Assessing the Safety Implication of Alternative Speed Limits in California

Research Team: Michael Zhang (lead) and Sarder Rafee Musabbir

UC Campus(es): UC Davis

Problem Statement: Assessing the impact of raising truck speed limits on freeways has become an important issue, given the potential to increase economic productivity, and the associated safety concerns regarding higher speed limits. There is a recent discussion regarding whether truck operating speed has a significant influence on the frequency and severity of crashes on the freeway based on two speed limit policies: differential speed limit (DSL); and, uniform speed limit (USL). DSL policy sets lower maximum speed limits for buses and trucks, whereas USL policy sets uniform speed limits for all vehicles. California ascribes to DSL policy and sets the speed limit for trucks at 55 mph, compared to 65 mph for passenger cars. However, there is no compelling evidence to support the widespread application of DSL. Only a handful of States (California, Oregon, Washington, Idaho, Montana, Michigan, Indiana and Arkansas) practice this policy.

Project Description: This project combined the statewide crash data (SWIRTS) and traffic data (PeMS)to develop statistical models to determine the safety impacts of alternative speed limits on California highways. The models examined whether various factors about crashes, including average traffic speed and truck-involvement, correlated with outcomes such as crash severity. The models were then used to test the impact of four alternative speed limit policies (B-E) on the predicted number of fatal crashes and unsafe-speed related crashes in urban and rural areas. The policies were: (A) Existing differential speed policy for cars (65 mph) and trucks (55 mph); (B) Raising the speed limit on interstates for trucks from 55 to 65 mph; (C) Raising the speed limit on interstates from 55 to 75 mph for trucks and 65 to 75 mph for cars; (D) Lowering the existing differential speed on interstates from 55 to 50mph for trucks and 65 to 60 mph for cars; (E) Raising the existing differential speed on interstates from 55 to 70 mph for trucks and 65 to 80 mph for cars. The policy analysis shows a difference in the predicted number of crashes (fatal, unsafe speed) in and between urban and rural areas. The percentage increase/decrease in predicted fatal crashes in rural areas is lower than urban areas across all policy alternatives.

Status: Completed

Budget: $74,020

Project Partner(s): California Assembly Transportation Committee, California Department of Transportation (Caltrans)

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