obtaining a
more balanced design in this area is to lower the level
of service on Sections 23-24 and 27—28.

The other major design criteria is a level of
service of at least level D for all roadways. It has
been shown that two sections on the inner roadways, and
four sections on the intersecting freeway do not meet
this standard. It has already been stated that due to
existing conditions it is doubtful if the level of serv-
ice on the intersecting freeway can be raised. However,
the level of service for the two sections with level of

service E on the dual-dual freeway can be improved.

Results

The previous sections discussed the shortcomings
of the freeway concerning design balance and level of
service. Recommendations will now be considered to re-
lieve the problems that have been encountered. In order
to obtain a balanced design the level of service for Sec-
tion 7-8 should be lowered from B to C. This may be done
by increasing the through traffic on the inner roadway or

by decreasing the number of lanes. Since the traffic

24

volumes are predetermined, and can not be changed, the
only solution would be to decrease the number of through
lanes. Due to possible future operational requirements

it is necessary to carry the three lanes across the bridge
and to the next ramp.

An alternative to building only two lanes would
be to construct three lanes and then paint out the extra
lane between the exit and entrance ramps as shown in
Appendix A, Figure 5. This should have the same affect
as having only two lanes, but the third lane will still
be available if operational difficulties are experienced.
The same solution is also recommended for use on the
westbound inner roadways at Section 12—13, even though
it is Operating at a high level C.

The other problem with the dual—dual freeway is
the fact that Sections 14-15 and 9—10 are operating at
level of service E instead of the desired level D. Pos-
sible solutions are adding a lane or decreasing the
volume capacity ratio. Since physical factors control
the width of the right of way available increasing the
number of lanes is impractical. Thus, decreasing the

volume capacity ratio is the only method of raising the

25

level of service under these circumstances. By inSpection,
the volume capacity ratio can be decreased by decreasing
the demand volume, or by increasing the capacity. As men-
tioned previously the demand volumes are fixed and can not
be adjusted; therefore, the only approach left is to in-
crease the capacity. Inspection of the formula for deter-
mining the volume capacity ratio [2] indicates that the
capacity is equal to 2000 - N ° W . TL' In this equation
N equals a constant (3); W equals a constant (1.00), and
TL equals (.95). The only way of increasing capacity
would be to increase the adjustment factor for trucks to
unity. This would be possible only if the percentage of
trucks on the inner roadway is reduced to zero. Therefore,
it is recommended that truck traffic be prohibited on the
inner roadways. If this is done the level of service for
Sections 9—10 and 14-15 will be raised'to level D.

For the intersecting freeway the solution to the
problem of design balance is the same as for the dual-dual
freeway. That is, reduce the number of lanes beyond the
nose of the exit ramp as shown in Appendix A, Figure 5.

It is particularly important to retain the three lane

width in this area because this freeway is carrying such

26

high volumes of traffic. An error in assignment could
have disasterous results if a restriction is built into
the freeway. The level of service E for the sections
will have to be accepted without improvement since the
existing situation makes it impossible to either increase

the number of lanes or decrease the demand volume.

Weaving Section

 

Analysis

 

Within the interchange being analyzed there are
sections for which maximum service volume is determined
by the sections ability to allow vehicles to successively
merge and diverge. This maneuver is called weaving and
is another of the Operational characteristics that can
determine the level of service along a freeway.

The basic situation involving weaving is where
two vehicles on separate roadways enter, cross paths, and
exit on separate roadways. As this movement is being
accomplished there is a time and place where both vehicles

occupy the same lane. This restricts the maximum volume

27

that can weave at any given section to the maximum ca-
pacity of a single lane [2]. Therefore, it is apparent
that in order to accommodate any large amount of weaving,
additional lanes will have to be added to the freeway
section.

In addition to width of weaving section another
variable that can increase the amount of weaving a sec-
tion can accommodate is the length of the weaving sec-
tions. This fact can be deduced by observing the opera-
tion of a weaving section at high volumes. For example,
with low traffic volumes a vehicle may almost always enter
the weaving area, find a gap, and complete the maneuver
without conflict. This requires a weaving distance only
as great as is required to change lanes (normally 600 feet
at freeway Speeds) [9]. However, as the volumes increase
the driver approaching the weaving section may find there
are no gaps in the lanes to weave into and he must adjust
his speed and wait for a gap in the traffic. While the
driver is waiting for a gap to develop the vehicle con-
tinues in its original lane increasing the length of weav-
ing section required. There is a volume at which it is

no longer desirable to increase the length of weaving

28

section and a condition of stOp and wait for a gap de-
velOps. This volume will ordinarily occur when the sec-
tion is Operating at capacity and is in the realm of
unstable flow.

In developing a method of analysis for a weaving
section it is necessary to first identify the sections
that will be analyzed for weaving. It should be noted
that the basic factor that identifies a weaving section
is vehicles entering a common roadway from two or more
entrance flows, and afterwards Splitting into two or more
exit flows. Sections 3-4, 5—6, 18—19, and 20-21 meet
this requirement and are analyzed as weaving sections.

The same sections are also analyzed as exit and entrance
ramps with an auxiliary lane. The actual level of service
for the section will be the lowest level of service ob-
tained from the two analyses.

The analysis of a weaving section can be completed
in a straightforward manner once the sections have been
.identified. The method to be used will proceed as follows:
A. Assume a level of service and relate this to the

quality of flow desired. (Table III, Highway
Capacipy Manual.) [2].

 

Results

29

Using the weaving volume and the quality of flow
as determined above, enter Figure 7.4, Highway
Capacity Manual and determine the length of weav-
ing sections required for this volume [2].

