NCPA - National Center for Policy Analysis


February 26, 2007

Conventional wisdom states that traffic congestion cannot be reduced by building additional roadway capacity.  This theory holds that new roadways create or induce additional travel demand and that, as a result, there is no point to building more roadway capacity, since new roads cannot reduce traffic congestion.  However, this theory is wrong, says Wendell Cox and Alan Pisarski in the Reason Foundation's Privatization Watch magazine.

Houston, for example, which had the worst traffic congestion in the United States in 1985, undertook several measures to increase roadway capacity:

  • By 1993, traffic congestion-related delays had declined 40 percent.
  • In more recent years, even while some policies have not been continued, traffic congestion remains below the 1986 peak and similar to far smaller Portland (which had less than one-fifth Houston's traffic intensity in 1986).
  • Further, average roadway travel per capita increased only 10 percent in Houston from 1984 to 2002, less than one-third the national rate.

The emptiness of this "induced traffic" theory is further demonstrated by the experience elsewhere:

  • Since the mid-1980s, Phoenix has built more new urban freeways than any other major urban area in the last two decades.
  • Yet travel volumes in the area increased only 20 percent per capita from 1984 to 2002, well below the national urban average increase of 32 percent.
  • Perhaps even more significantly, Portland, with its adopted anti-freeway policies, experienced a 52 percent per capita increase in car use over the same period.

In fact, where metropolitan roadways have been substantially improved, virtually no "induced traffic" effect has been noted, say Cox and Pisarski.  Of course new roads do attract new traffic.  But research indicates that, even where a small, induced demand factor can be identified, it largely disappears when considered in terms of driving time (vehicle hours) instead of distance (vehicle miles).

Source: Wendell Cox and Alan Pisarski, "Can't Build Our Way Out?" Reason Foundation, January 3, 2007.


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