As cities across the US build bike lanes, their decisions are often seen as a move to give space to bikes at the expense of cars. But data tells us this isn't always true: In New York City, for instance, bike lanes have actually shortened cars' travel times on several streets, while simultaneously encouraging people to bike and making it safer.

How can this be? Two words: road diets.

The video above — created by city planner Jeff Speck and artist Spencer Boomhower — nicely explains four types of road diets, a trend that has become increasingly popular in US cities over the past few decades.

These diets can take several forms, but the basic idea is that by removing traffic lanes, cities can free up space for bike lanes and reduce the frequency of crashes. Narrowing lanes from 12 feet to 10, meanwhile, makes drivers less likely to speed, and in doing so has also been shown to cut down on crashes that involve drivers, cyclists, and pedestrians.

All this sounds like it would cause tons of congestion and make it harder to drive. But in many cases, it can be done without actually increasing the time it takes a driver to travel a given distance — and can sometimes even decrease it.

One reason is that short bursts of speeding, interspersed with waits at traffic lights, don't actually shorten travel times all that much — they just make streets more dangerous. Another is that if a street carries fewer than 20,000 cars per day, it generally doesn't need a second lane to maintain a smooth flow of traffic. Many of the roads targeted for diets carry far fewer cars to begin with.

What's more, though they allot less space to cars, many road diets use that space much more efficiently. For instance, road diet No. 2 in the video above (the most common type) removes a driving lane in both directions, but introduces a dedicated left turn lane for cars going in either direction.

This allows cars to wait to turn left without holding up traffic behind them — and is one reason why road diets and bike lanes along Broadway and Eighth and Ninth Avenues in New York City shortened travel times while significantly reducing cyclist injuries.