Determine the number of lanes required for the
demand volume of the section. The equation

V(k-1)VWZ

N: sv

will determine the number of lanes required [2].
In this equation N equals number of lanes; k
equals weaving influence factor (use k = 3.0 for
sections where Operations are represented by curve
III, IV, and V), Vw2 equals minor weaving volume
and SV equals maximum lane service volume. Table
7.1, Highway Capacity Manual, can be used to de-
termine maximum lane service volume for a given
quality of flow [2].

From the results of B and C above, it is possible
to determine the sufficiency of the number of
lanes and the length of weaving sections for the
given quality of flow.

Table VII, Appendix C is a tabulation of results

for the weaving areas within the study sections. The re—

sults indicate that all weaving areas have an adequate

number of lanes. The length of weaving area required in

all cases is between quality of flow III and IV which

provides a level of service D.

In order to improve the level of service for any

of the weaving sections it is necessary to increase the

30

length of the sections. Since physical characteristics
are such that it is impractical to lengthen the weaving

sections level of service D is acceptable.

Two—Lane Entrance and Exit Ramps

 

Analysis

Generally, in the design of interchange ramps and
turning roadways it is desirable to have only one-lane
exit and entrance ramps. However, in the case of an in-
terchange involving two freeways turning volumes in excess
of one-lane capacities are frequent and it may be neces-
sary to provide two—lane entrance and exit ramps. Proce-
dures for analyzing two—lane ramps are not well-defined
and it is necessary to develOp a method of analysis using
known information concerning the lateral placement of
traffic in the vicinity of a ramp.

Two-lane exit ramp§.--In analyzing two—lane exit
ramps, one basic fact emerges, a long parallel decelera-
tion lane is required if smooth flow is to be maintained

[9]. With high ramp volumes it is impossible for lane 1

31

to carry all freeway traffic destined for the exit ramp.
A certain number of ramp bound vehicles must travel in
lane 2 of the freeway. This requires vehicles to weave
across lane 1 through traffic to get to ramp lane A.
Safety considerations require the elimination of this
type of driving hazard.

The parallel deceleration is a feature that can
be easily incorporated into the design of the road to
eliminate this condition. This additional lane allows
vehicles bound for ramp lanes B and A to move laterally
to a position where they are not required to weave through
the freeway traffic. Assuming that all ramp bound traffic
is in either freeway lane 1 or 2 upstream from the exit
it can be seen that the parallel lane must be at least
600 feet long, or the distance required for lane 2 ve-
hicles to weave to lane 1 and lane 1 vehicles to weave
to the parallel deceleration lane. In order to provide
a margin of safety 800 feet is considered to be the min—
inum length for such a lane if lane A is to carry any
amount of traffic [2].

As in the problem with single-lane exit ramps it

is necessary to check the volume of traffic in each lane

32

at certain sections upstream of the exit. In the case of
the two lane exits for the dual-dual section of freeway
it is necessary to find the lane 1 and auxiliary lane
volumes at the selected sections and check these volumes
against the allowable diverge volumes set forth in the
Highway Capacity Manual [2].

In order to determine the spot volumes for any
lane at any given section on the freeway it is necessary
to account for the traffic that is passing that section.
The traffic could be made up of three types of vehicles:
vehicles entering from a ramp upstream, vehicles that
desire to exit at a ramp downstream, and vehicles which
have not been involved in a ramp movement within 4000
feet. Tables II, III, IV, and V, Appendix B, Showing
lane distribution for ramp and thru traffic have been
prepared and will be used as an aid in evaluation of the
Spot volumes.

The use of these tables require three assumptions:
(1) Distribution is under pressure of high volume in lane
1; (2) For all two—lane ramps lane B carries 1800 VPH and
lane A carries the remainder of the assigned traffic, and;

(3) Spot volumes will be checked at 500 feet intervals and

33

the measurement of the areas of influence can be rounded
to the nearest 500 feet interval.

In the analysis of two-lane exit ramps it is ne—
cessary to treat two different types: an exit with an
auxiliary lane provided and one without auxiliary lane.
0n the dual-dual section of freeway the two-lane exits
have auxiliary lanes and the analysis for these exits
are different than for the exits without auxiliary lanes.

For the two-lane exit ramps with auxiliary lanes
the spot volume for any lane at any section is the sum
of the entrance, exit, and through traffic in that par—
ticular lane. To obtain the spot volumes for the effect
of the adjacent entrance ramp, Table V, Appendix B is
used. For example, in a freeway section with a 4000 foot
auxiliary lane, 1000 feet downstream from the entrance
nose Table V Shows 56 percent of the entrance ramp volume
remaining in the auxiliary lane. The entrance ramp
traffic in lane 1 at the same point downstream is also
obtained from Table V and is the sum of the ramp traffic
that moved to lane 1 and not to lane 2, and the percentage
of ramp traffic that has moved from the auxiliary lane to

lane 1 in the preceeding 500 feet.

34

After determining the effect of the adjacent en-
trance ramp on the required sections, the next step is
to find the exit ramp volume at the same section. In
determining the lane 1 distribution of the exit ramp
volume at any point upstream of the nose, Table IV, Ap—
pendix B is used. Since this is a two—lane exit, it is
necessary to distribute the volumes for both lane A and
B separately and then combine these volumes for the total
exit bound traffic at the section. A special case of a
two-lane exit ramp is one that has a parallel decelera-
tion lane in addition to the auxiliary lane. In this
case the parallel deceleration lane is treated as if it
were an auxiliary lane and the auxiliary lane were lane
1. Thus at a point 1000 feet upstream from the exit nose,
93 per cent of all the lane B traffic is in the auxiliary
lane and 7 per cent is in lane 1. In the next 500 feet
0.80 of the 93 per cent will move to the parallel decel-
eration lane and in the 500 feet prior to the nose the
remaining 0.20 of the 93 per cent and the 7 per cent that
was in lane 1 will move to the parallel deceleration lane.

