Data on the speed profiles of trucks on sustained upgrades can be combined with safety estimates to quantify the increased accident rates caused by slow-moving trucks and the changes in accident rate with distance up the grade. Truck performance and speed data were taken from recent field measurements and were evaluated using the truck performance equations presented in NCHRP Report 185. The effect of speed differences on accident rate is based on the relationships developed by Solomon. The results show that there is a pronounced increase in accident rates of passenger cars and trucks in the traffic stream only when a sizeable portion of the truck population falls to speeds of 22.5 mph or less. The results indicate that, from a safety standpoint, there is little apparent need for truck climbing lanes on moderate upgrades (2%) or in the first portion of steeper upgrades. However, the results must be interpreted cautiously in light of limitations in the solomon data that were found during the analysis. In particular, the solomon data do not show how accident involvement rates change within the very important speed range from zero to 22.5 mph, and these data may represent sections with more intersection-and driveway-related accidents than would typically be found on a sustained grade. Further research is needed to quantify relationships between speed differences and accident involvement rates that are specifically applicable to sustained grades.
Abstract