Detours and induced demand

Bill Lindeke writes in MinnPost about Detours and induced demand

Probably the craziest detour in Minnesota history was the impromptu rerouting following the Interstate 35W bridge collapse. Needless to say, the 2007 bridge tragedy was completely unexpected, and forced state agencies to reroute 150,000 cars per day into other parts of the Twin Cities’ freeway system.

As it turned out, however, the detours worked surprisingly well. A University of Minnesota study determined that average commute times increased only marginally, less than half a minute on average.

Those unexpected results stemmed from two counterintuitive facets of detour construction. The first is the phenomenon of “induced demand,” or the dynamic relationship between road capacity and people’s willingness to drive. Simply put, the more available capacity on the road, the more likely that people will choose to drive; conversely, the less capacity, the less likely the car trip. That means that expanding roads might create more traffic, while reducing travel lanes might decrease traffic at the margins.

According to David Levinson, the engineering professor at the University of Minnesota who worked on the study, the I-35W bridge was a great example of “reduced demand” (the opposite of “induced demand”) where “about 1/3 of the trips across the river ‘disappeared,’ and were either foregone or went to different destinations.”

The second conclusion is that the Twin Cities’ road system actually has a well-connected road network, relatively speaking. During a detour, most Twin Cities drivers are able to find alternative routes.