A young man speeding along I-66 recently pulled alongside a silver Mustang with a flat black air scoop, gunned his engine, and a race was on.

The man thought he had won -- until he saw the flashing red light of the Mustang, an unmarked state police patrol car, and got a speeding ticket.

Virginia State Police have joined other state police departments in adding sports cars to their stable of patrol vehicles to help round up speeders who blithely blow past at 80 miles an hour or impatient truck drivers who tailgate.

But while several other states use sports cars, Virginia is one of the only states to use unmarked sports cars. The North Carolina Highway Patrol also uses unmarked sports cars. A year ago, the highway patrol bought 50 unmarked Mustangs, and officers say they have been pleased with the results.

The appeal of the unmarked sports cars is obvious. "People just ignore you," said Virginia State Trooper R.F. Dyer. "They blow right past you and won't even take a second look."

The unmarked sports cars are not universally popular with police, however. Their most unsettling drawback has been spotlighted in two incidents this month in which young women have been stopped by men who were not police, though they were in what appeared to be unmarked cars that had flashing red lights.

One of the women, stopped Thursday morning in West Springfield, was told by a man posing as a Prince William County policeman that she had run a red light. He then pulled her from the car, beat her and threatened to rape her before fleeing when another motorist appeared.

In Virginia, state police fielded about 1,400 patrol vehicles that wrote nearly 250,000 traffic tickets in the first six months of 1984. Though only seven of those were unmarked sports cars -- Mustangs and Camaros -- they accounted for nearly 7,100 of those tickets, about six times the average for other patrol cars.

Those who don't like the unmarked cars are vehement. "Unmarked cars are just downright dangerous," said Gerald S. Arenberg, executive director of the Miami-based National Association of Chiefs of Police. In fact, officials say, about half the states forbid the use of unmarked police patrol cars of any sort.

Arenberg says the unmarked cars make it too easy for civilians to masquerade as police officers and thereby jeopardize the public.

"You can buy a siren for $150 and a flashing light for $39," Arenberg said. "I would certainly worry that the person pulling me over would be a phony."

"In the dark, I can see where that would cause a citizen some concern, especially for women," said Dyer, who regularly patrols the Northern Virginia area in one of the sports cars.

Dyer says police are perfectly willing to follow jittery motorists to a lighted area or to a gas station -- at the legal speed limit.

Virginia State Police in fact recommend that motorists keep their windows up and their doors locked until they are satisfied that a car with flashing lights is actually a police vehicle.

Virginia State Police are required "to be in full uniform, from that Stetson hat all the way down," said 1st Sgt. W.D. Carter, who supervises the Braddock Road barracks of state police. "And if there are any questions, they all carry IDs."

The two men who impersonated police officers recently were wearing civilian clothes, police said.

The typical reaction from those stopped by the sports cars, say police and prosecutors, is amused surprise and a slight sense that police are somehow violating the rules, but only rarely anger.

"The reactions range from those who are amused that police are out driving a sports car to the few who scream 'entrapment!' " said Dyer.

The plaintive cry of many motorists caught by unmarked cars is: "I didn't know it was a police car," according to assistant Fairfax commonwealth's attorney Stephen H. Moriarty Jr.

"And the judges always respond, 'Well, I assume you didn't know it was a cop car. That's the point,' " Moriarty said.

Maryland uses unmarked vehicles of various kinds to gauge the speed of cars on the highway, but does not use sports cars, according to a spokesman.

A number of states use sports cars that have standard police markings. The California Highway Patrol, for example, has 400 Mustangs with 5.5-litre engines. All have clear police markings.

"The purpose of not having unmarked vehicles is to be out, to be visible and to influence people by that visibility," said Kent R. Milton, a spokesman for California Highway Patrol.

Police who use the sports cars -- both marked and unmarked -- also say their speed cuts the amount of chase time on the highway. A 5.5-litre Mustang has a top speed of about 130 miles an hour and extremely quick acceleration, police say.

Moreover, police say, the sports cars get twice the mileage of standard police cruisers.