When traffic congestion gets especially nasty, the first thing planners think to do is expand road capacity. More lanes should ease the pressure, right? Except, that doesn’t work. As Eric Jaffe points out over at Atlantic Cities, traffic tends to expand to fill capacity. He cites a new paper in the American Economic Review that finds that traffic “increases proportionately to roadway lane kilometers for interstate highways and probably slightly less rapidly for other types of roads.” (Scott Olson/Getty)
Why is this? The demand for space on the roads is high. More lanes just cause people to drive more. Habits shift, too: The guy who’d previously left work at 6 in the morning to beat the traffic now decides to leave a little later, closer to rush hour. Overall congestion stays roughly constant. A second option for planners, of course, is to expand public transportation. If there are more buses and subways, that should free up space on the roads, right?
No again. Here are the authors of the AER paper, Gilles Duranton and Matthew Turner: “We find no evidence that the provision of public transportation affects [vehicle miles traveled. We conclude that increased provision of roads or public transit is unlikely to relieve congestion.”
