As commuters clogged the eastbound lanes of Arlington Boulevard (Rte. 50) between 8:30 and 9:30 yesterday morning, the traffic light at N. Irving Street clicked from green to yellow to red about 25 times.

Sgt. Thomas Hoffmann of the Arlington police didn't take his eyes off it.

Hoffmann and Officer David Herbstreit were watching for red-light violators as part of a new police crackdown in Arlington aimed at reducing the accident rate at some of the county's most troublesome intersections.

Using computerized data to pinpoint the trouble spots and peak times for accidents, Arlington police joined other area police forces this month in cracking down on certain types of traffic violations.

In Arlington, they are staking out the six intersections with the highest accident rates, as well as several others that residents have complained about.

Shortly after 8:30 a.m., Hoffmann and Herbstreit rushed into the street on foot and waved a red Honda Prelude to a side road. The driver, 50-year-old Francisca Alonso, said it was a good idea to crack down on red-light violators. But she insisted the light she had driven through was yellow.

"When you have so much traffic and you are tense . . . you follow the car that is in front of you. It was yellow; I'm sure it was yellow," she said.

Herbstreit wrote her a $35 ticket -- his second of the day -- and Alonso drove off into a thinning parade of eastbound automobiles.

Motorists who run red lights have been an object of concern recently throughout the area. Last week, District of Columbia Police Chief Maurice T. Turner announced that his department will beef up ticketing of speeders, jaywalkers and red-light violators beginning next month.

And the Metropolitan Council of Governments recently called for stiffer penalties for motorists who run red lights twice or more.

Arlington County Board Member Albert C. Eisenberg, a member of the COG public safety committee, said red-light violators have become a greater problem as traffic has increased.

"We deal with a stressful environment," he said. "People just don't want to hang around waiting for lights. They'd rather save that minute or two."

Arlington police are not tallying the number of motorists they stop for running red lights, but several officers who have been watching the intersections said they were writing fewer tickets yesterday than they had on previous mornings.

When the stepped-up enforcement campaign started, according to Deputy Chief David Reiten, as many as seven motorists an hour might be stopped for running red lights. "It seems like it's taking a lot longer before we find someone" running a red light, Herbstreit said.

Although the officers were able to flag down early violators, when rush hour wound down the speeds started picking up.

Hoffmann checked his watch: 8:45. "We're going to have to go to chase mode," he announced. The two straddled gleaming Harley-Davidsons, revved the engines and watched the light.

Moments later a silver Pontiac Fiero whizzed through the intersection, and Hoffmann rolled after it, his motorcycle light spinning and siren wailing. He was back in five minutes.

"Another denial: 'But the light was yellow,' " he said, and shook his head.