Incentives for drivers who avoid the busiest streets

PALO ALTO, California - London, Singapore, Stockholm and a few other cities battle heavy traffic with a "congestion charge," a high fee for driving in crowded areas at peak hours. But drivers generally hate the idea, and efforts to impose it in the United States have failed.

Balaji Prabhakar, a professor of computer science at Stanford University in California, thinks he has a better way.

A few years ago, trapped in a traffic jam in Bangalore, India, he reflected that there was more than one way to get drivers to change their behavior. Congestion charges are sticks; why not try a carrot?

So this spring, with a $3 million research grant from the United States Department of Transportation, Stanford deployed a system designed by Dr. Prabhakar's group that allows people driving to the notoriously traffic-clogged campus to enter a daily lottery, with a chance to win up to $50, just by shifting their commute to off-peak times.

The program is popular and will expand next fall to encourage people to park farther from the busiest parking structures.

Amaya Odiaga, the director of business operations for Stanford's physical education department, now drives to campus a few minutes earlier and says she has won $15. Her commute now takes as little as 7 minutes, down from 25 minutes.

Dr. Prabhakar has conducted a variety of experiments using incentives to get people to change their behavior in driving, taking public transit, parking and even adopting a more active lifestyle. Unlike congestion pricing, which is mandatory for everyone and usually requires legislation, "incentives can be started incrementally and are voluntary," he said.

Singapore is considering a system he and his students designed that offers lottery participation or a fare discount to public transit riders who travel at off-peak times. A trial run begun in January lowered rush-hour ridership by more than 10 percent. (Given a choice between discounts and lottery, riders overwhelmingly chose the lottery.)

Still, Charles Komanoff, a transportation expert who has designed a computer model of New York traffic, said he had reservations about incentive systems as an alternative to congestion pricing.

"The incentives will be far too small," he wrote in an e-mail, adding: "You really do need big disincentives (big sticks). Little carrots won't do the job of changing drivers' decisions" in New York or in San Francisco.

Dr. Prabhakar said congestion pricing and his incentive system need not be mutually exclusive, and he noted that even a small reduction in vehicles at a given time can have a big effect on traffic flow. And conversely, added Frank Kelly, a mathematician at the University of Cambridge in England who specializes in traffic networks, "when the system is close to critical levels, very small increases in traffic can create time delays for everyone."

Prabhakar's Stanford experiment adds a social network component to the lottery, making it a game where friends can see one another's "good" behavior, which tends to reinforce commitment. Dr. Prabhakar's first experiment was in Bangalore in 2008, when he created a system to encourage employees at the software company Infosys to choose different travel times to its suburban campus. The system significantly lowered congestion. Dr. Prabhakar said the power of his method was that only a small change could have a drastic effect.

"This is one of the nicer problems," he said. "You don't have to change everyone's behavior; in fact, it's better if you don't."

The New York Times