Lane A traffic would be distributed in a Similar manner.

35

The other type of two-lane exit ramp that is en-
countered in this analysis is a two-lane exit without an
auxiliary lane. In addition to there being no auxiliary
lane the traffic distribution problems are further Sim—
plified by the fact that there is no entrance ramp traffic
to account for at the check sections.

To determine the spot volumes at any 500 feet
sections is a matter of determining the through traffic
and the exit bound traffic at the check point. For this
type of exit several assumptions are made in addition to
the ones already set forth: (1) Lane 1 is carrying only
traffic bound for ramp lane B; and (2) Lane B traffic is
in lane 1 or 2 and lane A traffic is in lane 2 or 3. If
these assumptions are made the volumes can be determined
directly from Table III, Appendix B.

Two-Lane Entrance Ramp§.--The two-lane entrance

 

ramp is similar to the two-lane exit ramp with some of
the Same assumptions required to obtain the Spot volumes
at a section downstream from the nose of the entrance
ramp. As was the case for the two-lane exit ramps it is
assumed that: (1) lane 1 has high volumes of flow; (2)

ramp lane B is assigned 1800 VPH and ramp lane A is

36

assigned the remainder; and (3) the length of the section
can be rounded to the nearest 500 feet without error.

The spot volume per freeway lane in the case of
the dual-dual freeway ramps is made up of the volume of
traffic originating at an adjacent entrance ramp; the
volume of traffic in the lane destined for an exit ramp;
and the through volume in the lane. For example, dual-
dual freeway section 5-6 is adjacent to a two-lane en-
trance ramp and the spot volumes would be determined as
follows: Entrance ramp lane B vehicles would be dis-
tributed to lane 1 as indicated in Table V, Appendix B
and in the same manner would be distributed again out of
lane 1 into lane 2. Entrance ramp lane A is a direct
entry into lane 1 and would also be distributed to lane 2
using Table V. Thus any point in lane 1 has three en-
trance ramp volume segments: Lane B traffic that has
previously entered lane 1 and has not moved to lane 2;
Lane B traffic that has moved into lane 1 in the preceding
500 feet; and lane A traffic that has not left lane 1.
These three segments give the spot volume at any point
due to the entrance ramp traffic. The remainder of the

Spot volume at any section is due to exit ramp traffic.

'37

In order to obtain the spot volumes for the different
sections it is assumed that 100 per cent of the ramp
bound traffic is at the nose of the entrance ramp and
as this traffic moves downstream it is assumed that the
traffic moves to lane 1 and then to the auxiliary lane
as if two auxiliary lanes were provided.

The other two-lane entrance ramps are on the
intersecting freeway and do not have an auxiliary lane.
Instead lane A traffic is assumed to enter directly into
lane 1, then move to lane 2 and lane 3 as indicated in
Table III, Appendix B. Lane B traffic is distributed
using the same table; however, it is assumed that the
lane B traffic will move only to lane 1 or 2. A con—
trolling factor for lane 1 traffic is the fact that the
percentage of traffic in the right lane cannot be less

than the percentage indicated in Table II, Appendix B.

Results

Tabulated in Appendix C, Table VIII through XV
is a volume distribution Sheet for each two-lane entrance

and exit ramp within the study area. On these sheets

38

are the spot volumes for each lane. The spot volumes,

as shown on the Sheet, are made up of the traffic volumes
contributed by the entrance ramp, exit ramp, and through
traffic and are calculated using the methods previously
described. By inspection of the volume distribution Sheet
it is possible to pick out the section and lanes that have
volumes in excess of those allowed for the desired level
of service. In a prior section of this thesis it was
stated that the desirable level of service was D. At

this level 1800 VPH would be the maximum volume per lane
at any given time. If this volume is exceeded, either a
lesser level of service must be accepted or the physical
layout of the ramp would have to be altered.

Two-Lane Exit Ramps.-—Volume distribution Table

 

VIII, Appendix C, indicates that ramp 2 on the dual—dual
freeway has a volume of 2210 VPH in the auxiliary lane,
1000 feet upstream from the ramp nose. This point coin-
cides with the beginning of the parallel deceleration
lane and indicates an insufficient length of parallel
deceleration lane. An additional 500 to 1000 feet of
lane is recommended in order to allow more ramp-bound

vehicles to move out of lane 1 to the parallel

39

deceleration line. Ramp 20, the other two—lane exit on
the dual-dual freeway, does not have a spot volume in
excess of 1800 VPH and is considered to be adequate.

On the intersecting freeway the Spot volumes in-
dicate a different Situation from the two—lane exit ramps
on the dual-dual freeway. In this case assigned traffic
volumes are so high that it is impossible to reduce the
spot volumes in lanes 2 and 3 below 1800 VPH without re-
ducing the total volume of traffic by metering or by in-
creasing the number of lanes.

Two—Lane Entrance RampS.—-Of the four two-lane
entrance ramps, all have Spot volumes in excess of 1800
VPH in at least one lane. On the dual-dual section of
freeway, Table IX, Appendix C, ramp 5 has a spot volume
of 2170 500 feet downstream from the nose. Table X,
Appendix C, has a maximum Spot volume of 2060 VPH in the
auxiliary lane 500 feet downstream from the nose. These
spot volumes indicate that the 500 feet prior to the nose
of a ramp is the area in which maximum congestion occurs.
The easiest way to eliminate this congestion is to in-
crease the number of vehicles that use lane A and thus

decrease the number of vehicles making multiple weaves.

