Yes, Bike Lanes Can Reduce Car Capacity. But They Probably Don’t Worsen Traffic.
D.C.'s latest protected cycletrack, on 1st Street NE, is nearly finished. For bikers, this means a faster, safer way to travel through NoMa. For drivers, it's yet another reason to gripe about bike infrastructure slowing down car traffic. Or is it? FiveThirtyEight has a study today on this very issue. The takeaway is this: Yes, converting a car lane to a bike lane means less space for cars to drive, without substantially reducing the number of cars driving there. This would appear to mean slower traffic, except it usually doesn't. It all depends on the street's volume-to-capacity ratio—the number of cars driving there as a proportion of the number of cars that could drive there at maximum capacity. Traffic doesn't really start to slow down until this ratio passes 0.75. In Minneapolis, America's most bike-friendly city and the location of this study, turning a car lane into a bike lane did increase the volume-to-capacity ratio on all 10 roads examined, sometimes substantially. But it never pushed it past the 0.75 marker, meaning that congestion never got severe. In the worst cases, it probably got harder to change lanes, but in most cases, there wasn't a discernible impact on drivers' ability to move speedily. In other words, the impact on drivers was minimal. The impact on cyclists who use those routes, on the other hand, was likely substantial. Between 4 and 5 percent of Minneapolitans commute by bike. That's probably too small a number to make a noticeable dent in car traffic, but a big enough one to be worth catering to. It's also one that's likely to increase as bike infrastructure improves—protected cycletracks provide a sense of safety that encourages more people to bike—perhaps to levels that will start diminishing road congestion. In D.C., according to a recent Washington City Paper/Kojo Nnamdi Show poll, 4 percent of likely Democratic voters in the April 1 primary commute by bike. (That figure may actually underestimate the total, given that the poll was conducted by landline and surveyed likely voters—a demographic that skews older than many cyclists.) Again, that's probably not quite enough to produce a meaningful decrease in car traffic, but it could get there in the future with improvements to cycling infrastructure. And in the meantime, as long as the city's not putting bike lanes on its most congested roads, there's probably not much harm. I've put in a request to the District Department of Transportation for volume-to-capacity ratios on 1st Street and other cycletrack locations, and will update if I receive those.
Photo from @DDOTDC
This article originally said that the City Paper/Kojo Nnamdi Show poll queried D.C. residents. In fact, it was a poll of likely Democratic primary voters in D.C.