How Not To Measure Traffic Congestion, Redux Again

Yesterday, TomTom released its annual rankings of the levels of congestion in world and U.S. cities. Predictably, they generated the horrified, self-pitying headlines about how awful congestion is in the top-ranked cities. Cue the telephoto lens shots of bumper-to-bumper traffic, and tales of gridlock.

As we've long pointed out, there are big problems with the travel time index TomTom and others use to compare congestion levels between cities. Most importantly, some cities have much shorter commute distances than others—meaning that even if traffic moves slower at the peak hour, people spend less time commuting. For example, Houston has an average commute distance of 12.2 miles, while Portland has an average commute distance of 7.1 miles, according to the Brookings Institution. So even if Portland's "congestion index number" is slightly higher (26 percent) than Houston's (25 percent)—at least according to TomTom—average commute times are much shorter in Portland because of its more compact land use patterns. In effect, the travel time index, expressed as a percentage of total commute times, discounts the pain of traffic congestion in sprawling, car-dependent cities. That's why its a lousy guide for talking about how well transportation systems work. The same problems plague the rankings released by Inrix two weeks earlier.

Credit: Nick Douglas, Flickr

Plus, as Felix Salmon pointed out a couple of years ago, the TomTom data has a special bias: it’s chiefly gathered from people who've bought the devices, who almost by definition are not typical commuters. It's highly likely that they represent those who drive the most, and who drive most in peak traffic (hence the value of Tom-Tom’s services); but data gathered from these devices is not necessarily typical of the experience of the average commuter.

TomTom, as you know, is in the business of selling real-time traffic data and navigation assistance to motorists. So in many respects, its rankings may be less a serious and balanced effort to assess congestion than they are to drum up demand for its product. The company’s top three suggestions for coping with traffic congestion are use a real time navigation system (like a TomTom device), “dare” to try alternate routes suggested by...your Tom Tom device, and "check traffic before you leave"—you get the picture.

Refreshingly, TomTom admits in its press release that building more highway capacity will do nothing to alleviate the congestion it identifies—we doubt that this is the part of the study that highway advocates will share with a wider audience. Nick Cohn, the company's "traffic expert," tells us:

Traffic congestion is a fact of life for every driver. And as we reveal the latest Traffic Index results this year, we can see that the problem is not going away.

We should not expect our transport authorities to simply ‘build away’ congestion. Studies have shown over the years that building new motorways or freeways does not eliminate congestion.

Even though Nick Cohn apparently knows better—and really just wants us to buy his company's product—what TomTom and its peers are doing is feeding a profoundly distorted view of traffic congestion problems. Those in the highway lobby routinely use this kind of data to try and scare us into spending billions for new highway construction projects that are often un-needed, or which do nothing to reduce congestion.