Safety in numbers and safety by design: A ‘virtuous cycle’

By Chris McCahill

Two recent studies reiterate what makes safer walking environments: more pedestrians, according to one; and well-connected networks of local streets, according to the other. Taken together, these studies build upon growing evidence that the safety benefits of cities designed for walking and biking are self-reinforcing and extend to drivers as well.

Researchers from the University of Minnesota analyzed 448 intersections in Minneapolis for a study published recently in Accident Analysis & Prevention. With all else being equal, they found that crash risk (crashes per person or per vehicle) goes down as volumes increase. This was most pronounced for pedestrians, whose crash risk goes down significantly as the number of people goes up (but increases with more car traffic). The authors call this “safety in numbers.”

However, as study co-author Brendan Murphy explained to Streetsblog: “We don’t have good statistical evidence to show that if a place is safe, people will walk—or in the other direction, that if people are walking, they make the place safer.” In other words, places that attract large numbers of pedestrians might be inherently safer to begin with.

Another study published recently in the Journal of Safety Research points to certain design features that play some role in making places safer and, subsequently, more attractive for pedestrians: narrow streets and frequent intersections. The study compares traffic fatalities, intersection densities, and length of road by type in 16 random U.S. cities. The authors found that cities with more intersections per mile of road have fewer traffic deaths and pedestrian deaths per capita. They also found that cities with more non-arterials (typically local roads with one lane in each direction) have lower death rates, while each mile of arterial road significantly increases those risks.

Several studies published by University of Connecticut researchers from 2010 to 2012 reached similar conclusions—i.e., that highly-connected street networks are safer and also encourage walking and biking—highlighting the interrelationships. Norman Garrick, co-author of those studies, told SSTI that there is safety in numbers, but that it’s important to understand what makes more people comfortable walking and biking in the first place.

Co-author Wesley Marshall, now a professor at the University of Colorado-Denver, explains: “If you design compact and connected networks and streets that encourage slower driving speeds, you get more walking and bicycling. If you have slower speeds and more pedestrians and more bicyclists, then you get better safety for all modes.” He calls this a “virtuous cycle” between design and safety.

Chris McCahill is a Associate Researcher at SSTI.