I hang out in city-planning circles and this is something we talk a lot about. Of course, shorter commutes have been predicted since I was in college and gas was predicted to someday top $1 per gallon.
I think the only way polycentric development lowers vehicle use is in a 19th century paradigm: everyone is so poor and unskilled that they live in a rent house and work at their local mill.
Once you have employers wanting to hire the brightest workers from the entire labor market, and once people have the technological means to live in the best school district and work wherever they make the best salary, a polycentric metropolis increases traffic geometrically. The trend can be abated somewhat so long as it’s possible to concentrate a huge number of jobs in a compact area served by radial transit—the Chicago and Boston model.
There’s a sort of naïve appeal to the idea that, in a polycentric metropolis, commutes would be reduced because people could work close to where they live. But this assumes a single breadwinner who will stay forever at a single job close to home or move when he takes another job. In fact, what happens is that people choose home locations based on things like school districts or big yards and then two workers drive long distances in opposite directions to work. There have actually been a couple of serious studies of this question, both of which disprove the theory:
“All else being equal, commute distances and times for auto drivers are longer in most polycentric regions than in monocentric urban areas.”
Schwanen, T., Dieleman, F.M. & Dijst, M. (2004). The Impact of Metropolitan Structure on Commute Behavior in the Netherlands: A Multilevel Approach, Growth and Change, 35 (3) 333. DOI: 10.1111/j.1468-2257.2004.00251.x
“Our analysis clearly indicates that, once travel mode and key social factors are controlled for, the shift from a monocentric to a dispersed city form is responsible, in the Québec metropolitan area, for increasing commuting time.” Vandersmissen, M.H., Villeneuve, P. & Thériault, M. (2003). Analyzing Changes in Urban Form and Commuting Time, The Professional Geographer, 55 (4) 463. DOI: 10.1111/0033-0124.5504004
And that’s just looking at work trips. The tide of rising expectations means that people today want the region’s best restaurant, best musical performance, best library, best food store, etc.—NOT the one closest to them.