Adding Neighborhood 20 MPH Zones Isn’t a Zero-Sum Game

An exhaustive report published in the British Medical Journal found that traffic injuries declined in London's 20 mph zones and, to a lesser but still significant extent, on the streets immediately adjacent to the zones. Image: British Medical Journal

The Brooklyn Paper ran one of its trademark neighbor-vs.-neighbor stories today, turning a weekend public workshop about implementing a 20 mph zone in Park Slope into an occasion for more conflict-driven reporting:

Greenwood Heights activists claim drivers heading south on Sixth Avenue already speed up once they cross the Prospect Expressway and hit a five-block stretch between 20th and 25th streets with no stop signs.

“It’s already treacherous,” said resident Sarah Raskin. “This would divert unsafe driving from one neighborhood to another.”

Sounds like streets in Greenwood Heights need traffic calming too. And in fact, the Greenwood Heights residents quoted in the Brooklyn Paper seem to be saying they’d welcome a slow zone that encompasses their neighborhood.

It would be great to see a blanket 20 mph speed limit — pioneered by NYC DOT in the Bronx neighborhood of Claremont — extend to many neighborhoods at once. But if Park Slope gets a slow zone before Greenwood Heights, or if Greenwood Heights gets a slow zone before Park Slope, research suggests both neighborhoods will still be better off.

The definitive piece of research on 20 mph zones was published in the British Medical Journal in 2009. Reviewing 20 years of data, researchers found that London’s 20 mph zones, a patchwork of neighborhoods that expanded gradually over many years, prevent 27 traffic deaths and serious injuries annually. Within the zones, serious traffic injuries and deaths fell 46 percent, and children sustained 50 percent fewer casualties.

Significantly, the authors reported that the data “suggests that casualties inside 20 mph zones are not being displaced to nearby roads.” And on top of that, they found a spillover effect, with traffic injuries and deaths declining eight percent in areas adjacent to the slow zones (within 150 meters, or about two NYC blocks).

Adding slow zones is not a zero-sum game.