40

Since it is the nature of drivers to prefer lane B of the
ramp, Special design details have to be made to encourage
drivers to use lane A of the ramp. By aligning the ramp
in such a fashion that direct entry into the freeway can
be made from lane A, traffic will choose the path of least
resistance and move to the left decreasing the multiple
weaving downstream from the ramp nose.

On the intersecting freeway Tables XIII and XIV,
Appendix C, Show a spot volume in excess of 1800 VPH. This
is to be expected considering the high volume of assigned
traffic for this road. It appears that there is no way
short of metering or adding lanes that can be recommended

to increase the level of service D.

Single-Lane Ramps

 

Analysis

The Single-lane ramp is another segment of design
that Should be investigated to determine its effect on
capacity and level of service. Ramp capacity is generally

determined by the design limitation at one of the

41

following locations: the entrance or exit point at the
freeway, the ramp prOper, or the terminus of the ramp
with the surface street system. In most cases it is the
merge or diverge volumes at the freeway that will control
the capacity of the freeway. AS for the ramp proper, it
is assumed that the maximum design volume will be 1800
VPH. Figure 4, Appendix A indicates the maximum assigned
volume for any one—lane ramp in the dual-dual section of
freeway is 1410 VPH; therefore, ramp proper will not be
the limiting location for capacity analysis. The other
location that may determine the maximum service volume
is the terminus of the ramp with the surface street.
In the section being analyzed the ramps do not terminate
on surface streets but at another merge or diverge, and
would be analyzed as normal ramp merge or diverge areas.
Several procedures have been developed for deter-
mining the merge or diverge volume for one-lane ramps.
However, regardless of the procedure used, the information
required is the same. As is the case with the two-lane
ramps, the volume of traffic at any point on the freeway
adjacent to the ramp is the sum of three possible traffic

elements. These traffic elements are as follows:

42

A. Traffic in the lane being evaluated that is des-

tined for an exit ramp.

B. Traffic that originates at an entrance ramp.

C. Traffic that is in the lane, but has not been
involved in a ramp movement within 4000 feet,

and is considered to be through traffic.

Evaluation of each of these elements will be carried out
by the same method used for the two-lane ramps. Spot
volumes are determined at selected points adjacent to
the ramp and the merge and diverge volumes and these

volumes are checked against those in the Highway Capacity

 

Manual [2]. As is the case for two-lane ramps, Tables II
and III, Appendix B, are used in the determination of
spot volumes. Table II is used to determine lane 1
through volume. Table III is used to determine lane 1
volume upstream and downstream of the off-ramp nose.

A Special case that arises in the analysis of
the single-lane ramps in the dual-dual freeway iS tWO
Off—ramps or two on-ramps located adjacent to each other.

In this situation it is necessary to add another element

43

to the group that makes up the traffic for any one point.
This additional element is the effect of the adjacent
ramp on the Spot volume being checked. For example,
Figure 1, Appendix A shows ramps 8 and 9 of the dual-dual
freeway sections are adjacent entrance ramps. To eval-
uate the spot volume in lane 1, 500 feet downstream of

the nose of ramp 9 it is necessary to find:

A. The through volume, using Table II, Appendix B.

B. The number of vehicles from ramp 8 that are in
lane 1 at the check point. Use Table III, Ap~

pendix B.

C. The number of vehicles from ramp 9 that are in
lane 1 at the check point. Use Table III, Ap-

pendix B.

Thus, total volume in lane 1 at a point 500 feet down—
stream from ramp 9's nose equals 180 + 270 + 950 = 1400
VPH. This example illustrates the procedure that will

be used on all of the one-lane exit ramps in this study
section. An assumption that is made when using Tables II

and III is that the percentage of traffic should yield

44

volumes near 1800 VPH. However, if the volume in the
right lane is considerably less than 1800 VPH, then the
section is obviously satisfactory and the actual dis—
tribution is of no significance.

In a situation such as is encountered in the
analysis of the one-lane ramps for the dual—dual section
the most critical section that is encountered is a point
either 500 feet upstream of the off-ramps or 500 feet
downstream of the on-ramps. This is the case only when
there is no traffic that is weaving through the ramp
bound traffic as for adjacent on-off or off—on ramp com—

binations.

Results

Appendix C, Tables XVI, XVII, XVIII, and XIX are
tabulations of the Spot volumes for the one-lane ramps on
the dual—dual section of freeway. It is noted that the
maximum lane 1 spot volumes for ramps 7, 8, 9, 12, and 13
are well below the maximum allowable value of 1800 VPH
for level of service D. These ramps are thus considered
to be of adequate design and there are no recommendations

to improve their Operation.

45

For ramp 14, it is noted that the maximum spot
volume in lane 1 is 1880 VPH. This is Slightly greater
than the allowable maximum of 1800 VPH for level of
service D. To improve service level at this ramp it is
recommended that the nose of ramp 14 be moved 500 feet
to the east to reduce the maximum Spot volume of 1720
VPH. This value is within the allowable volume for level
of service D and will insure smooth Operating conditions

on the inner roadway.

Left-Hand Entrance and Exit Ramps

 

In the design of the interchange under considera-
tion there are two-left hand exits and two-left hand
entrance ramps. These ramps allow outer roadway traffic
to move to the inner roadways and inner roadway traffic
to move to the outer roadways. It is anticipated that
these ramps will be carrying high Speed traffic and
should be designed to cause as little disturbance as
possible. Studies indicate that Speed in all lanes ad-

jacent to the left-hand entrance ramps are higher than

46

the comparable right-hand entrance ramps [11]. This
operational characteristic must be kept in mind in the
design of the ramps and can be used to aid the flow
through these areas. In general, there has been a re—
luctance in the past for most highway agencies to use,~
or recommend, the design of the left—hand ramps. Thus,
the amount of data that is available is limited [12].

