On a Sunday in late September, at about a quarter past 11 in the morning, a police car flicked on its emergency lights and raced ahead of traffic heading eastbound toward Denver on Interstate 70. No doubt one or two drivers near the front of the herd thought they were getting pulled over. But the officer driving didn't have speeding tickets in mind. Instead, the cruiser settled in front of the pack and immediately began to travel at exactly 55 mph.
The goal was to act as a pace car for the 2,000 or so drivers making the 27-mile trek from Silverthorne to Empire Junction, toward Colorado's big city. The officer's primary job was to make sure no eager lead foot burst out ahead of the others — in short, to keep everyone in the pack near 55 mph. The event was repeated every ten minutes or so until a quarter past 3 that afternoon. It was the second in a series of trial runs for a traffic management program called "rolling speed harmonization."
Congestion on I-70 is a regular problem. It's not unusual for the average speed on this stretch to drift between 10 and 30 mph. On big traffic days — say, after a heavy snow or when peak ski season brings tens of thousands of drivers toward Denver — things can get even worse, often backing up for miles west of the city. But on that Sunday in late September, despite packs of traffic that reached nearly 2,300 cars, highway travelers averaged a smooth 55 mph from Silverthorne to the Eisenhower-Johnson Memorial Tunnel, and a smoother 60 mph from the tunnel to Empire Junction.
Of course drivers complained about being slowed down. Still, the trick clearly worked. In fact, when the officers stopped getting out in front of the packs, travel speeds dropped from 60 down to 30 mph, according to the Colorado Department of Transportation. These results, combined with those of a previous trial in August, convinced officials to adopt the strategy on Sundays in early 2012, during the upcoming ski season.
Rolling speed harmonization is a congestion-fighting strategy that eases flow by reducing the variation of vehicle speeds on a road. The basic idea is that holding cars to similar speeds maximizes the capacity of a given roadway and reduces the likelihood of a collision (which, of course, creates further congestion). While a few cars go slower than they would have otherwise, cars as a whole reach their destination much faster. In the parlance of ski country, it asks everyone, regardless of preference for black diamond or bunny hill, to keep the moderate pace of a blue square.
Take, for instance, what happened at the Eisenhower tunnel. Typically, backups at the tunnel stretch back some 4,000 feet. That problem worsens as the line lengthens, because creating a tunnel clog is easier than loosening one: officials find that every minute the tunnel is jammed requires six to eight minutes of recovery time. During the September trial run, however, the tunnel queue extended back only about 1,000 feet, reports the Denver Post. The speed pacing prevented officials from having to "meter" the tunnel — in other words, to halt the entrance of cars entirely until the traffic knot unwinds.
Writing about the technique over at Slate, Tom Vanderbilt likens the situation to pouring rice slowly through a funnel:
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.
While rolling speed harmonization has received limited attention in the United States, it's been used in European countries for decades, often during road construction or in bad weather conditions. According to a report on these efforts [PDF], prepared last year for the Federal Highway Administration, the results have been quite positive. In Germany, harmonization has resulted in both lower accident rates and a 5 to 10 percent increase in road capacity. Denmark and the Netherlands have had similar success. In England, the practice even reduced vehicle emissions between 4 and 10 percent.
The first step to good road harmony is alerting drivers of the situation. In Europe, signs warning of an impending "queue" are given prominent display. That reduces the likelihood of accidents caused by the initial decrease in speed. From there, according to the highway administration report, the key is getting drivers to buy into the system — "building users' experience and trust in the general traffic management concept" — or, short of that, getting police to strongly enforce it. Sustaining compliance was a problem with a recent harmonization trial on the Wilson Bridge in Virginia, according to Vanderbilt. Drivers didn't change their speeds, and police didn't make them. The result was a much longer run to the proverbial ski lodge.
Image courtesy Flickr user Kristin Brenemen