Danny Staley, a 20-year-old service station attendant, rode blissfully in a cloud of gas fumes and rock 'n' roll yesterday as he guided his stereo-equipped, midnight black moped along Holmes Run Parkway in far western Alexandria.

When he approached the entrance of Brook Valley Park, he abruptly shut off the bike's motor and pedaled the heavy machine down a park trail.

Staley knows that these days the city parks, where it is illegal to ride motorized vehicles of any sort, have eyes. "I heard there are a lot of police out here on bikes," he explained, pedaling the moped, which is legal.

Since the mid-1970s, Alexandria police have routinely used down-sized motorcycles, or trail bikes, to help run down purse snatchers and apprehend suspects who proved too elusive for officers on foot or in cruisers. But in recent years, Alexandria police have shifted their use of the department's six trail bikes to the spaghetti network of city-maintained biking and jogging trails concentrated in the west end of the city.

This summer, according to police officials, officers on trail bikes have almost eliminated what used to be a constant headache for those who use the parks and those who try to maintain them: the youths on trail bikes whose frequently reckless riding shattered the quiet and left thousands of dollars worth of damage to creek beds and freshly seeded grounds.

"Every spring, what happens is that the juveniles get out of school, start drinking and come out in the park with their motorcycles and mopeds, trail bikes, dirt bikes," says Cpl. Michael Crabill, one of eight officers who rotate duty on the trail bikes. "It really gets to be a problem if you don't get on top of it."

And getting "on top of it," says Crabill, a nine-year veteran of the department and one of the first to be trained to use the trail bikes, means logging more than 80 miles a day on the bikes in a patrol area that takes police through more than 500 acres of park land in the city.

"It means high visibility," he says.

"Being seen is definitely a deterrent," says officer Garry Ground, who says that despite his height of almost 6 feet 3 inches, he enjoys his trail bike duty.

"The kids might call us the kiddie cops of Alexandria or ask us, 'Hey, officer, where did you get that moped?' but we make a difference," Ground says.

Alexandria Deputy Chief Arlen Justice credits the bike patrols for helping to make city parks safer.

"We haven't had the problems on the bike trails this year we have had in previous years," he says. "I'm sure the presence of the uniform officers on trail bikes has some effect on the situation."

In past summers, members of the bike patrol have issued citations for trespassing, driving under the influence, destruction of private property and vandalism. This year they haven't issued any, but they have had occasion to give chase or to call in assistance.

"We can hide and sit back in the bushes until we see somebody and then we'll come out of there and call for a cruiser and there's no place for them to go," Crabill says.

Jack Rovey, director of Alexandria's Cameron Run Regional Park, has nothing but praise for the trail bike patrols.

He says that when construction was under way on the park's wave pool, the piles of dirt were an open invitation to youths on dirt bikes who turned the site into a race course, destroyed park property and impeded construction in the process.

"But the police bikes made a big difference," he says. "The police could go where the kids went. The problem disappeared."