Seattle is sprouting. Census Bureau figures released earlier this year show that the city added 15,000 people between summer 2017 and summer 2018, a 2.1 percent jump from the prior year, marking the Emerald City as the nation’s second-fastest-growing large city. Construction of multifamily units is down slightly over last year, but the cranes are still swinging and the jackhammers still jacking: By July of this year, the city had issued permits for 15,000 apartment units.

Jim Curtain puts it more succinctly. “The building out here is going gangbusters,” he says.

So it’s curious, perhaps, that Curtain, the director of the project development division at Seattle’s Department of Transportation, is helping to slow the city down. Way down. This week, the DOT, along with Mayor Jenny Durkan, said it plans to lower the speed limit on all major roads to 25 mph, down from 40 mph in some places. Many residential streets in the city already top out at 20 mph.

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Historically, transportation departments have sought to quickly move people from A to B. But Seattle’s is actually joining something of a nationwide trend. In New York, Philadelphia, Boston, and Portland, Oregon, officials have experimented with lowering speed limits, often to 25 mph or below.

The reason is simple, and science-driven: The World Health Organization has suggested that lowering speed limits by 5 percent means 30 percent fewer crashes that result in deaths. A study published earlier this year by the Insurance Institute of Highway Safety found that states raising their speed limits in the last 25 years had a deadly effect, leading to 36,760 additional road deaths. (States generally set maximum speed limits in the US—more on that later.)

Seattle is reducing speeds because it has seen traffic deaths tick up. Twenty-five people have died on Seattle roads this year, the most since 2016 and nearly double last year’s rate.

Still, it’s unusual for a city to lower speed limits all at once. Seattle’s trying to get it done by the end of next year. “Work orders are already being written,” says Curtain.

New York is the only major American city to recently lower speed limits citywide, to 25 mph, in 2015. The results there have been positive—though mixed. Traffic deaths are falling, hitting 200 last year, down 10 percent from the year previous and a record low. Still, pedestrian deaths were up in 2018, and research suggests that New York’s efforts at slowing traffic have not been as effective as other cities’—perhaps because they have not gone far enough. London, for example, has carved out dedicated “slow zones” with raised sidewalks, built mini-roundabouts, and extended curbs, transforming the driving and walking experience for city dwellers, and leading to a dip in traffic deaths. (New York also has “slow zones,” but they have fewer traffic-calming features than their brethren across the pond.)

Seattle plans to use other tools in its traffic-slowing toolbox, too. On some roads, it has narrowed lanes, a move that forces drivers to pay better attention and slow down while traveling. It has also added “speed cushions,” which sound very luxurious but are actually just speed bumps with gaps, allowing vehicles with wide wheelbases—like fire engines—to pass unimpeded. Seattle also announced this week that it would increase crossing times for pedestrian traffic signals, giving people on foot more time to get across the street before cars are allowed to turn. Curtain said that the city has also started coordinating with the police department, both on driver education and speed limit enforcement.