Traffic congestion datapoints of the day

July 10, 2012
congestion indices today, comparing 31 cities in Europe and 26 cities in the US and Canada.

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TomTom has released its first congestion indices today, comparing 31 cities in Europe and 26 cities in the US and Canada. (They call that North America, which is a bit disappointing, because I’d dearly love to see how Mexico City compares to other North American cities, and it’s not on the list.) The rankings are interesting, but even more interesting, to me, are the way that the rankings have changed over the past year.

Consider Edmonton, for instance: a town in the midst of a massive oil boom, where road construction can’t even begin to keep up with population growth. That was obvious back in September 2009, in the city’s transportation master plan:

As Edmonton evolves from a mid-size prairie city to a large metropolitan area, it is inevitable that congestion levels will increase, particularly during peak periods. Physical, financial and community constraints in many areas make it unfeasible or even undesirable to build or expand roads to alleviate congestion.

TomTom doesn’t give data as far back as 2009, but at least we can see what direction the city is moving in. Last year, Edmonton had a congestion index of 24%, which means that on average, travel times were 24% longer than they would take if traffic were flowing freely. That meant Edmonton was the 8th most congested city on TomTom’s list. This year, the Edmonton congestion index has plunged to just 13%, placing Edmonton 23rd out of the 26 cities, with an enormous decrease particularly during the evening rush hour:


I have no idea why traffic in Edmonton has improved so much over the past year; certainly I wouldn’t have been at all surprised if it had gotten worse rather than better. But the point here is that there’s an important stochastic element to congestion. Consider New York: in 2008, Mike Bloomberg proposed a congestion charge, which passed muster with city legislators but which was ultimately killed in Albany. Again, we don’t have data for what congestion was like in 2008. But between 2011 and 2012, congestion rates in New York overall fell from 23% to just 17%: a very impressive improvement. And today, New York is only the 15th most congested city on the list — behind metropolitan areas like Tampa, Ottawa, and San Diego.

What’s happened in New York to cause the drop in congestion? You can’t say higher gas prices, since those are a nationwide phenomenon, and don’t explain the drop in relative congestion. Plus, congestion in North America overall has stayed stable at 20% even as gas prices have risen. So if it’s not gas prices, what is it? Could it be all those bike lanes? Could it be that John Cassidy needs to eat some crow, and admit that bike lanes reduce congestion, rather than increasing it?

Perhaps: the jury’s still out. And maybe what we’re seeing here is more a function of random variation, and less a function of anything under the control of New York’s Department of Transportation.

What this report does tell me is that it’s going to be very difficult indeed to judge how effective any congestion-charging system is, just by looking at what happens to congestion after such a charge is introduced. I’m sure that if Edmonton had introduced a congestion charge at the beginning of 2011, the city would have claimed a huge amount of credit for the drop in congestion that resulted. But in fact, as we’ve seen, that drop in congestion would have happened anyway.

I’m planning to talk to the people at TomTom next week, and I’ll ask them whether they have any bright ideas when it comes to separating out causative factors for changes in congestion. In the meantime, we now at least have reasonably reliable league tables for the least pleasant cities to drive in. In North America, you want to avoid Los Angeles and Vancouver; in Europe, you want to avoid pretty much every major city. (Stockholm and London, with congestion charges, both have 27% congestion rates, putting them on a par with the very worst US cities.) But especially avoid driving in Warsaw, Rome, and Brussels. They’re even worse than LA.

Update: JCortright, in the comments, makes the excellent point that these numbers are much better at showing congestion changes within a city than they are at comparing congestion between cities. If you have a 45-minute commute in Atlanta, for instance, as measured on a congestion-free basis, and you’re stuck in traffic for an extra half an hour, then that’s 67% congestion. Whereas if you’re stuck in traffic for 15 minutes on a drive that would take you 15 minutes without traffic, that’s 100% congestion. So this methodology makes denser, smaller cities (like Europe’s) look worse.


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