It's 5:59 p.m. on a normal workday. A dozen cars suddenly pull off to the side of the road and stop. A minute later, the whole group resumes the evening com-mute as if on cue.

Such abrupt stops are a side effect of "road pricing," a program in this Norwegian city in which drivers have to pay to enter the center of town between 6 a.m. and 6 p.m. The experiment is part of a European Union study.Like many cities around the world, Trondheim, founded 1,001 years ago, is struggling to cope with a steady increase in health-damaging smog and noise from traffic jams on streets often more suited to horse-carts than cars.

Even a small change in traffic can have a large impact on travel time and pollution, according to Norwegian studies.

Seeking to ease the traffic choke hold by discouraging driving, this city of 140,000 people installed the world's first fully automatic "toll ring" in 1991. Unmanned electronic toll booths deduct a fee from a windshield-mounted gadget on cars that pass during the business day.

Dozens of similar projects are being tested or opened around the world by cities battling the private automobile, with mixed results.

In Trondheim, cars sometimes screech to a halt on the highway just before rush hour ends because it's free to pass toll stations between 6 p.m. and 6 a.m. as well as on weekends. During the toll hours, it costs up to 12 kroner ($1.60) to drive into the toll zone.

"The aim in `congestion pricing' is to take the peak off traffic during the rush hour," said Terje Tretvik at the SINTEF research institute in Trondheim, about 300 miles north of Oslo.

He said the tolls reflect the environmental, road-building and social expenses of heavy traf-fic "so drivers personally feel the cost."

If sudden stops in the evening and commuters speeding to beat the start of tolls in the morning are the negative effects, "The Trondheim Package" has also shown benefits.

When the system was introduced, peak rush hour traffic immediately dipped 10 percent as people adjusted their schedules to avoid the toll.