Highways Work Best When You Force Cars To Crawl Along at 55 MPH

How we get from here to there.
Oct. 12 2011 3:19 PM

Go Slow To Go Fast

Why highways move more swiftly when you force cars to crawl along at 55 mph.

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The program “showed such good results,” says Wissel, “we said, ‘Why don’t we do this more often?’ ” And so the program has been tested twice on a clear, dry stretch of I-70. There were no crashes, and CDOT is hinting (though numbers are still being crunched) that throughput—how many vehicles are moving through any one section of highway at a given time—improved during the experiment.

One to key to explaining why is the merge zones. At the entrance to the Eisenhower Tunnel, CDOT notes, every minute of backup translates to eight minutes of recovery time. This is another law of traffic: It takes exponentially longer to get out of a traffic jam than to get into one. Rather than having drivers go full-tilt into a jam at the tunnel entrance, drivers approach more slowly; even though their speed may be temporarily reduced, the system is now processing vehicles faster. It’s the famous rice-and-funnels effect popularized by former Washington transportation commissioner Doug MacDonald: The slower your pour the rice, the faster it gets through the bottleneck.


Colorado’s program is an exercise in what’s known as “active traffic management.” Rather than just posting static speeds and fixed infrastructure, and letting drivers work things out for themselves (a more passive approach), the idea is to shape traffic algorithmically based on changing conditions—automatically slowing drivers ahead of a construction work zone, opening up a shoulder when peak congestion levels are hit. There are a bounty of studies from Europe showing that technologies like “variable speed limits,” which generate specific speeds depending on traffic conditions, can, under the right conditions, help reduce crashes and even improve highway throughput (even as mean speeds are lowered).

But it can be hard for the individual driver to appreciate the subtle beauty of system optimality. “I actually got caught up in it Sunday,” said one online commenter at the Denver Post’s website. “I'd have to say it did nothing except congest/pack the drivers together.” This, of course, is part of the point: to reduce “headways.” Traffic engineers know, for example, that a highway can move more vehicles per hour at 55 mph than 85 mph. Another commenter sounded a typical refrain: “Get rid of the idiots who drive too slow and you can get traffic through the tunnel faster.” This would presumably involve removing truck traffic, which struggles mightily with the upgrade, from the highway. Which is fine, unless you possess a crazy desire for, say, a functioning civilization.

And the less a driver appreciates these larger dynamics, the less he’s likely to heed a new suggested speed without a pace car enforcing the limit. In the blunt lament of one report, “altering driver behavior is recognized as the greatest challenge with a VSL [variable speed limit] system.” In a much-touted program on the Capital Beltway, the Virginia Department of Transportation unveiled a VSL program ahead of a series of quasi-permanent work-zone merges for construction on the Woodrow Wilson Bridge. But as Michael Fontaine, a senior research scientist for the Virginia Center for Transportation Innovation, told me, “the results generally did not show significant improvements in flow.” One reason: Drivers rarely changed their speed. Fontaine notes several problems: Enforcement was only periodic; Jersey barriers alongside the road left few places for highway patrol to park; portable messaging signs were often hard to see. As a VDOT report concluded, “education of the VSL system should be supplemented with robust enforcement to maximize speed compliance. Without proper enforcement and police involvement, motorists will likely not comply with lower posted speed limits approaching the work zone, inhibiting speed harmonization.”

In Europe, enforcement is typically done by automated speed cameras and similar technologies. As Wissel notes, “that’s not going to sell very readily over here.” Where speed cameras are rather common, if not exactly loved, in the U.K. and Europe, they have been a tougher sell in the United States, particularly in these regulation-averse times. We Americans relish our freedom—even if it’s the freedom to drive into a traffic jam we ourselves have exacerbated. But with the era of highway building essentially over, and traffic volumes rising, we are likely to see more, not less, active traffic management. As sprawling urban regions are pitched on the knife edge of paralysis, the lesson becomes clear: Traffic is too important to leave to drivers.


Illustration by Rob Donnelly.

Tom Vanderbilt is author of Traffic: Why We Drive the Way We Do, now available in paperback. He is contributing editor to Artforum, Print, and I.D.; contributing writer to Design Observer; and has written for many publications, including Wired, the Wilson Quarterly, the New York Times Magazine, and the London Review of Books. He blogs at howwedrive.com and lives in Brooklyn, N.Y. You can follow him on Twitter at www.twitter.com/tomvanderbilt.