HOW EFFECTIVE is it when Washington's police force suddenly moves out en masse, sets up roving roadblocks and arrests more than 400 people in a 7-hour citywide crackdown on drug trafficking? If anyone believes this is enough to drive drug traffic out of the city, just take a look at the streets maybe a week later. Nevertheless--and taking into account serious concerns about the dangers of heavy-handed "sweeps" or "dragnets" that may harass innocent bystanders or infringe upon civil liberties--doesn't a carefully planned operation of this sort accomplish something?

Yes--even if the effects are not forever. In neighborhoods where visible drug traffic and all its attendant burglaries, assaults and threats of violence are not a daily way of life, perhaps it's easier to sit back and conclude that the police are wasting their time and courting constitutional problems with these operations. But last Saturday's Operation "C-Note Sevenfold," conducted in all seven police precincts, appears to have undergone months of planning specifically aimed at protecting individual liberties.

Authorities emphasize that the roadblocks that were set up were not discriminatory nor were they conducted as fishing expeditions to search car trunks and glove compartments. They were intended 1) to make drug arrests after observations of apparent illegal activity and 2) to allow plainclothes and undercover officers to scour side streets, actually observe drug sales and make arrests.

D.C. Council Chairman David Clarke is understandably concerned that the use of roadblocks can be abused and can lead to violations of people's time and privacy. But last Saturday, police put great emphasis on making roadblock interviews brief and avoiding traffic tieups. Mayor Barry, noting residents' concerns about drugs and related crimes, says any "temporary inconvenience of citizens who were innocent is worth it in the long run," so long as it is within court guidelines. Certainly these qualifications are critical to every operation of this kind.

All these arrests do put yet another strain on a sorely stretched judicial and correctional system, but is that an excuse for letting crimes continue unchallenged? Should a police officer call to see if there's an available cell before covering a beat or making an arrest? Should society just chalk off certain criminal activities as unstoppable and let them go on?

No, no and no. As columnist Courtland Milloy notes in today's District Weekly section, "one thing the raid has shown is that crime remains a major concern in Washington and the dramatic short-term solutions, such as police raids, do momentarily raise the morale of citizens and police." Certainly, there are other things to be done to get at the conditions that feed drug sales as well as use. But if authorities do not respond enough, there is another dangerous possibility: as District delegate Walter Fauntroy has said, "the worst thing that could happen is for people to feel so frustrated they don't believe the law works for them. . . . We don't want people taking the law into their own hands."