Go Slow To Go Fast

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

Go Slow To Go Fast
Lower highway speeds may get you where you’re going faster, as counterintuitive as it sounds, thanks to speed harmonization.

Illustration by Rob Donnelly.

There is no more common lament voiced by the American driver than of the one about the “idiot” in the “fast lane” who’s slowing down traffic. If everyone could just drive faster, the thinking goes—if we could only cull the weak gazelles in our furiously charging migration—we could stamp out congestion.

We equate speed in traffic with efficiency. In the U.K., the Tory government is currently advocating raising the speed limit on certain motorway sections to 80 MPH, anticipating a massive windfall in economic productivity and time saved. It’s speed as the health of the state! (Though not all projections are so rosy.)

But one thing that tends to be lost on the individual driver, who through the proscenium of his windshield commands what he believes to be an empirically incontrovertible perspective on the ground truth of traffic, is that sometimes you have to go slower to go faster.

This is the thinking behind some recent trials on Colorado’s vital, increasingly congested I-70 mountain corridor. Once the number of cars on the road reached a certain level (initially, 1,100 vehicles per hour per direction), highway patrol vehicles, riding in tandem with lights ablaze, set an artificially low travel rate of 55 mph—on a highway where cars and trucks might travel 70 and 30 mph, respectively—“pacing” a series of vehicle platoons on a segment of the highway. Gone was the furious weaving, the sudden squalls of brake lights—this was a NASCAR pace lap.

Welcome to “rolling speed harmonization.” As one report describes it, speed harmonization “holds that by encouraging speed compliance and reducing speed differential between vehicles, volume throughput can be maximized without a physical increase in roadway dimensions.” 

Colorado’s rolling speed harmonization, where a lead highway patrol vehicle deliberately slows traffic to a more efficient pace

Courtesy Colorado Dept. of Transportation.

The concept plays, in part, on one of traffic engineering’s core truths: Big speed differentials are dangerous. This is laid out in the “Green Book,” the bible of the American Association of Surface Highway Transportation Officials. “Crashes are not related as much to speed as to the range in speeds from the highest to lowest,” the book states. “Studies show that, regardless of the average speed on the highway, the more a vehicle deviates from the average speed, the greater its chances of becoming involved in a crash.”

The common retort to this statement by the “average driver” is precisely as described in the opening paragraph: See, just get everyone going the same fast speed and it will be safer! There are a number of problems with this interpretation, but let’s take the one most relevant to I-70: When there is a long queue of suddenly stopped vehicles (whose speed is zero), the higher the speed of following traffic is, the greater the differential, and thus the greater the crash risk.

The I-70 mountain corridor is a rather unusual piece of highway. As Ken Wissel, a transportation engineer with the Denver firm Stantec, who oversaw the project, describes it, I-70 has two of the highest peaks in the entire Interstate Highway system within 25 miles of each other. There are four major ascents, a two-mile-long tunnel that dips under the Continental Divide, a terrifying descent that features one of the country’s most-used emergency truck ramps, and a number of merge zones where traffic must jockey as the highway goes from three to two lanes before entering the tunnels. To complicate matters there’s snow, a lot of snow (“We had 600 inches last year,” Wissel says); and traffic, a lot of traffic.  “We end up with some real long queues,” Wissel says. Backups as long as 30 miles have been reported.

These conditions are exacerbated on weekends during ski season. “If you go past Denver International Airport, and take a look at the rental car lots, about the only thing for rent is an SUV or four-wheel-drive cross-over,” Wissel says. “There’s a lot of inexperienced drivers who come here for a vacation.” It’s the “curse of the SUV,” he continues: “They have enhanced traction going uphill, to the point where it takes away some of the sense of how dangerous that road is. We have a lot of spinoffs, drivers hitting the median.”

Enter Operation “Icy Falcon” and Operation “Snow Tortoise.” That’s Colorado Department of Transportation slang for a program, rolled out several years ago, that used highway patrol cars to pace civilian cars in inclement weather, both to avoid chain-reaction crashes once a crash occurs and to prevent crashes from happening the first place. (The Colorado State Patrol claimed it was able to cut crashes in half.) The program’s primary impetus was safety, but Operation Snow Tortoise also mitigated congestion: no crashes, no congestion.

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.