Safety Performance Functions For Rural County Road Segments In Michigan

Safety performance functions (SPFs) were developed for rural two-lane county roadway segments in Michigan.  Five years of crash data (2011 - 2015) were analyzed for greater than 6,500 miles of rural county roadways, covering 29 of Michigan’s 83 counties and representing all regions of the state.  Three separate models were developed to estimate annual deer-excluded total and injury crashes on rural county roadways: 1.) paved federal aid segments, 2.) paved non-federal aid segments, and 3.) paved and gravel non-federal aid segments with fewer than 400 vehicles per day.  To account for the unobserved heterogeneity associated with differing county design standards, mixed effects negative binomial models with a county-specific random effect was utilized.  Not surprisingly, the county segment SPFs generally differed from traditional models generated using data from state-maintained roadways. County federal aid roadways general showed greater crash occurrence than county non-federal aid roadways, the Highway Safety Manual (HSM) two-lane rural roadways model, and rural state highways in Michigan.  County non-federal aid paved roadways showed crash occurrence rates that were remarkably similar to the HSM base rural two-lane roadway model, while gravel roadways showed greater crash occurrence rates.  The presence of horizontal curves with design speeds below 55 mph had a strong association with the occurrence of total and injury crashes across all county road classes. Increasing driveway density was also found to be associated with increased crash occurrence.  However, lane width, roadway surface width, and paved shoulder width had little to no impact on total or injury crashes.