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Drivers pay more attention.

Just in time for a warm-weather holiday weekend, we’re reminded (via the Transportationist’s David Levinson) that, statistically speaking, it’s a lot safer to walk and ride a bike where lots of other people walk and ride bikes. The “safety in numbers” concept feels obvious today, but strong evidence in its favor didn’t emerge until a 2003 study. Let’s take a closer look at this classic.

Health consultant Peter Lyndon Jacobsen gathered two main sets of data: how much people walk and bike, and how often walkers and bike riders collide with drivers. Unlike past analyses, which had compared such trends at the level of individual intersections, Jacobsen analyzed them on a much larger scale. He looked at 68 California cities, 47 towns in Denmark, and 14 countries in Europe.

The results couldn’t be clearer. In each case, the population data suggested that the likelihood of a pedestrian or cyclist being struck by a car decreased as walking and riding increased. It was true of per-capita injuries in California cities circa 2000:

(Injury Prevention)

And of per-capita injuries in Denmark towns between 1993 and 1996:

(Injury Prevention)

And of per capita cycling deaths in Europe in 1998:

(Injury Prevention)

In other words, he concluded, cars are less likely to strike walkers and cyclists where there are more walkers and cyclists. That’s not to say fewer total people get injured in these places; on the contrary, the data suggested that a place where walking or cycling doubled would result in a third more pedestrian or rider injuries. But the risk of injury in these same places would fall 34 percent. And the flip side holds true, too. If the number of walkers and cyclists halved, fewer total injuries occur, but the risk of injury goes up 52 percent.

Jacobsen calls the result “unexpected.” His analysis can’t explain for sure why there’s safety in numbers, but he offers a theory: drivers become more attentive when there’s a lot of bikes and pedestrians around. (The same expectation of driver caution guides the “shared streets” movement.) He concludes in Injury Prevention:

Since it is unlikely that the people walking and bicycling become more cautious if their numbers are larger, it indicates that the behavior of motorists controls the likelihood of collisions with people walking and bicycling. It appears that motorists adjust their behavior in the presence of people walking and bicycling.

So be careful on city streets this holiday, but don’t be too resentful of the crowds. If you’re walking or riding a bike, they might be keeping you safe.

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