Know when to give your road a diet?


Three-lane road design in Minneapolis
East Hennepin Avenue in Minneapolis. Photo: Hennepin County

Public works departments and DOTs across the country are constantly grappling with how to best improve safety for all road users. With alarming upticks in injuries and deaths on our roads during the pandemic, especially for pedestrians and bicyclists, the demand for implementable solutions continues to rise.

One option increasingly being put into practice is the road diet—an approach that converts a wider road into a narrower design. Four-lane to three-lane road diets are very common. In such 4-3 conversions, a middle turn lane is added between two opposite direction lanes. Other forms of road diets include the one being considered on Hennepin Avenue South in Minneapolis, which proposes elements such as bus lanes, bike lanes, and wider pedestrian facilities in the place of traffic or parking lanes.

Road diets are recognized as proven safety interventions by the Federal Highway Administration. They also check many of the boxes prioritized by the USDOT’s new safe systems approach to traffic safety. Road diets can reduce risks to pedestrians by shortening crossing distances, installing pedestrian islands, and creating more space for protected bike and pedestrian facilities. For drivers, diets can help discourage risky maneuvers and eliminate some of the most dangerous turn types, such as left turns across two lanes of traffic.

Man biking with children along 66th Street in Richfield, MN
66th Street in Richfield, MN. Photo: Hennepin County

Researchers at the University of Minnesota and partners at Hennepin County recently discussed road diets at a CTS Transportation Safety and Traffic Flow Research Council webinar. The event allowed Professor Gary Davis (Department of Civil, Environmental, and Geo- Engineering) to share key findings around road diet performance. It also gave Hennepin County transportation planner Jordan Kocak the opportunity to talk about the real-time outcomes the county has seen on 66th Street, which recently underwent a road diet. The researcher-practitioner conversation shows how research and practice iterate off one another to improve our transportation systems. 

Davis’s work showed that road diets on moderately trafficked roads had little impact on the number of vehicles moving through or their speed. Kocak verified this with his discussion of the 66th Street diet, showing how traffic continued to flow at similar speeds while safety outcomes improved significantly for all users.

Davis’s project also showed how the metrics engineers and planners use to evaluate and guide projects impact their final form. Davis’s study responded to a survey of practitioners who wanted to know how a 4-3 conversion would affect level of service for car traffic. Conducting a similar study but foregrounding other considerations—such as safety outcomes or transit throughput—could provide additional decision-making points for implementers to consider alongside traditional items like vehicle throughput. And such an approach offers another set of interesting research questions for U researchers and our practitioner partners to explore in the future.


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Michael McCarthy