Abstract

Highway speed limits are increasing across the United States (Figure 1). There is also a national trend toward uniform speed limits, within states, for both passenger vehicles and trucks. California is one of only seven remaining states that sets different speed limits, with lower speed limits for trucks than passenger vehicles. While higher speed limits provide operational benefits by shortening travel times and fostering economic benefits—especially for the trucking and logistics industries—they can also increase the likelihood and severity of crashes since higher vehicle speeds require longer stopping distances and generate more energy during a collision.
While there is no consensus on the optimal speed limit, research generally shows that lower speed limits reduce the frequency and severity of crashes. Likewise, there is mixed evidence on whether universal (i.e., same for trucks and passenger vehicles) or differential speed limits are safer. While some evidence suggests that setting lower speed limits for heavier trucks that take longer to stop has safety benefits, others argue that differential speed limits create bottlenecks may cause more crashes as cars attempt to overtake slower-moving trucks.
The research team used traffic simulation and statistical models to estimate the costs of crashes, travel time, and vehicle operation for various speed limit scenarios in California. Analysis focused on the economic and comprehensive costs associated with fatal and severe traffic crashes, as outlined by the National Safety Council. They derived key metrics such as traffic speeds, travel times, and distances by simulating typical highway operating conditions. These insights helped calculate the overall economic and operational impact across the state’s highway system.