There is substantial evidence that demonstrates that building new roads often increases congestion. A well-established body of research shows that new lanes tend to get filled up with new traffic within a few years, particularly if surrounding routes are also congested. This phenomenonoften called "induced traffic"occurs when road capacity is expanded near congested routes and drivers flock to the new facility hoping to save time, even if they have to travel a great deal farther to achieve it. Also, the new roadways tend to draw people who would otherwise avoid congested conditions or take alternative modes to their destinations. The result is an overall increase in the total amount of driving and the total number of automobile trips in the regionnot just the redistribution of traffic from surrounding areas.
This theory has been strongly supported by empirical evidence. Since the 1940s, dozens of traffic studies have found that traffic inducement does indeed occur. New studies continue to support this hypothesis. The most notable of these covers 30 urban counties in California from 1973 to 1990. The authors, UC Berkeley researchers Mark Hansen and Yuanlin Huang, found that at the metropolitan level, every 1% increase in new lane-miles generated a 0.9% increase in traffic in less than five years, which led them to conclude that "With so much induced demand, adding road capacity does little to reduce congestion."
The time has come for transportation officials to stop making congestion relief claims to bolster highway proposals. Not only has road construction proven to be an ineffective congestion relief strategy, but it is an expensive one as well.
International transportation research has yielded other promising insights: the reduction of roadway capacity actually reduces traffic in most cases because people shift to transit, walking, bicycling and other modes of travel. In 1998, British researchers analyzed 60 road closures worldwide and found that on average, traffic decreased by 25 percent when a road was closed. In some cases, they found that an astonishing 60 percent of the driving trips disappeared.
One example of this was Londons Hammersmith Bridge, which in February 1997 was closed to all traffic except buses and cyclists. Londons Transport Department surveyed people who used the bridge a few days before it closed, and then contacted the same people in the weeks following the closure. Of the commuters who used the bridge to get to work, some switched to public transit and others chose to walk or bike. Overall, 21 percent no longer drove to work. And remarkably, congestion in the surrounding areas has not markedly increased. The results of these studies led researchers to say that "we conclude that measures which reduce or reallocate road capacity, when well-designed and favoured by strong reasons of policy, need not automatically be rejected for fear that they must inevitably cause unacceptable congestion."