MTR 61, More Highways = More Traffic

Mobilizing the Region
Issue 61 January 12, 1996

More Highways = More Traffic

A recent study from a group at the University of California, Berkeley supports the argument that new highways or highway capacity generates additional motor vehicle traffic. While many might view this as common sense, most highway agencies in the United States deny it, arguing that increases in overall driving (measured in vehicle miles traveled -- VMT) are driven by forces external to the transportation system. The question is a critical one, because it bears directly on whether building new highway lanes will ease traffic congestion or simply lead to the same congestion on a bigger scale. The latter effect could sharply tilt any cost-benefit measure away from a costly urban or suburban roadway expansion project (to the extent that a city or metropolitan area employs such analysis). A government commission in Britain concluded last year that, under many circumstances, additional roadway lanes do induce extra traffic, and that finding is central to today's transportation policy debates in the U.K.

The new study attempts to isolate VMT increase stemming from a greater "roadway supply" from that resulting from other variables. It developed a "panel" data set from thirty urban California counties for the 1973-1990 period to correlate year-to-year variations in traffic and available highway lane-miles.

The main finding was that adding lane-miles does induce substantial new traffic. In metropolitan areas, a 1% increase in lane miles soon induces a .2 percent VMT increase, building to a .9% increase four years later. "Therefore, it appears that adding road capacity does little to decrease congestion because of the substantial induced traffic." An outstanding question is whether traffic attracted to new highway capacity is new trips or is displaced from local street networks. The study also estimates the contributions to VMT growth from various sources during the late 1970s and 1980s. Lane-mile growth has become progressively less important as a spur to VMT, in some measure due to declining additions to the roadway system. Other factors, like population, employment, the cost of driving, transit provision and other trends have become proportionately larger contributors to VMT growth. The latter finding does not, however, reduce the attention that must be paid to the conclusion that highway expansion is an ineffective tool to combat traffic congestion. Public officials in the Tri-State area need to ask whether the very expensive highway widening projects underway or planned here are an out-and-out waste of the public's money over the long run. The study's findings were presented by Mark Hansen in the Fall, 1995 Access: Research at the University of California Transportation Center.