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

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.” 

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.