In the design of the left entrance or exit ramps
there are major problems that must be considered if the
design is to be adequate. Following are some of the

major problems encountered:

A. Because of a tendency of drivers to use the left
lane of the freeway more than the other lanes,
any disruption in the left lane will effect the

volume of the other lanes as well [9].

B. Entrance vehicles must merge into the high Speed
and high volume lane. This increases the possi—

bility of an accident on the freeway [11].

C. There is an increase of weaving and hazardous
maneuvers in an area adjacent to left-hand

ramps [11].

47

D. Trucks generally are positioned in the right-hand
lane and the weaving required to enter the left-
hand exit ramp will result in serious disruptions

for conditions of high truck volume.

E. Accident frequency appears to be greater in the
area adjacent to a left-hand exit and entrance

ramps [13,14].

The above mentioned items should be considered if
the design of the left-hand exits and entrances are not
to cause operational problems. Once these items have
contributed their part to the design, a method must be
presented that will permit a quantitative and qualitative

analysis of the selected design.

Analysis

In establishing a method to determine merge, di-
verge, and weaving volumes, the approach is basically
the same as for a right—hand ramp. The difference is
through traffic will be in the left-lane instead of the

right. AS with any ramp the elements that are required

48

to make the analysis are the through traffic volumes,

the on—ramp traffic volume, and off-ramp traffic volumes.
In the Situation presented, the combination of

traffic volumes for an entrance-exit combination with

auxiliary lane will apply. As stated in the Highway

Capacity Manual [2], some point between the ramp there

 

will be a critical section which will have the maximum,
number of vehicles per lane merging, diverging, or weav-
ing as the case may be [2].

To find the locations of these critical sections
Tables IV and V, Appendix B are used. These tables make
it possible to determine the percentage of vehicles from
the on-ramp, or the vehicles destined for the off—ramp,
that are occupying a particular lane at some given posi—
tion between the two noses. For example, at a point 500
feet downstream from the entrance ramp nose, 65 per cent
of the entrance ramp volume will be in the left lane of
the freeway, and at a point 1000 feet upstream from the
off-ramp nose, 30 per cent of the off-ramp vehicles are
in the left-hand freeway lane. Tables XX and XXI, Ap—
pendix C demonstrate a complete breakdown by lane of all

traffic within the section under study. By observation

49

of the lane volumes the critical locations can be readily

identified.

Results

For freeway section 3—4 maximum lane values are
as follows: (1) merge volume—-1330 VPH, (2) diverge
volume——1130 VPH, and (3) weaving volume——1510 VPH. In
this case weaving volume is the greatest. When this value
is compared to the values in Table 8.1 in the Highway Ca-

pacity Manual [2], a level of service D is obtained for

 

the section.

Likewise for freeway section 18—19 the maximum
values for merge, diverge and weaving volumes are 1220
VPH, 1360 VPH, and 1340 VPH respectfully. In this sec-
tion the diverge value is the greatest and Table 8.1 gives
a level of service C [2].

With the levels of service obtained from the anal-
ysis of these sections it is apparent that an adequate
level of service is maintained and no improvements are

recommended.

CHAPTER IV

CONCLUSIONS AND RECOMMENDATIONS

Conclusions

 

Increasing demand for freeways has led to the
development of a new method of freeway design that allows
high volumes of traffic on a Single right of way and,
yet, limits the number of lanes in any direction to four.
This thesis has been concerned with the capacity and
level of service analysis of such a freeway, a dual-dual
freeway. An attempt was made to isolate the various ele-
ments of the dual-dual freeway and to analyze each of
these elements for capacity and level of service. The
desired results of the design were a level of service D
for all dual-dual freeway elements, and a balanced design.

From the analysis of the dual-dual freeway the
following elements of the design need improvement if a
balanced design with a level of service D is to be

achieved.

50

51

Dual-dual freeway

1.

Sections 7—8 and 12-13 have a level of service
that is too high to achieve a balanced design
for the overall freeway. This is a result of
the movement of a large volume of traffic from
the inner roadway to the outer roadway.

Sections l4~15 and 9-10 have too low a level of
service to meet the requirement set forth in
Chapter II.

The intersecting freeway, an existing facility,
has a level of service E and is not subject to
improvement.

Weaving areas

1.

The weaving areas as defined in Chapter III,
have a sfifficient number of lanes, and are of
sufficient length to maintain a level of serv-
ice D. However, the quality of flow could be
increased by increasing the length of the
weaving area.

Two-lane exit ramps

1.

Ramp 2 on the eastbound outer roadway has a
Spot volume in excess of 1800 VPH. This could
result in a complete breakdown of flow at this
ramp and steps Should be taken to remedy this
situation.

The two-lane exits on the intersecting freeway
are in areas that have Spot volumes greatly in
excess of 1800 VPH. However, it is impossible
to improve the situation without undertaking
major revisions on the freeway itself.

Two-lane entrance ramps

1.

Ramp 5 and ramp 1? have spot volumes 500 feet
downstream from the nose in excess of 1800 VPH.

52

2. As was the case for the two-lane exit ramp
there is too much assigned traffic to obtain
a level of service D on the intersecting free-
way downstream from entrance ramps.

E. Single-lane ramps--Ramp 14 has a spot volume
greater than 1800 VPH due to the effect of the
adjacent ramp. This Should be improved if a
smooth flow of traffic is to be maintained.

F. Left-hand entrance and exit ramps-~The limiting
factor for these ramps is the length of weaving
areas between the exit and the entrance ramps.

As noted in Chapter III this is adequate for

level of service D. Therefore, it is assumed

that these ramps function in an efficient manner.

