Seattle think-tank Sightline Institute has some bad news for city planners: investing more in public transit might not be the answer to reducing road congestion. Director of Programs Clark Williams-Derry posted on the institute's Daily Score blog last week a list of reasons he believes transit is not the solution to a city's traffic woes that we hoped it would be:

Transit advocates sometimes argue that bus or rail investments can help ease traffic, by getting people out of their cars.

Yet as far as I can tell, the evidence for this isn't so good.

This paper by researcher Antonio Bento and colleagues suggests that significant increases in bus service have essentially no effect on vehicle travel. Rail service increases do decrease vehicle travel, but by a surprisingly modest amount.

This paper by researchers from the University of Toronto found -- unsurprisingly -- that increases in road capacity were quickly matched by increases in traffic volumes. But it also found that increases in transit service had no effect on traffic volumes. In the authors' words: "these results fail to support the hypothesis that increase provision of public transit affects [vehicle miles traveled]."

And this study from a University of California-Davis found that higher residential densities and greater land use mix did decrease vehicle travel -- but found no statistically significant link between better transit service and less driving

I'm sure that there's more literature on this issue, some of which finds stronger connections between transit and vehicle travel. But in general, based on what I've found I have to align myself with Anthony Brooks and transit planner Jarrett Walker, who both argue that transit investments have little impact on how much driving goes on in a crowded urban area. To quote Walker:

To my knowledge...no transit project or service has ever been the clear direct cause of a substantial drop in traffic congestion. So claiming that a project you favor will reduce congestion is unwise; the data just don't support that claim.

Transit is good for an awful lot of things. It helps move people to where they want to go; it gives people who prefer not to drive, or who can't drive, a decent transportation option for many trips. It can reduce a region's reliance on risky fossil fuels; and on and on. But for folks who hope that transit investments will offset the impacts of road expansions -- well, sadly, I don't think the evidence lines up that way.

Read more on the Sightline Institute's blog the Daily Score, or check out Clark Williams-Derry's previous pieces on The Tyee.