Highway Expansion Isn't Pointless
Catch phrases such as, “You can’t pave your way out of traffic congestion” or “Build it and they will come” often arise from the perception that road construction merely attracts more drivers. Previous studies put forth in academic literature present the concept of induced travel and have been taken hold of by anti-highway groups. This theory has been called into question by recent research.
Induced traffic is defined as “the additional travel resulting from a transportation improvement, which would not have otherwise occurred."[1] It describes new traffic generated by roadway improvements, which creates, rather than relieves, congestion.
A 1995 study conducted by the University of California offered compelling evidence that added road capacity unleashes new travel. According to the study, for every ten percent increase in road capacity, there is a nine percent increase in traffic volumes within three to four years. In other words, nine-tenths of added road capacity was absorbed by new trips. Essentially, this suggests that any new construction is a pointless endeavor.[2]
But, does expanding road capacity actually lead to such traffic increases, or does capacity follow traffic?
A recent study of Chicago’s I-294 and I-88 highways found that major population gains occurred in proximity to the expressways over a decade before the construction of the highways.[3] The population gains spurred the construction of the highways. This is an example of supply chasing demand — induced investment rather than induced travel.
A complex blend of factors influences travel growth and it is difficult to separate the effects of induced travel from other changes in travel behavior. “Road investments by themselves do not increase volumes,” states Regional Planning Professor Robert Cervero of University of California, Berkeley. “Only by incurring a benefit, like faster speeds, will traffic increase. Behavioral shifts, for one, have a huge impact on road use."[4] With greater traffic capacity, individuals change their driving behavior, namely the routes and types of trips they take.
Some of these changes in behavior induce new travel, but other changes merely redistribute existing travel.[5] New roadways may encourage drivers to make longer trips. For example, individuals who used to shop at the small, expensive grocery store nearby because of congested roads may now travel further to shop at a less expensive supermarket.
In a culture where the car is vital to our way of life, it is easy to believe that increased highway capacity generates new traffic. New research seems to indicate, however, that increasing road capacity mainly redistributes traffic. So although drivers may respond to decreased highway driving times by driving more on highways, highway expansion has a beneficial effect on overall congestion as trips are shifted away from other roads.
Notes
[1] Todd Littman. “Determining Generated Traffic External Costs,” (Victoria, BC: Victoria Transportation Policy Institute, 1995), 3.
[2] Robert Cervero, “Are Induced-Travel Studies Inducing Bad Investments?” Access, Number 22, Spring 2003.
[3] Ibid.
[4] Ibid.
[5] “Assessing the Issue of Induced Travel,” July 2001 see www.secondcrossing.org.
Jen Melby is a former research intern with The Buckeye Institute.