Over all the analysis of the dual-dual freeway
section and its various elements point out the fact that
a dual-dual freeway can be effectively used to obtain high
volumes of traffic within a limited right of way. If the
standards set forth in Chapter II are followed, a level
of service D may be easily Obtained. The only problem
with this type of design is providing for the high turning
volumes at a major interchange. However, if adequate

parallel acceleration and deceleration lanes are main—

tained this problem can be eliminated.

53

Recommendations

As noted in the conclusion, various sections of

the dual-dual freeway are not up to the standards that

were originally set forth. In order to eliminate these

sub—standard sections the following recommendations are

suggested:

A.

Dual-dual freeway

1. In order to provide a balanced design section
7—8 and 12-13 Should have the number of lanes
for traffic reduced from three to two. Since
other circumstances require the construction
of three lanes, it may be possible to achieve
the same effect by painting out one lane.
Figure 5, Appendix A is a Sketch of such a
solution.

2. Section 14—15 and 9—10 need increased capacity
to raise the level of service for these sec-
tions from E to D. By prohibiting truck traf-
fic on the inner roadways this result can be
achieved.

Two-lane exit and entrance ramps

1. Ramps 2, 5, and 17 have spot volumes in excess
of 1800 VPH. In all three cases this can be
prevented by increasing the length of the
parallel deceleration and acceleration lane
to 1500 feet.

Single-lane ramps

1. Ramp 14 should be moved 500 feet to the east
in order to lessen the effect of the adjacent
ramp. This will reduce all Spot volumes to
less than 1800 VPH.

54

The recommendations noted above are all specific
points required to improve a certain section in the free-
way design. A more general recommendation for this type
of facility is to provide adequate Space to construct the
interchanges along the dual-dual freeway. It is at the
interchanges that weaving, diverging, and merging takes
place with the greatest frequency and these maneuvers re—
quire space.

One other general recommendation that will help
this type of facility function in the best possible manner
would be the exclusion of truck traffic from the inner
roadways. If this is done the high speed inner roadways

are certain to develop less friction and a higher capacity.

BIBLIOGRAPHY

BIBLIOGRAPHY

Institute of Traffic Engineers. Capacities and Limita—

 

tions of Urban Transportations Modes. Washington:
Institute of Traffic Engineers, 1965.

Highway Research Board. Highway Capacity Manual.
Washington: Highway Research Board, 1965.

American Association of State Highway Officials. ‘A
Policy on Geometric Design of Rural Highways.
Washington: American Assoc. of State Highway
Officials, 1964.

Michigan Department of State Highways. Road Standard
Plans and Standard Guides. Lansing: Michigan
Department of State Highways, 1964.

 

Michigan Department of State Highways. I—96 Freeway,
Planning and Route Location Study: City of De—
troit. Vol. I. Lansing: Michigan Department of
State Highways, 1964.

 

American Association of State Highway Officials.
Highway Design and Operational Practices Related
to Highway78afety. Washington: American Associa-
tion of State Highway Officials, 1967.

 

American Association of State Highway Officials. A_
Policy on Arterial Highways in Urban Areas.
Washington: American Association of State High-
way Officials, 1957.

Newman, L. and Moskowitz, K. "Effects of Grade on
Service Volume," Highway Research Record, 99:
224-243, 1965.

Newman, L. and Moskowitz, K. Notes on Freeway Capa-
city. Traffic Bulletin 4. Sacramento: State of
California, Division of Highways, 1962.

56

10.

11.

12.

13.

14.

15.

16.

17.

18.

19.

57

Hess, J. W. "Capacities and Characteristics of Ramp—
Freeway Connections," Highwangesearch Record,
27:69-115, 1963.

 

Worrall, R. D. and others. ”Study of Operational
Characteristics of Left Hand Entrance and Exit
Ramps on Urban Freeways," Highway Research Record,
99:244-274, 1965.

 

Greenshields, B. D. "A Study of Traffic Capacity,"
Proceedings of Highway Research Board. Bulletin
14:448—474. 1934.

 

Fisher, R. L. "Accident and Operating Experience at
Interchanges," Highway Research Bulletin, 235:
125—138, 1959.

Michigan Department of State Highways. A Comparison
of Left Hand Entrances and Exits with Right Hand
Entrances and Exits on a Divided Limited Access
Highway. Lansing: Traffic Division, Michigan
Department of State Highways, 1958.

Berry, D. J. and others. "A Study of Left Hand Exit
Ramps on Freeways,” Highway Research Record, 21:
1—16, 1963.

Hess, J. W. "Ramp Freeway Terminal Operation as Re-
lated to Freeway Lane Volume Distribution and
Adjacent Ramp Influence," Highway Research Record,
99:81—116, 1965.

Normann, O. K. ”Operation of Weaving Areas," High-
way Research Bulletin, 167:38-41, 1957.

Malo, A. F.; Mika, H. S.; and Walbridge, V. P.
"Traffic Behavior on an Urban Expressway," High—
wangesearch Bulletin, 235:19-37, 1960.

 

Keese, C. J.; Pinnell, C.; and McCasland, W. R. "A
Study of Freeway Traffic Operations," Highway
Research Bulletin, 235:73, 1960.

 

20.

21.

22.

23.

24.

25.

26.

27.

58

Drew, D. R. and Keese, C. J. "Freeway Level of
Service as Influenced by Volume and Capacity
Characteristics," Highway Research Record, 99:
1—49, 1965.

 

Furutome, Ichiro and Moskowitz, Karl. "Traffic
Behavior and Off Ramp Design,‘I Highway Research
Record, 21:17—31, 1963.

 

Wattleworth, J. A. and others. ”Operational Ef-
fects of Some Entrance Ramp Geometrics on Free-
way Merging," Highway Research Record, 202:79-
113, 1966.

