University of California Institute of Transportation Studies

Highway speed limits inherently represent a tradeoff between safety and mobility. While higher speed limits shorten travel times and foster economic benefits (especially for the trucking and logistics industries), they can also increase the likelihood and severity of crashes, as higher vehicle speeds require longer stopping distances and generate more energy during a collision. Highway speed limits are increasing nationwide. While there is no consensus on the optimal speed limit (Figure 1), research generally shows that lower speed limits reduce the frequency and severity of crashes. Likewise, there is mixed evidence on whether a universal speed limit (trucks and passenger vehicles subject to the same speed limit) or a differential speed limit (trucks subject to a lower speed limit than passenger vehicles) is safer. While some evidence indicates that setting lower speed limits for heavier trucks that are slow to stop has safety benefits, other research suggests that differential speed limits create bottlenecks that may actually cause more crashes as cars attempt to overtake slower trucks. California is one of only seven states that set differential speed limits.