State DOTs are helping locals set lower speed limits

By Chris McCahill 

In response to mounting safety issues, more transportation agencies are recognizing the importance of managing traffic speeds—a shift from long-held practices that prioritize vehicle speed. In the long run, this will require widespread changes in road design that reinforce lower travel speeds through physical and visual cues. Until then, however, authorities are turning to more immediate strategies like setting lower speed limits in urbanized areas. As the leading authorities across much of the U.S., state DOTs are stepping up to the task. 

In Minnesota, state law establishes standard speed limits on most roads: 10 mph in alleys, 30 mph on urban streets, and 55 mph on most other local roads. Prior to 2019, local governments could establish speed zones with lower speed limits, but they would need MnDOT to conduct a speed study and get approval from the Transportation Commissioner.  

In 2019, the law was updated to make it easier to change local speed limits if local authorities use accepted best practices, post appropriate signs, and implement the changes in a clear and consistent way. And MnDOT has released guidance for doing so. 

Traditionally, speed limits are set using the “85th percentile rule.” First, a traffic study is conducted, then speeds are set within 5 mph of the observed 85th percentile speed—the speed that only 15% of drivers exceed. That approach has come under scrutiny as researchers explain the rule of thumb was only ever meant to be a starting point and organizations like NACTO point out that the rule rewards the fastest drivers. 

The new MnDOT guide outlines a process for locals involving public outreach, data collection, and coordinating with law enforcement. In lieu of the 85th percentile speed, they recommend using the 50th percentile—an approach backed by research and federal guidance—or the 10 mph pace speed, based on a narrow range that represents most drivers. 

Other state agencies offer similar guidance. MassDOT, for example, offers resources for speed management, setting target speeds, and lowering speed limits. And in 2020, a group of engineers from WSDOT and local agencies across the state of Washington outlined recommendations for managing speeds and reducing traffic injuries that emphasized road design and speed limits. The findings from that report reflect conditions across most of the country: 

Year after year thousands of people die or become seriously injured while using Washington State roads. Driver speed is directly linked to the likelihood of a crash and to crash severity. The current system is not bringing about the desired goals of reducing injuries and eliminating traffic deaths. […] Key changes needed to lower operating speeds include modifications to the existing geometric design speed approach and typical approach to setting speed limits.

Photo Credit: Chad Elliott via Flickr, unmodified. License.