Smart cruise control eliminates traffic jams

We've all been there, wilted behind the steering wheel while our car inches and jerks forward like an ice chunk through a straw: The traffic jam, the bane of commuters in major cities around the country.

But such infuriating backups could be avoided—at least on single lanes that don't require merging—if just 20 percent of cars were equipped with adaptive cruise control (ACC), says adjunct physics Professor L.C. Davis. Doing computer simulations on his laptop from programs he wrote, Davis studies the origins of traffic jams and the impact that vehicles equipped with ACC have on traffic flow.

ACC detects the presence of a preceding vehicle and measures the distance as well as relative speed using a forward-looking sensor, and it automatically adjusts the vehicle speed to keep a proper range. ACC is an option on some high-end vehicles.

In a recent paper published in The American Physical Society's Physical Review, Davis demonstrated a "spectacular" reduction in single-lane jams if just 20 percent of the vehicles in a 600-car platoon were equipped with ACC.

Davis's paper also revealed other interesting traffic jam behavior. For instance, with all other factors being equal, faster traffic is more "sensitive" and will jam more quickly than slower vehicles. At 30 meters per second (about 67 mph), when Davis changed the 188th car in the platoon from ACC to no ACC, a jam formed immediately. But when 10 percent to 33 percent were changed to ACC at 15 meters per second (about 34 mph), traffic slowed more gradually and did not experience an abrupt jam.

"There is different behavior at lower speeds than at higher speeds," Davis says. "At 30 meters per second you get a discontinuous change. You either get a jam or you don't. At lower speeds you get something in between."

The good news is that equipping even a small percentage of vehicles with ACC appears to improve traffic flow greatly. But the news is bad, Davis says, when merging traffic is added to the mix.

Even though traffic jams can be prevented, the introduction of ACC doesn't improve the average speed through an on-ramp area as much as Davis had hoped, he says. There are two kinds of congestion, he notes: jams on single-lane roads, and the slowdowns that occur where two roads merge.

Much research has been done on traffic flow and jamming behavior, with conflicting results. Davis says his modeling shows that even the best of those earlier results are optimistic. For example, a European study of the German Autobahn suggests that if just 10 percent of vehicles had ACC, it would reduce travel time due to jams by 80 percent. But Davis's paper disagreed with those findings, and showed that concentrations of 10 percent or less of ACC vehicles would not improve traffic flow.

Davis is a retired manager with Ford Motor Co. and a seasoned rush-hour commuter. He started modeling traffic behavior in 2001 at Ford and joined the University in 2002.

What does Davis's research suggest commuters can do to reduce traffic jams now?

"I think what we should do is encourage drivers, when they start coming toward a place where one lane ends, to start moving over," Davis says.

He notes that drivers who zoom ahead of the pack to sneak abruptly into the line of merging traffic often cause traffic jams. "If everyone is obeying and getting in early, one person can still ruin the entire thing."

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