The Carmageddon Myth

Joe Cortright takes a closer look at the effects of the closure of Seattle's Alaskan Way Viaduct, a roadway that handled almost 100,000 vehicle trips a day. He finds that the peak-hour traffic patterns two days in do not look that much different than typical days in the past.

In fact, some surrounding roadways look less congested — the result, says Cortright, of drivers changing or postponing trips as they adjust to the new street network:

It’s a corollary of induced demand: when we build new capacity in urban roadways, traffic grows quickly to fill it, resulting in more travel and continuing traffic jams. What we have here is “reduced demand” – when we cut the supply of urban road space, traffic volumes fall.

Cortright notes that the phenomenon of Carmageddon predictions that never play out has happened before in other cities with major road projects, including Los Angeles, Atlanta, and Minneapolis. 

He adds that what happens in Seattle in coming weeks could provide important lessons about the perceived impacts of adjusting roadway capacity. "For example, this ought to be a signal that road diets, which have been shown to greatly improve safety and encourage walking and cycling, don’t have anything approaching the kinds of adverse effects on travel that highway engineers usually predict."