It was midnight, and Arlington police officer Keith A. Bryan was beginning the prime hours of his patrol when the car came into view. It was a Datsun 240-Z, and it was headed down Wilson Boulevard the wrong way, with its head lights off.

But it was more than just the glaring traffic violations that made Bryan start off in pursuit of the car. To Bryan, that type of traffic violations pointed to one conclusion: that the man at the wheel of the car probably was drunk.

On weekend nights, drunk drivers are Bryan's chief preoccupation. He is assigned to the newly-formed "cocktail patrol," a four-man squad that drives along Arlington's streets Friday and Saturday nights searching for drunk drivers.

Last month, this patrol helped push the monthly arrest total of drunk drivers in Arlington to 150 - more than twice the average monthly arrest total for this offense in 1977.

As Bryan began to question the Datsun's driver, an army officer in his mid-30s, he could smell alcohol on the officer's breath. Even so, the exhibited few of the normal characteristics of a drunk. He was calm, coherent and friendly, explaining to Bryan that he had missed the one-way sign at the corner and accidentally turned the wrong way.

He also told Bryan that he lived less than two blocks away - but did not know Wilson Boulevard at that point is one way.

When questioned about his drinking that evening, the officer said he just came from Georgetown. But he admitted to having only the "two beers," the standard story that Bryan and his fellow "cocktail patrol" officers hear from the drivers they arrest.

Although the army officer's eyes were bloodshot, he was steady on his feet and his words came out crisply and clearly. But when he blew up the test balloon Bryan gave him, the guage attached to the balloon turned bright green. (The chemical in the indicator guage normally is pale yellow, but the alcohol on the breath of someone who has been drinking turns it green.) The color on the guage legally, gave Bryan probable cause to arrest the officer for drunk driving, which he did.

Bryan said later that his initial suspicions about the driver's condition were intensified after he heard the time-honord story about the "two beers" and the man's claim that he lived only two block away. "(The account) was crookeder than a dog's hind leg," Bryan said later.

He got out his handcuffs and made the arrest.

For the Army officer, it was humiliating experience. He was handcuffed on the street, frisked, and taken to the police station in a police van - all standard procedures with such an arrest. His car was impounded.

Once in the station he waited 20 minutes before taking the Breathalyzer test - enough time for any excess alcohol in his month to evaporate. That test, the only official test given by the police, showed that the Army officer had a blood-alcohol concentration of .09 percent.

Under Virginia law, a driver is not automatically considered to be drunk unless the BAC is .10, so the decision of whether to charge the officer was a judgement call for Bryan. He went ahead.

Later, Bryan expressed some reservations about the arrest, especially since the man was helpful and seemed upset about what was happening. But, he said, when he was contemplating the decision he remembered an accident the week before when a drunk driver ran into a parked car and seriously injured a man changing a flat tire, while the man's wife and young son watched.

"That's what this program is directed at, thses borderline people . . . " Bryan said. "He may be able to make it home fine if he's the only car on the road, but if a little kid runs in front of him or a car stops he may not have control."

Peter J. Larkin, director of the Arlington Alcohol Safety Action Program (ASAP), said the state-funded "cocktail patrol" was formed in June because the number of traffic deaths had risen to eight so far this year. Four of these involved drinking drivers. The county had only 10 fatal accidents, in all of 1977, he added.

Arlington also has the highest traffic density in the state, Larkin said. Since 900,000 to 1 million cars travel through the county daily, national statistics indicate that approximately 18,000 drinking drivers are using the county's roads every day. Many of these are just passing through, as they come from bars in the District of Columbia on their way home.

Unless an arrested driver wants to challenge the drunk driving charge in court - where, if convicted, he faces a mandatory revocation of his license and a minimum $500 fine if he loses - he will be referred to ASAP. If a driver has no history of problem drinking, he probably will be ordered to join the ASAP education program to learn about the effects of alcohol. The course lasts about four weeks, and costs the driver $200, Larkin said.

If a driver successfully completes the assigned program and is not picked up for drunk driving again, his drunk driving charge is reduced and shows on his record as a lesser traffic violation, equivalent to reckless driving.

After he was booked, the army officer arrested by Bryan, admitted that he had more than two drinks, and was really in no condition to drive. He just did not realize it at the time, he added.

"It's a learning experience," he said. "It's very beneficial to me . . . The point of just having those handcuffs on is enough . . . The point is, it's personally degrading."

The army officer was released without bond after the booking, and Bryan took the man and a companion of his to the companion's car so that they could go hoon of his to the companion's car so that they could go home . Then Bryan went back out on patrol.