Back in July, 2015 a report that bike sharing didn't do much for congestion or air quality made some headlines on some more wonky media. But the authors own conclusions undermined that reporting
Ricci found that bike-share users don’t bike instead of driving so much as they bike instead of taking transit or walking. She writes that, “although Dublin bikes users reported considerable behavioral change, the prevailing trend showed a large modal shift (80.2 percent) from sustainable modes of travel to the bicycle, particularly from walking (45.6 percent) and including transfer from bus (25.8 percent) and rail (8.8 percent).” Still, nearly 20 percent of Dublin bikes users say they now drive less. Other European and American cities saw far lower rates of mode shift. In London only 2 percent of users shifted away from cars. In Lyon, France, and Washington, D.C., it’s 7 percent.
It certainly seems there is SOME reduction in driving and that is likely to lead to SOME reduction in congestion. And about a month later a pair of researchers from DC and the University of Richmond did a study showing that having 7% of CaBi users drive less results in about a 2-3% reduction in driving. They determined this by looking at traffic data over a 2 year period during which bike share docks were being installed and comparing the traffic before and after installation.
Our empirical results indicate that the average treatment effect of the presence of bikeshare docks is a 2 to 3% reduction in traffic congestion. In addition to these results, we also find evidence of a potential spillover effect, in which docks increase congestion in neighboring locations, perhaps as they lead drivers to find alternative routes to avoid bicycle traffic.
This fall they revised their study, and the news is better
Estimates from our preferred models indicate a 4% reduction in congestion due to the presence of a bikeshare...This would reduce annual congestion costs for Washington area automobile commuters by approximately $57 per commuter, and total costs by $182 million...[and]...In terms of social benefits, a 4% reduction in traffic congestion for our study area would imply an annual benefit of roughly $1.28 million from reductions in congestion-induced CO2 emissions
This value ignores any benefits from cleaner air (like NOx emissions), private cost-savings from modeswitching and any health benefits that may accrue to bicycle commuters. They also found that congestion mitigation occurs primarily in areas with relatively high congestion and that there was actually almost no spillover effect
Better identification indicates that there is little or no congestion spillover from adjacent treatment.
All good news, and should add to the rapid development of bikesharing in just about every urban area in America and the world.