All-way stop signs reduce crashes by one-third

By Chris McCahill

From 2009 to 2016, the District Department of Transportation (DDOT) in Washington, DC, converted 60 intersections from two-way stops (allowing free flow on the major route) to all-way stop control. A new study in the Transportation Research Record looks at 53 of those intersections with available data and shows promising safety benefits.

Engineers and residents sometimes oppose adding stop signs because they delay traffic. But as many transportation agencies wrestle with persistent crashes and embrace Vision Zero, design interventions that improve safety are beginning to take precedence over speedy vehicle throughput. This recent study shows that stop signs can cut certain types of crashes, including the most serious ones, by more than half.

By comparing treatment sites to similar two-way stop intersections, the study shows crashes were lowered by 36 percent overall after all-way stop control was introduced, and crashes with injuries dropped 42 percent. The treatment seems to be most beneficial at skewed intersections, according to the study, where overall crashes dropped 64 percent and injury crashes dropped 74 percent. The impacts were also significant at locations with parking on both sides of the streets (-49 percent and -56 percent) and at four-way intersections (-43 percent and -54 percent). Intersections with missing sidewalks saw a 78 percent reduction in crashes, which the researchers think might have to do with the small number of sites. There was no significant impact on injury crashes at those locations.

Breaking down crashes by type, the study shows some mixed results. Straight-on crashes with pedestrians, for instance, seem to increase four-fold with all-way stop control. But these were only 31 out of 921 crashes (three percent), making it difficult to say if this would hold true with a larger sample. Meanwhile, right angle crashes, which make up one-third of the total, saw a substantial 83 percent drop. So while these nuances point to some unanswered questions, the evidence is overwhelmingly positive.

Chris McCahill is the Deputy Director at SSTI.