Rethinking how urban speed limits are set improves safety for people in a number of ways. Even changing the posted speed limit sign creates safety benefits and allows cities to provide more and better safety treatments, and improve overall quality of life.
A growing body of research shows that speed limit changes alone can lead to measurable declines in speeds and crashes, even absent enforcement or engineering changes. For example, a 2017 Insurance Institute for Highway Safety study in Boston found that just reducing the citywide speed limit to 25 mph from 30 mph reduced speeding overall and dramatically decreased the instances of high-end speeding (vehicles traveling faster than 35 mph).
Similarly, in Canada, researchers at The Hospital for Sick Children found measurable safety gains after Toronto lowered speed limits from 40 kilometers per hour (~25 mph) to 30 kilometers per hour (~20 mph) on a number of local streets.
Recent efforts in Seattle underscore this pattern. There, the Department of Transportation saw significant speed and crash reductions when they lowered the speed limit to 25 mph and increased the density of speed limit signs on select streets.
Reducing the posted speed limit unlocks a variety of engineering and design tools that can further increase safety on a street and support other policy goals. Typically, the posted speed of a street dictates what infrastructure and safety elements can be included in the final street design. For example, if the posted speed is 30 mph, a wider curb radius will be required than if the posted speed is 25 mph. The wider curb radius increases exposure and risk for people walking and biking. All too often, essential pieces of safety infrastructure—raised crossings, bike lanes, corner bulb-outs—are ironically ineligible for inclusion in a street redesign because drivers are currently going too fast. In effect, the street is too dangerous to build safety infrastructure.
Reducing posted speeds creates opportunities for safer street designs that also support other policy goals. Similar to curb radii decisions, often infrastructure that supports transit and other sustainable modes like biking and walking, cannot be included in a design if the posted speed is too high. City policies around safety, economic sustainability, equity, carbon emissions reductions, and increased transit, bike, and walk mode share are interconnected. Rethinking speed limits unlocks the door for better design and safer streets, which increases opportunities for all.