Safety Performance Functions for Low-Volume Rural Stop-Controlled Intersections

Abstract

This study involved the development of safety performance functions for rural, low-volume, minor road stop-controlled intersections in Michigan. Facility types included three-leg stop-controlled (3ST) and four-leg stop-controlled (4ST) intersections under state or county jurisdiction and were sampled from each of Michigan’s 83 counties. To isolate lower-volume rural intersections, major roadway traffic volumes were limited to the range of 400–2,000 vehicles per day (vpd). Data were compiled from several sources for 2,023 intersections statewide. These data included traffic crashes, volumes, roadway classification, geometry, cross-sectional features, and other site characteristics covering the period of 2011–2015. Random effects negative binomial regression models were specified for each stop-controlled intersection type considering factors such as driveway density, lighting presence, turn lane presence, and intersection skew, in addition to volume. To account for the unobserved heterogeneity between counties, mixed effects negative binomial models with a county-specific random effect were utilized. Furthermore, unobserved temporal effects were controlled through the use of a year-specific random effect. Separate models were developed for fatal/injury crashes, property damage crashes, and select target crash types. The analysis found that skew angles of greater than five degrees led to significantly greater crash occurrence for both 3ST and 4ST intersections, while greater than two driveways near the intersection led to significantly greater angle crashes at 4ST intersections. Other factors were found to have little impact on crash occurrence. Comparison with the Highway Safety Manual (HSM) base models showed that the HSM models over-predict crashes on 4ST intersections and 3ST intersections with volumes between 1,200 and 2,000 vpd.

Crash Modification Functions for Rural Skewed Intersections

“…Nonfederal aid three-leg intersections had 29% lower crash occurrence, whereas nonfederal aid four-leg intersections had 20% lower crash occurrence. This echoes the findings of a study by Stapleton et al ( 18 ) into the safety performance of low volume rural stop-controlled intersections in Michigan. The perceived safety benefit of nonfederal aid intersections could possibly be attributed to a greater proportion of more familiar “local” drivers, despite otherwise lesser design standards and maintenance status.…”

Safety Performance of Rural Offset-T Intersections

“…At offset-T intersections, angle crashes account for less than a quarter (23.8%) of FI crashes, which is more similar to the default distribution for three-leg stop-controlled intersections in the HSM. This finding is consistent with previous investigations which conclude that either offset-T or three-leg intersections have fewer angle crashes than four-leg intersections ( 2, 3, 6, 7, 10, 11, 13 ). Not surprisingly, rear-end crashes and single-vehicle crashes were more prominent at offset-T sites, likely because of the unique geometry.…”

“…A spatially based algorithm was developed in ArcGIS to generate nodes based on the occurrence of intersecting lines within the All Roads shapefile. Further details about this process are described elsewhere ( 13 ). After nodes were generated, any intersection node located within Michigan’s Adjusted Census Urban Boundary zone was removed, to limit the data solely to intersections in rural areas.…”

Comprehensive Investigation of Commercial Motor Vehicle Crashes at Signalized and Unsignalized Intersections in Kentucky

“…Stapleton et al ( 5 ) adopted random effects NB models to develop SPFs for crashes occurring at 4-legged stop-controlled (4ST) and 3-legged stop-controlled (3ST) intersections in Michigan for the period 2011–2015. The results demonstrated that the annual average daily traffic (AADT) on major and minor roads and skew angle (greater than five degrees) increased crash frequency at both 3ST and 4ST intersections.…”