Prince William motorists will soon find virtual obstacle courses on some residential streets, as traffic engineers install devices to slow cars, addressing mounting complaints about speeders.

Transportation planners call the measures "residential traffic calming"--altering the road to slow drivers down. The devices include raised humps, traffic circles and islands in the middle of the asphalt that force cars to snake around them.

Starting in the spring, they will be installed on eight roads across the county--one in each magisterial district--in an experiment to see whether fast cars in this fast-growing county can be slowed.

"We're trying to make it more inconvenient for motorists to drive through a residential neighborhood," said Steve Stevens, a county transportation engineer. "Whether we increase the program will be up to the public."

The calming measures complement two other efforts underway to slow speeders and curb a growing problem of "cut-through" traffic, motorists who use quiet residential streets as short cuts to main roads. Over the past three years, the county has installed four-way stop signs and announced steep fines of an additional $200 on a total of 50 streets as deterrents to speeding.

The first of the new devices, two speed humps, were installed on Eastlawn Avenue in Dale City in October, mainly to lower speeds that had reached up to 10 miles per hour above the 25 mph limit, as well as to discourage cut-through traffic from Cardinal Drive to Dale Boulevard.

Smoketown Road in Lake Ridge also is scheduled to get speed humps. The county's Public Works Department is reviewing other potential roads for calming techniques, choosing from a list of problem areas noted by the Board of County Supervisors in consultation with neighborhood groups. Construction should begin in the spring once streets are selected.

The $120,000 project is a joint effort by Prince William and the Virginia Department of Transportation, which launched the residential calming program in 1997 to help counties grapple with speeders. Since then, about half a dozen jurisdictions across the state have installed the anti-speeding measures, including Fairfax and Arlington counties.

But traffic engineers have learned the hard way that altering streets with obstacle courses for drivers can cut both ways. Montgomery County's ambitious effort a few years ago to slow drivers with half a million speed humps was met with anger from frustrated motorists and rescue workers, who said the devices slowed emergency vehicles. Many of the humps eventually were removed.

Under VDOT rules, three-quarters of the residents of a given street must agree to any traffic calming measures.

"With all the sprawl in places like Prince William County, neighborhoods want to get more involved," said Joan Morris, a VDOT spokeswoman. "They want to be part of the solution."

Residents of Forestdale Avenue in Dale City have already petitioned the county to install signs announcing the $200 speeding fines. But under the new traffic calming program, the street also will get a raised crosswalk to slow cars, said Supervisor John D. Jenkins (D-Neabsco), who represents the district.

And a similar device will be installed on Knightbridge Drive in the Westridge development in Lake Ridge, where a steep hill, a crowd of densely packed town houses and a network of pedestrian paths have made residents particularly concerned about speeding vehicles.

"It goes back to people's aggressive driving, and besides the raised crosswalk, we need more enforcement," said Supervisor Mary K. Hill (R-Coles), whose district includes Knightbridge Drive. "I was getting so many complaints."

As they select other roads for traffic calming devices, county and state engineers will consider several other techniques besides speed humps and raised crosswalks. They include chicanes, a form of sidewalk extension that alternates from one side of the street to another to create curves that slow cars, and chokers, sidewalk extensions that create the illusion of a narrower street without altering its width. Traffic circles also are options.