 

Tipton, William E. "An Investigation of Factors
Affecting the Design Location of Freeway Ramps,"
Highway Research Record, 152:1-36, 1967.

 

Alexander, ChristOpher and Manhieim, M. L. "The
Design of Highway Interchanges: An Example of
a General Method for Analyzing Engineer Design
Problems," Highway Research Record, 83:48—75,
1965.

 

May, A. D. and Wagner, F. A. A Summary of Quality
and Fundamental Characteristics of Traffic Flow.
Lansing: Michigan State University, 1960.

Legarra, John. "Progress in Freeway Capacity
Studies," American Highways, 38:11, 21-23, July,
1958.

 

Johnson, Roger T. "Freeway Fatal Accidents 1961 and
1962," Highway Research Record, 99:117-156, 1965.

 

APPENDIX A

GENERAL PLAN OF DUAL-DUAL FREEWAY

AND

TYPICAL CROSS SECTIONS

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APPENDIX B

VOLUME DISTRIBUTION TABLES

66

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67

TABLE II
LANE DISTRIBUTION LANE 1

a
(NO RAMP MOVEMENT WITHIN 4000 FEET)

 

 

Percent of traffic in Lane 1

Thru Roadway (one Direction)

Volume

 

VPH 2 Lanes 3 Lanes 4 Lanes

 

< 1500 20

1500-2000 25

2000-2500 30

2500-3000 35

3000-3500 40 6

3500-4000 10

4000-4500 14 8

4500—5000 18 9

5000—5500 9

5500-6000 10

6000-6500 10

 

aKarl Moskowitz and Leonard Newman, Notes on Freeway Ca-
pacity, Traffic Bulletin No. 4 (Sacramento: State of
California Department of State Highway, 1962) Figure l.

 

68

TABLE III

a
PERCENTAGE OF RAMP TRAFFIC IN LANE 1

 

 

 

 

ENTRANCE RAMP EXIT RAMP
Dist. Per cent of Dist. Per cent of
Downstream Ramp Traffic Upstream Ramp Traffic
From Ramp Nose in Lane 1 From Ramp Nose in Lane 1
0 O 0 0
500 100 500 100
1000 60 1000 93
1500 32 1500 79
2000 18 2000 63
2500 14 2500 46
3000 12 3000 29
3500 11 3500 16
4000 10 4000 ll

 

a . . . .
Karl Moskow1tz and Leonard Newman, ibid., Figure 3.

69

 

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APPENDIX C

VOLUME DISTRIBUTION RESULTS

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74

TABLE VIII

VOLUME DISTRIBUTION RAMP 2

TWO-LANE EXIT RAMP SPOT VOLUME

 

 

 

 

 

 

LANE 1 AUXILIARY LANE
Dist.
Upstream Entr. Exit Thru Total Entr. Exit Total
Ramp Ramp Ramp Ramp
Feet VPH VPH VPH VPH VPH VPH VPH
4000 0 560 600 1160 800 O 800
3500 120 440 600 1160 680 450 1130
3000 300 460 600 1360 450 790 1240
2500 340 470 600 1410 280 1160 1440
2000 300 440 600 1340 180 1530 1710
1500 240 310 600 1150 110 1880 1990
1000 220 240 600 1060 40 2170 2210
500 170 70 600 840 20 1050 1070
O 140 0 600 740 O 650 650
ENTRANCE RAMP VOLUME = 800 VPH
EXIT RAMP VOLUME = 2450 VPH
FREEWAY VOLUME = 3810 VPH

75

TABLE IX

VOLUME DISTRIBUTION RAMP 5

TWO—LANE ENTRANCE SPOT VOLUME

 

 

 

 

 

 

LANE 1 AUXILIARY LANE
Dist.
Downstream Entr. Ex1t Total Entr. Ex1t Total
Ramp Ramp Ramp Ramp
Feet VPH VPH VPH VPH VPH VPH
0 810 0 810 1800 O 1800
500 1300 870 2170 900 0 900
1000 1210 350 1560 260 700 960
1500 630 110 740 50 980 1030
2000 300 O 300 0 1090 1090
ENTRANCE RAMP VOLUME = 2610 VPH
EXIT RAMP VOLUME = 1090 VPH
FREEWAY VOLUME = 4430 VPH

76

TABLE X

VOLUME DISTRIBUTION RAMP 17

TWO-LANE ENTRANCE RAMP SPOT VOLUME

 

 

 

 

 

 

Dist. LANE 1 AUXILIARY LANE
UPStream §:;;° ::;; Thru Total ESEE' ::;: Total
Feet VPH VPH VPH VPH VPH VPH VPH
0 0 90 200 290 650 0 650
500 100 60 200 360 1990 70 2060
1000 230 110 200 540 1580 120 1700
1500 580 160 200 940 1150 210 1360
2000 960 170 200 1330 530 340 870
2500 810 160 200 1170 260 470 730
3000 620 140 200 960 150 600 750
3500 440 90 200 730 70 710 730
4000 360 0 200 560 0 800 800
ENTRANCE RAMP VOLUME = 2450 VPH
EXIT RAMP VOLUME = 800 VPH

FREEWAY VOLUME = 3810 VPH

77

TABLE XI

VOLUME DISTRIBUTION RAMP 20

TWO-LANE EXIT RAMP SPOT VOLUME

 

 

 

 

 

 

LANE l AUXILIARY LANE
DiSt' E t E 't E t E 't
Downstream n r. X1 Total n r. X1 Total
Ramp Ramp Ramp Ramp
Feet VPH VPH VPH VPH VPH VPH
O 180 810 990 0 1800 1800
500 370 890 1260 60 1660 1720
1000 660 950 1610 150 1370 1520
1500 540 960 1500 540 890 1430
2000 O 1110 1110 1090 0 1090
ENTRANCE RAMP VOLUME = 1090 VPH
EXIT RAMP VOLUME = 2610 VPH
FREEWAY VOLUME = 4430 VPH

