About induced travel

Widening Highway 1 will not reduce traffic congestion. The widening of I-880 in San Jose illustrates the problem:

In a front page story in the San Jose Mercury of January 23, 2004, headlined “Where's the relief?” (see above image), Gary Richards writes:

“When the bottleneck on Interstate 880 near Brokaw Road was unplugged two months ago with the addition of a third lane, traffic experts said it would shave 18 minutes off the afternoon southbound commute.


Instead of saving time, commutes have lengthened by perhaps 18 minutes. Where traffic once thinned out past Highway 101, the freeway has turned into a solid sea of red brake lights from 4:30 to as late as 9 p.m.”

Reason and experience support this view: The number of vehicles simply expands to fill the available road surface.

The Master Transportation Study, released in July, 2003 by the City of Santa Cruz, cites (in Section IV) the “Triple Convergence Principle”:

When road capacity is increased, total travel time will ultimately equalize over time, until traffic moves at the previous levels of congestion. Expansion of roadway capacity cannot eliminate periods of frustrating slow speeds, due to drivers who previously:

  • Used alternative routes during peak hours switch to the improved roadway (spatial convergence);

  • Traveled just before or after the peak hour start to travel during those hours (time convergence); and

  • Used public transportation during peak hours now switch to driving, since it has become faster (modal convergence).

Similar conclusions are reached by those who have analyzed the phenomenon of Induced Travel, which is defined as any increase in vehicle miles traveled (VMT) that results from an infrastructure change such as increase in road capacity.

In a study by Robert Noland, published in 2001 in Transportation Research, the phenomenon is clearly described. His paper provides a useful introduction, with many portions easily grasped by the lay reader. Here it is. Summarized, Noland's conclusions are these:

“The results of the analyses presented clearly demonstrate that the hypothesis of induced demand cannot be rejected. Increased capacity clearly increases vehicle miles of travel beyond any short run congestion relief that may be obtained. The methods employed all found statistically significant relationships between lane miles and VMT.

Lane miles are found to generally have a statistically significant relationship with VMT of about 0.3 to 0.6 in the short run and between 0.7 and 1.0 in the long run.”

Induced travel is a key fact in transportation planning, but Caltrans still refuses to account for it in making traffic projections for the widening of Highway 1.

See also our paper entitled “Why Widening Highway 1 Won't Work”.