Want to Make Your City Better? Lower the Speed Limit.

Illustration by Tyler Hoehne

This spring, Anne Hidalgo, the new mayor of Paris, announced that the whole city would gradually lower the legal speed limit to just under 20 miles per hour (30km/h). Just a few streets that operate like highways will be exempt.

In adopting this prudent policy, Paris follows most of the major cities in the U.K., where the “20’s Plenty” campaign—aimed at reducing speed limits in residential urban areas—began. Central London has had a 20 mph speed limit for almost a year, where there are already more than 400 of these reduced speed zones marked by signs displaying a simple “20,” surrounded by a red circle.

The concept has even found its way to the United States. In a decision applauded by safety advocates all over the five boroughs, New York City was recently granted permission from state lawmakers to lower the default citywide speed limit from 30 to 25.

So why are some of the world’s premier cities hitting the brakes? Organizers of 20’s Plenty say there are a number of benefits and practically no downside to slower urban streets.

“It will increase the value of our homes. It will increase the number of people walking and cycling,” said Anna Semlyen, one of the campaign’s organizers, in a video by filmmaker Elizabeth Press. “It’s safer. It’s a better quality of life. It doesn’t have major effects on anybody’s journey times.”

And there’s solid data to back up Semlyen’s assertions. For example, after London established its 20 mile-per-hour zones, serious traffic fatalities and injuries fell an amazing 46 percent.

Crash severity is a simple function of physics: the weight, speed, and direction of the moving objects determines the amount of damage done. Thus, bigger objects (like cars and trucks) do more damage at higher speeds in a crash, especially in densely populated areas. In New York, for example, car collisions are a leading killer of children. That’s a pretty sad fact in a city where less than half the population owns cars. The charge to lower New York’s speed limits has been led, unsurprisingly, by the families of victims of traffic accidents.

Controlling speed is the surest way to reduce traffic fatalities. If a car traveling 20 miles per hour hits a pedestrian, researchers have found that the chances of survival are almost assured, or about 95 percent. But if that car is traveling 40 miles per hour, the pedestrian will almost certainly die; the fatality rate is about 85 percent.

Fast moving cars can also limit city dwellers’ movement. Calmer streets invite people who were put off by traffic and other road hazards to increase active travel, like walking and biking, and cut down on driving. Downtown areas become more pedestrian-friendly, and foot traffic increases, creating a boon for local businesses. In some cities in the U.K., active travel increased 12 percent [PDF] after speed limits were reduced from 30 to 20 miles per hour.

Slower traffic means better quality of life for city dwellers, and allows people who live in urban centers like Paris, London, and New York City, to worry less and enjoy more freedom. These cities, so famous for offering the best in cosmopolitan living, are all getting wise to the idea that slowing cars can improve their most basic appeal. It’s only a matter of time before other cities around the U.S. begin to catch on.