Research Team: Michael Zhang (lead) and Sarder Rafee Musabbir
UC Campus(es): UC Davis
Problem Statement: Setting an optimum speed limit on freeways for all types of vehicles, including trucks, is a critical task for highway management agencies. There are two schools of thought for setting speed limits. One is the uniform speed limit (USL), which mandates the same speed for all types of vehicular traffic. The other is the differential speed limit (DSL), which specifies distinct speed limits for cars and trucks. California along with six other states follow the DSL policy that sets a lower speed limit for trucks compared to passenger vehicles. There is no conclusive evidence on whether DSLs are safer or more economical than USLs or vice-versa for urban and rural areas. Thus, the safety and operational impacts of alternative speed limit policies are an open-ended question and to answer it will require a rich dataset with traffic attributes, roadway geometry, crash data, travel time, and other data points.
Project Description: This project estimates the safety and mobility impact of changing speed limits on California highways. Four different DSL scenarios and four uniform speed limit (USL) scenarios are tested for each of the simulation segments. The results show a decrease of travel time but an increase of fuel consumption as the speed limits get higher. The safety cost of crashes and operational costs were also estimated based on the simulation models. In general, as the speed limits are increased, the safety costs increase with the predicted increases in crashes, particularly severe and fatal crashes. The operational costs, on the other hand, generally decrease as the speed limits are increased. However, the amount of operational cost decreases are subject to greater uncertainty than the safety cost estimates are, due to uncertainties in sampling and demand estimation and in negligence of construction costs of roadway and signage changes to accommodate the new speed limits. From the economic perspective in this study, raising speed limits on rural California highways could reduce monetary costs, as savings in operational costs would exceed losses from more crashes.