Lowering speed limits can reduce crashes

By Chris McCahill

Speed limits are often based on observed 85th percentile free flow speeds. Setting them lower, even to address safety concerns, can be difficult once engineering recommendations have been made. A new study, however, bolsters the case for doing so by showing that setting limits just below the observed speeds can reduce crashes, including the most serious ones.

This newest study from Penn State looked at 12 road segments in Montana. Four of the roads had speed limits that matched the engineering recommendations. Three roads were posted at 5 MPH below the recommended speed and another five were 10 to 25 MPH lower.

Using regression analysis and considering a variety of road characteristics, the researchers found that reducing speed limits to 5 MPH below the recommended speed could reduce crashes by 50 percent. Fatal and other serious crashes drop by almost as much. For roads set 10 MPH below the recommended speed, total crashes dropped slightly but fatal and injury crashes were slightly higher. Setting speed limits any lower could increase crash risk, according to the study, but the sample size was too small to draw meaningful conclusions in this range.

Figure 1. Crash modification factors (CMFs) based on posted speed limits.

One reason for these safety benefits could be improved compliance with the posted limits, according to the study. In general, compliance was highest in locations with higher speed limits, but compliance increased when speed limits were set 5 to 10 MPH below the engineering recommendation. In other words, lowering speed limits within this range can reduce operating speeds, just as they are meant to.

These findings support recommendations from the National Transportation Safety Board that FHWA revise its guidance pertaining to 85th percentile speeds from the Manual on Uniform Traffic Control Devices (MUTCD). Their 2017 report notes, “there is not strong evidence that, within a given traffic flow, the 85th percentile speed equates to the speed with the lowest crash involvement rate on all road types,” adding that the original research is dated and might not stand up to scrutiny.

This new research suggests that lower speed limits, particularly when paired with heavy enforcement, can be effective tools for improving traffic safety. It does not, however, let road designers off the hook or negate the fact that speed limits should be intuitive to drivers, based on the context of a road. Both ITE and NACTO urge road designers to select maximum desired target speeds to base their designs on, so that the features of the road also help encourage lower speed travel.

Chris McCahill is the Deputy Director at SSTI.