Roadway design plays a major role in influencing driver behavior and operating speeds — often more than posted speed limits alone. To help agencies better understand this relationship and make informed design decisions, a recent study from MnDOT and the Minnesota Local Road Research Board (LRRB) examined which physical elements have the most influence on driver speeds.
Researchers developed speed reduction factors (SRFs) that measure the extent of speed reduction due to various roadway geometric features. As a result, these factors will help MnDOT and local agencies compare design alternatives and make data-driven decisions that align with safety goals, particularly for vulnerable users. Findings showed that the most effective speed-reduction strategy was a roundabout, which had an SRF of 7 mph for vehicles within 500 feet, followed by a raised median, which had an SRF of 3.1 mph.
For easy access to key findings, researchers created a fact sheet and accompanying YouTube video titled “Driving Speeds: What Makes a Difference?” These resources cover the basics of design speed and the roadway elements that have the greatest impact, plus a few common myths about road design.
