Safety Performance Functions for Rural Two-Lane County Road Segments

Safety performance functions (SPFs) were developed for rural two-lane county roadway segments in Michigan. Five years of crash data (2011 to 2015) were analyzed for greater than 6,500 mi 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 vpd. To account for the unobserved heterogeneity associated with differing county design standards, mixed effects negative binomial models with a county-specific random effect were 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, whereas 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.

Field Performance of Chip Seals for Pavement Preservation

Over time, new pavements deteriorate due to the effect of traffic loads and the environment. If appropriate treatments are applied during the early stages of deterioration, it is possible to extend the service life of the pavement without incurring in costly rehabilitation or reconstruction activities. Chip seals are preservation treatments that can help protect the pavement structure, reduce the rate of pavement deterioration, improve skid resistance, and address minor surface problems. As part of the National Center for Asphalt Technology Pavement Preservation Study, chip seal test sections were placed in a low traffic volume road (Lee County Road 159) in Auburn, Alabama. The location consists of a two-lane county road that provides dead end access to a quarry and an asphalt plant, resulting in a high percentage of heavy loads. At the time of treatment, the existing pavement was 14 years old and consisted of a 5.5 in. hot-mix asphalt layer over a 6.0 in. granular base. Treatments were applied in the summer of 2012 and have been in service for approximately 4.5 years. During this time, cracking, roughness, rutting, and macrotexture data were collected weekly to evaluate pavement performance. The results determined that the performance of the treated sections is highly dependent on the initial condition of the pavement, particularly the percentage of area cracked. Pavements that are treated while still in good condition tend to remain in that category for a longer time. Macrotexture may also be used to evaluate the functional performance of the chip seals.