Suggested Citation:
Musabbir, Sarder Rafee and Michael Zhang (2020) Assessing the Safety Implication of Alternative Speed Limits in California. Institute of Transportation Studies, University of California, Davis, Research Report UCD-ITS-RR-20-03

This project used statewide crash and traffic data to develop four statistical models to determine the safety impacts of increasing speed limits for trucks and cars on California highways. The models examined whether various factors about crashes—including average traffic speed, involved vehicle type, weather, etc.—correlated with other crash characteristics of particular policy relevance: namely, fatal crash, truck-related crash, speeding-related crash, and crash severity. The fatal crash model was then used to predict the probability of fatal crashes in urban and rural areas under four possible speed limit policies: (A) maintaining the existing speed limits of 65 mph for cars and 55 mph for trucks; (B) increasing each of these by 5 or 10 mph; (C) increasing the current truck speed limit to equal the car speed limit of 65 mph; (D) following policy C and then increasing the uniform 65 mph speed limit by 5 or 10 mph. The obtained probabilities were then used to forecast the number of fatal crashes under these policies. For the cases with speed limit increases (B-D), the corresponding increases in the predicted number of fatal crashes are small in rural areas and are far less than in urban areas. This suggests that increasing the truck speed limit towards 65 mph to a uniform speed limit (Policy C) in rural areas will not likely increase the frequency of fatal crashes. For urban areas, all speed limit increases are likely to increase the number of fatal crashes in comparison to Policy A (the current policy), but the increase in fatal crashes with Policy C (65 mph for trucks and cars) over Policy A is 1%. Policy C, therefore, is considered the best choice in balancing safety and mobility for both rural and urban areas in California.

Keywords: Speed limits, trucks, highway safety, policy analysis, crash risk, forecasting, traffic speeds, logits