78

 

 

 

 

 

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HONOR. HHHQH. QCMH HMDOHH. SHQB OCMH OCMH HMfiOB DHQH. OCMHH EMOHUWG3OQ
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H mzmH m mzeH m mzmH

 

 

WZDHO> 80mm OSOO OUZOOBZW OZOHIOBB mm OZOO ZOHEDOHOBOHQ OEDHO>

HHHN HHO<E

mm> ONmm MEDHO> N<3mmmm

80

 

 

mm> oeom u mzaHo> mama mozemezm
mmHm mOHH ONOH mmHm mme omm omeH 0mm 0mm oom ooom
mmHm memH oem mmHm mom ooe QNNH omm ohm omm oomH
omom ommH oom omHm omo can own OOOH o omoH OOOH
ommH ommH o ommH omo oemH o oomH o oomH oom
ommH ommH 0 com com o o o o o o
mm> mm> mm> mm> mm> mm> mm> mm> mm> mm> ummm

4 m m m
HONOR. DHSR. OCOHH HONOR. SHSR. OCOH @COH HONOR. SHHRR. OCOH EOOHNOFS’OQ
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m OZOH N OZOH H OZOH

 

 

WZDHO> 80mm m2<m OUZOOBZM OZOHIOZE 5N m2<m ZOHBDOHOBOHQ MZDHO>

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81

mm> OmNm

MZDHO> Mmzmmmm

 

 

 

 

 

mm> Oth H MEDHO> mzfim EHXO
OMO omO OONN OOO Ohm ovm ONNN OOmH OMO OOON
OMHH OMHH OOHN OOO ONO Own ONON OOmH Omv OOON
ONVH ONwH OOON OOO OOm Omm OOOH OOmH OmN OOmH
OOOH OOOH OOOH OOO OMH OOOH OhOH OOmH OO OOOH
OOOH OOOH OOOH OOO O OBHH OOmH OOmH O OOm
OOOH OOOH OOO OOO O O OOmH OOmH O O
mm> mm> mm> mm> SOD mm> mm> mm> mm> ummm
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Hmuoa OOOH Hmuoa Dune OOOH OOOH Hmuoe SHOE OOOH Emmnummb
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MEDHO> 90mm HEOO BHXW OZOHIORB ON OEOO ZOHBDOHOBOHQ MEDHO>

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82

TABLE XVI

VOLUME DISTRIBUTION RAMP 7

ONE-LANE EXIT RAMP SPOT VOLUME

 

 

 

 

 

LANE 1
Up::::am EXit Thru Total
Volume Vol.
Feet VPH VPH VPH
O 0 180 180
500 1410 180 1590
1000 1310 180 1490
1500 1110 180 1290
2000 880 180 1060
2500 650 180 830
3000 410 180 590
3500 220 180 400
4000 155 180 335
EXIT RAMP VOLUME = 1410 VPH
FREEWAY VOLUME = 4400 VPH

TABLE XVII

VOLUME DISTRIBUTION RAMPS 8 & 9

ONE-LANE ENTRANCE RAMP SPOT VOLUME

 

 

 

 

 

LANE l
Dist.
Downstream Entrance Entrance Thru Total
Ramp Ramp Vol. Vol. Vol Vol

8 9 Ramp 8 Ramp 9 ° '
Feet Feet VPH VPH VPH VPH
0 0 O O 180 180
500 0 950 0 180 1130
1000 O 570 O 180 750
1500 500 300 950 180 1430
2000 1000 170 570 180 920
2500 1500 130 300 180 610
3000 2000 110 170 180 460
3500 2500 100 130 180 410
4000 3000 95 110 180 385
3500 95 100 180 375
4000 95 95 180 370

ENTRANCE RAMP 8 = 950 VPH

ENTRANCE RAMP 9 = 950 VPH

FREEWAY VOLUME = 4890 VPH

84

TABLE XVIII

VOLUME DISTRIBUTION RAMP 12

ONE-LANE ENTRANCE RAMP SPOT VOLUME

 

 

 

 

LANE 1
Dist.
Downstream Entrance Thru Total
Vol. Vol. Vol.
Feet VPH VPH VPH
0 0 180 180
500 1410 180 1590
1000 850 180 1030
1500 450 180 630
2000 250 180 430
2500 200 180 380
3000 170 180 350
3500 160 180 340
4000 140 180 320
ENTRANCE RAMP VOLUME = 1410 VPH O
FREEWAY VOLUME = 4400 VPH 0

85

TABLE XIX

VOLUME DISTRIBUTION RAMPS 13 & 14

ONE-LANE EXIT RAMPS SPOT VOLUME

 

 

 

 

 

Dist. LANE 1
Upstream
Ramp Ramp Exit Vol. Exit Vol. Thru Total
13 14‘ Ramp 13 Ramp 14 Vol. Vol.
Feet Feet VPH VPH VPH VPH
0 0 0 0 180 180
500 O 950 0 180 1130
1000 0 880 0 180 1060
1500 500 750 950 180 1880
2000 1000 600 880 180 1660
2500 1500 440 750 180 1370
3000 2000 270 600 180 1050
3500 2500 150 440 180 770
4000 3000 100 270 180 550’
3500 100 150 180 430
4000 100 100 180 380
EXIT RAMP 13 VOLUME = 950 VPH
EXIT RAMP l4 VOLUME = 950 VPH

FREEWAY VOLUME = 4890 VPH

86

 

Om> Oth

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87

Om> Oth

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