The problem with TTI's analysis is that it focuses narrowly on just one variable---how fast traffic moves on highways---while ignoring things like commute distance and land use. The result is that cities where people are able to live in dense neighborhoods with relatively slow traffic speeds rank worse than sprawling areas with vast highway systems. It may take me 15 minutes to travel five miles to work in the morning, but because I'm driving on 30-mph arterials, by TTI's reasoning, I'm worse off than someone who commutes 45 miles from a sprawling suburb but does so at 60 mph. As Streetsblog puts it, "It assumes, for example, that everyone should be able to speed as rapidly down the highway during rush hour as they could in the middle of the night."
For a more concrete example, according to the blog CEOS For Cities,
consider Nashville and Portland. According to the UMR, Portland has a worse traffic problem than Nashville, with a Travel Time Index of 1.23. and 36 hours of delay per year per traveler, compared to Nashville, which has a Travel Time Index of 1.15 and 35 hours of delay. But these data also mean that the average peak traveler in Nashville has to spend a total of 268 hours per year commuting compared to the commuter in Portland who travels only 193 hours per year. So the commuter in Portland travels 75 fewer hours annually because of shorter travel distance, due in large part to less sprawling development patterns. Consistent with conclusions presented in Driven Apart, the UMR completely misses the importance of land use planning as a key to reducing the burden of peak period travel.
Just last year, CEOs for Cities did its own traffic study, taking sprawl and travel times into account, and found that Portland actually ranks seventh on the list of cities with the shortest commutes. The worst cities, in terms of how much time an average commuter spends on the road each year, were Detroit, Indianapolis, Louisville, Raleigh, and St. Louis.
Why does the TTI study matter? Because planners and policymakers routinely use TTI's findings as a justification for investing in road expansion, rather than transit, sidewalks, or bike lanes---road expansion which, ironically, leads to longer commute times. (Locally, see this pro-car, anti-rail editorial in the Seattle Times, or this PI story from 2007, when "roads and transit" initiative backers used the institute's grim findings as justification for a massive proposed highway package. Bad information can be the basis for bad policy---especially when it's reported uncritically by media across the country year after year.