I’ve had Minneapolis on my mind lately, not just because I’ve just arrived to the city, but because I’ve been reading a fascinating new paper (available here) on the large-scale traffic effects of the tragic collapse of the I-35W bridge last year.

The paper, “The Traffic and Behavioral Effects of the I-35 W Mississippi River Bridge Collapse,” is by David Levinson, Henry Liu, Shanjiang Zhu, and Kathleen Harder (Levinson, whose “Transportationist” blog is a daily must-read of mine, also figures in a number of ways in Traffic).

This paper tackles one of those fascinating traffic issues that have come up now and again in different ways: What happens when, for some reason or another, whether on purpose or not, sections of road are removed? What effects do these “network disruptions” have on the entire system? In this particular case, the paper notes, I-35W carried some 140,000 people across the bridge every day. Neighboring bridges, in the wake of the new patterns that seem to have asserted themselves a year later, are carrying only 90,000 more cars. What happened to the others? Judging by the survey results collected here, the majority that were affected did one of two things: They changed their route, or they changed the time they left home. And the rest, it would appear, simply did not make trips.

This raises interesting questions about the traffic stream during peak hours. Do all those people need to be there, or is their desire to be there equal (e.g., would they pay a premium to make that trip during that time)? It also shows the amazing flexibility and cleverness people have in staking out new routes and strategies during these disruptions. When New York City suddenly had a “plus three” car occupancy requirement in the wake of the transit strike a few years back, we were all suddenly instant car-poolers. The new routing that Minneapolis drivers quickly took on invokes a classic principal from the world of transportation: Wardrop’s equilibrium. This states (and I’m simplifying here) that a single driver cannot, by his or her own action, find a better route than the one they are on. As a New Yorker, I sort of view this in a Yogi Berra kind of way: If there was a faster way to go, everyone else would be slowing it down already.

The missing capacity in the system, the authors note, is not without costs — to the economy, to people’s individual desire to travel, to individual’s commute times. But, they add, “travelers exhibited great flexibility in dealing with the changed traffic pattern.” Some of this was aided through novel infrastructure tweaks (like adding new lanes on other bridge crossings by simply making the existing lanes more narrow), but much of it just came down to individual decisions, and people’s willingness to change. The two words in the title say it all: Traffic is behavior. In any case, the work is ongoing and promises to yield more insights.

This entry was posted on Monday, August 4th, 2008 at 8:21 pm and is filed under Congestion, Drivers, Traffic Reports. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.