On a narrow, two-lane stretch of Georgia Avenue emergency flares pierce the darkness, marking a police checkpoint where uniformed Montgomery County officers with flashlights stop traffic and study every driver for signs of drunkeness.

A likely suspect is spotted. "Good evening, sir. This is a routine sobriety test," says Sgt. Mike Brownell. "Pull over to the side of the road and place your car in park, please."

If the driver had appeared sober, he would have been allowed to drive away. Those who show signs of having had too much to drink must perform a series of sobriety tests that may lead to their arrest for driving while impaired, driving under the influence or the more serious charge of driving while intoxicated.

The checkpoints represent the latest tactic in Montgomery County's campaign against the nation's leading cause of highway deaths -- drunk driving. Under a program begun 2 1/2 weeks ago, county police have been stopping traffic on roads known for high accident rates on weekend nights and occasionally during the week. The roadblocks so far have resulted in 56 arrests and area police consider them an important deterrent to the potentially lethal combination of car and alochol.

"This is an effort to make people aware that we are serious about stopping the drunk driver," said Montgomery police Capt. John Baker. In this country, Baker said, "We have more people killed by drunk drivers than from any other source. This is a matter of trying to save innocent lives."

Brownell said that the last 20 fatal accidents he has investigated involved drunk drivers.

Not everyone is so enthusiastic. Art Spitzer, legal director for the Washington chapter of the American Civil Liberties Union, says the program may violate the Fourth Amendment. "It certainly seems offensive and unconstitutional," he said.

While most police departments in the Washington area have increased their emphasis on stopping drunk driving, the others do not make indiscriminate traffic stops. They prefer instead to stop a driver only when there is probable cause to believe he or she is inebriated.

Montgomery police officials base their claim of legality of the "checkpoints" on a Supreme Court case, State of Delaware v. Prouse.

In that decision, the court ruled against random stops by Delaware police to check vehicle registrations. "But the court went on to say in their opinion that the same situation would not arise from a stop of all oncoming traffic for a limited purpose like this," said Assistant County Attorney Bruce Sherman, who works closely with the police department.

Spitzer disagrees that the case can be interpreted as sanctioning such activity. "Basically, you don't lose your Fourth Amendment protection by going out and driving your car," he said, adding that part of the opinion said that police cannot stop drivers unless there is reasonable suspicion that a crime has been committed.

County police say they have received nothing but support for the program since it was begun Oct. 30. Although more than 2,800 people have been stopped on the seven nights the checkpoints were in operation, no one has complained to them.

Besides the ACLU, the main critics have been the managers of some bars in the county who claim that the program is hurting their business.

"They set up less than a quarter of a mile from our front door the other night," said Tommy Tyler, general manager of the Silo Inn on Georgia Avenue. "On a normal night, we lost 25 percent of our normal activity. We're not sure it's fair and we're certainly against it."

Since January 1980, 110 persons have been killed in traffic accidents in the county and "a large portion, more than half, involved drunk drivers," according to Capt. John W. Baker of the Wheaton-Glenmont district.

Twenty-four of those deaths have been in the Wheaton District, and it was that statistic that prompted Baker to begin the checkpoints. So far, his district is the only one in the county to try them, but police officials say they may be used in other areas if the tactic proves effective.

Baker's officers have set up checkpoints on three roads with high accident rates -- Route 108, Georgia Avenue and Muncaster Mill Road.

Last Friday night, police staked out a two-lane stretch of Georgia Avenue, several miles north of the Capital Beltway. Emergency road flares were used to slow traffic in both directions, while as many as six officers flagged down everyone, including Greyhound and Metro bus drivers.

The first things the officers looked for were overt signs of drunkenness: slurred speech, an evident odor of liquor, bottles or cans in the car, awkward movements. If one or more of these were spotted, then the driver was asked to get out and perform sobriety tests.

"We make them recite the alphabet," said Baker. "One fellow only got seven letters. They have to stand off balance, close their eyes and touch their nose. One woman hit her cheek four times. Then they have to walk a straight line, toe-to-heel. For some, just getting their wallet out is an effort."

In several instances, drivers have been known to drive back to bars and nightclubs to warn drinkers that they'll be stopped if they take a certain route home.

One driver drove straight through the checkpoint.

"When we caught up to him," said Baker, "and asked him why he didn't stop when we asked him to, he said 'What's that about a party back there?' "