Colorado takes new approach to setting speed limits

By Aaron Westling 

The research is clear, increased driver speeds lead to more dangerous roads. For example, increasing the state maximum speed limit by 5 mph results in an 8% increase in the fatality rate on interstates and freeways, and a 4% increase on other roads. Speed is even more dangerous for pedestrians; research shows that a person hit by a car traveling at 35 mph is five times more likely to die than a person hit by a car traveling at 20 mph. These facts highlight the important role speed limits play in creating safe streets, and is one of the reasons the Colorado DOT (CDOT) is rethinking how it determines appropriate limits.  

Around the country, most DOTs use the 85th percentile rule when determining speed limits. This technique involves monitoring a roadway and identifying the speed at or below which 85% of the drivers are traveling, and then setting the speed limit by rounding from that speed to the nearest 5 mph marker. This method is tied to current driving tendencies and the design of a road, and often results in high speed limits regardless of whether that aligns with a DOT’s stated safety goals.  

In an effort to rethink how posted speed limits are decided and to right-size speeds for various types of roads and situations, CDOT is shifting to a new speed management theory. This will allow traffic engineers to take additional factors into consideration such as the purpose of the road, the number of vulnerable traffic users—including pedestrians and cyclists—who use it, and its design. According to Colorado Public Radio, CDOT has evaluated between 50 and 60 miles of roadway under the new criteria, and “about two-thirds of those studies have resulted in lower speed limits.”  

Allowing traffic engineers to use a range of data and considerations to determine the appropriate speed for a road may be most impactful in urban settings. In many cities around the United States, the most dangerous roads for all types of road users—but especially pedestrians and cyclists—are arterial highways, which are most often owned and operated by state DOTs. For example, in Milwaukee, WI, pedestrian deaths increased by 50% from 2021 to 2022, reaching the highest level since 1981. The two most dangerous roads in the city are both state-owned highways that cut through predominantly Black and brown residential neighborhoods. When asked about what the Wisconsin DOT is doing to reduce this trend a representative from the DOT explained that WisDOT typically approaches rural and urban highways with the same set of goals, including traffic flow. 

Source: NACTO, City Limits: Setting Safe Speed Limits on Urban Streets, 2020

CDOT is one of the first agencies to adjust its policies to treat the danger of travel speed with the urgency it deserves. This shift can act as a model for DOTs around the country that are looking at strategies to slow down drivers where doing so could save lives.  

Photo Credit: Isabella Fischer via Unsplash, unmodified. License