The Prince William Board of County Supervisors agreed Tuesday to make permanent a countywide youth curfew -- even though it has not had its desired effect on juvenile violence -- citing its popularity with police and parents alike.

The curfew, which was scheduled to expire at the end of June, has been in effect since 1997 as part of a pilot program aimed at fighting youth violence in the county.

A study presented to the board last week by Theodore Caplow, a sociology professor at the University of Virginia, found that although the curfew enjoys huge popularity among parents and is considered a powerful law enforcement tool by police, it has done nothing to curb youth violence -- the reason the supervisors instituted it two years ago.

Six unannounced police sweeps intended to publicize the ordinance were deemed cost ineffective by Caplow and overly harsh by residents. Teenagers found in violation of the curfew during the sweeps were taken to curfew centers, shackled by hand and foot and attached to a fixed object until a parent or guardian arrived.

The curfew makes it illegal for anyone younger than 18 to be in a public place from 11 p.m. to 5 a.m. Sunday through Thursday and from midnight to 5 a.m. Friday and Saturday. The ordinance includes various allowances for such late-night events as concerts and movies, providing that teenagers have written permission from a parent.

Under the curfew ordinance, a police officer who catches anyone in violation has the discretion to make an arrest, give a warning or issue a summons to appear in court. Violators face as many as 50 hours of community service for the first offense; a minimum fine of $200 and as many as 100 hours of community service for the second offense; and a minimum fine of $350 and as many as 200 hours of community service for a third offense.

Parents and business owners who knowingly permit minors to violate the curfew can be charged with a misdemeanor, which carries a maximum sentence of six months in jail or fines as high as $1,000, an element of the curfew that makes it among the toughest in the Washington area. In the past two years, 1,108 minors have been arrested for curfew violations through the sweeps as well as targeted and regular enforcement.