Safety on rural highways continues to be a serious concern in the United States. While only 20 percent of the population live in rural areas, approximately one-half of motor vehicle fatalities occur on rural roadways, resulting in a rural fatal crash rate that is approximately double that of urban areas. In many states, most rural arterial highways are owned by the state department of transportation. However, several states, including Michigan, possess a large rural county highway network. For example, nearly 75 percent of the approximately 120,000 miles of public roadways in Michigan are owned by one of the 83 county road agencies across the state.County-owned highways typically possess characteristics that differ considerably from those owned by the state department of transportation, which limits the usefulness of safety performance functions (SPFs) and crash modification factors (CMFs) generated based on state highways, including those found in the Highway Safety Manual (HSM). Thus, assumptions made from models generated using data from state highways may not apply county highways due to differences in traffic, design, and maintenance. As a substantial proportion of rural crashes occur on county roads, identification of factors affecting safety performance on rural county roads is critical to support highway safety improvement programs and development of design standards.A cross-sectional safety performance analysis was performed for county highway segments and stop-controlled intersections throughout rural Michigan, including both federal aid and non-federal aid highways, as well as paved and unpaved road surfaces. SPFs were developed using mixed effects negative binomial regression to determine the safety effect of various design elements and site characteristics, including cross-sectional and geometric characteristics, which were included in the models as fixed effects. Random intercepts were incorporated into the models to account for unobserved heterogeneity between counties and between individual sites.One particularly noteworthy contribution of this research was to investigate the impacts of horizontal curvature on safety performance. Curve radii data extracted from the Michigan roadway shapefile allowed for the safety performance effects of decreasing curve design speed to be assessed in an incremental manner. Horizontal curves on paved county roads with design speeds below 40 mph experienced crash occurrence that was more than four times greater than segments without substandard curvature. On unpaved roadways, such curves experienced three times greater crash occurrence compared to segments without substandard curvature. Deer-related crashes, however, were shown to be fewer in frequency along horizontal curves.For stop-controlled intersections, skew angle was a variable of interest. At rural four-leg stop-controlled intersections, skew angles between 10 and 39 degrees were associated with increased crash frequency at intersections across all intersection classes. Skew had the greatest effect when the major road was county non-federal aid, where skew angles between 10 and 39 degrees experienced 60 percent more crashes than intersections without skew. Considering federal-aid intersections, the skew effect was diminished by approximately one-half.As expected, county-specific SPFs differed from models previously developed for state highways, including the SPFs included in the HSM. Generally speaking, at intersections, county highways were found to experience fewer crashes per unit of traffic volume than state highways, with county non-federal aid highways showing the lowest crash occurrence. County highway segments tend to have higher crash frequency than state roads. However, this is not the case at all traffic volumes, which further shows the need for county-specific safety performance models.
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