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Study: Bike share has (slightly) reduced congestion in Washington, DC

(MrTinDC)

Bike-share systems are booming in cities across the country. One of their main benefits, according to boosters, is that by encouraging people to bike for transportation, they cut down on traffic.

But does that actually happen? Until recently, we had virtually no data on the impact of bike-share systems on traffic.

So in a new study, Timothy Hamilton and Casey Wichman of Resources for the Future examine the effect of Washington, DC's Capital Bikeshare network — and they say that on the whole, bike-share stations appear to reduce congestion in surrounding areas by 2 to 3 percent. Interestingly, though, the stations appear to slightly increase car traffic in adjacent neighborhoods, possibly by causing drivers to avoid bike-heavy areas.

How the study worked

Hamilton and Wichman looked at the impact of a bike-share station on congestion in each of the DC metro area's census block groups. These are areas designated by the US Census that typically have 600 to 3,000 people living in them. The idea was to use these, rather than smaller areas called census blocks, because block groups are large enough that you probably wouldn't end up walking to another one to get a bike.

The Washington, DC, area's census block groups.

(Hamilton and Wichman)

The researchers used traffic data from the University of Maryland's CATT Lab, which details the speed and volume of cars on a huge number of the DC area's roads. They specifically defined congestion as how slow cars actually traveled, compared with how fast they'd go on each road if they were allowed to flow freely, only restrained by speed limits.

Of course, bike-share stations aren't placed randomly: They're most often put in relatively dense, pedestrian-friendly neighborhoods, as well as ones that are disproportionately wealthy. So to combat this bias, the researchers sought to compare pairs of areas that were otherwise similar, but only one had gotten a bike share station. To do that, they matched the 82 areas that got stations sometime during 2010 with 82 control areas that didn't, but were comparable in terms of pre-2010 congestion levels, wealth, land value, and proximity to a metro station.

Now, there are a few caveats here. If you're using bike share instead of a car, you're going to reduce traffic on all the roads that make up your entire route — and not just the area in the vicinity of your station. (It's likely that most people bike across multiple blocks.) That said, if bike sharing really were reducing congestion, you'd still expect to see a disproportionate reduction in the block groups that contain bike-share stations — and thus serve as the start and end of trips — rather than those that don't.

The other big caveat is that this is just one study, in one city, over one period of time. DC's traffic trends might not be applicable to other cities'. What's more, the study's data only runs through 2012. Things might have changed since.

How bike share affected DC's traffic patterns

(Brittany Greeson/The Washington Post via Getty Images)

Hamilton and Wichman looked at the level of congestion in each area's roads during weekday morning rush hours in April, May, September, and October for each of the years from 2010 to 2012. And on the whole, they found that areas with bike share saw congestion decline by 2 to 3 percent during that period, compared with similar areas that didn't get stations.

This definitely isn't an earth-shattering change — depending on the free flow speed of the road, it works out to about a 2 to 3 percent increase in average speed — but when you multiply that across millions of drivers, that could mean a lot of time saved. "I was surprised at how big the effect was," Wichman told WAMU.

When he and Hamilton looked at individual roads, they found an especially big decline on those designed for faster traffic to begin with. Roads with 35 miles per hour or greater speed limits in areas with bike share stations saw the biggest congestion reductions, approaching 3 percent or more. On the surface, this makes sense — these commuting corridors are disproportionately slowed down by excess traffic, and people on bikes probably try to avoid riding on them, sticking to slower side streets.

Finally, the researchers found a smaller but measurable increase in congestion in areas that were adjacent to those with bike-share stations, whether or not they had stations themselves.

One possible explanation is that it's a spillover effect: Drivers, in theory, might learn that certain areas have lots of bikers and try to avoid them accordingly. But this finding didn't come from the same sort of comparison with control areas. So it's possible it's just a result of the fact that most of the areas adjacent to those with stations are in denser parts of the city, and became more congested between 2010 and 2012 for other reasons.

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