There's a new breed of commuter riding the restricted express lanes of Northern Virginia: drivers equipped with mannequins, babies and diplomatic immunity, all designed to evade even the most sophisticated enforcement efforts.

It took state troopers months to nab the elusive driver who breezed into Washington via the Shirley Highway express lanes with his three silent, blank-faced mannequins. And Arlington Deputy Chief David L. Reiten did a double take before speeding after the driver who'd stacked boxes topped with hats in his empty car seats.

"American ingenuity is amazing," says Reiten.

That native ingenuity has led to an urban game of cat and mouse between troopers and express lane commuters on Shirley Highway, Rte. 50 and I-66, where rush hour autos must carry four riders.

Enforcing the restrictions--graced with the technocratic acronym HOV-4--has tested not only the creative lawlessness of commuters, but the resilience of police, who must step quickly to catch the latest gimmick designed to outsmart them and, not incidentally, save a few minutes on the trip.

"You always say to yourself: 'Is that a fake body there or someone snoozing?' " said State Trooper C.E. Blosser. "You pretty much have to assume it's someone snoozing, because you can't pull over a car to see if everyone in it is alive."

Sometimes, however, it's the live ones that give police even bigger headaches. "You'll stop people that have a two-year-old in the back seat and a three-month-old under their arm," Reiten said. "They're meeting the basic requirements, but they're sure not following the intent of the law."

And then there was the driver who called State Police Sgt. David M. Smith. "She said she was expecting and wanted to know if that would count for two," said an incredulous Smith. "I told her it only counted for one until the joyful day."

Born of the gasoline shortage of the mid-1970s, Northern Virginia's car pool lanes were designed to encourage gasoline conservation in return for a ride in the fast lane. But they also have become an enforcement nightmare.

For instance, the State Police assigned 16 additional troopers to the Northern Virginia District in anticipation of the recently opened I-66. During the morning and evening rush, two to five troopers cruise the 9.6-mile stretch, and Arlington County police heavily patrol the highway in the county.

All the figures aren't in yet, but law enforcement officials say they issued 267 commuter-lane violations on the restricted stretch of I-66 during the last week in January--close to the average weekly allotment of tickets since the highway opened in late December.

Getting caught in the fast lane without four live people in the car costs the driver $35 in fines and court costs. The mannequins pay nothing.

Lawbreakers offer a variety of creative excuses when they are nabbed. Mention the signs that boldly mark each entrance ramp, and officers hear everything from "What sign?" to "I don't speak English," Blosser said.

But only one excuse works wonders: "I have diplomatic immunity"--an explanation that leaves police powerless and irritated.

"It's unfair, but what can you do?" asked Blosser as he watched a cream-colored Cadillac armed with diplomatic license tags whiz past yesterday. "We don't even bother to pull them over. It takes our dispatcher too long to look in the book to see if they're listing diplomatic immunity."

Sometimes, though, it is playful law-abiding citizens who are most bothersome.

Spotting a single-rider car yesterday, Blosser gave pursuit. Suddenly, a child's head popped up. Then another, and another, until the required four heads--smiling away--appeared through the windows. "The hide-and-bob-up trick," police call it.

Muttered an unamused Blosser: "Getting their kicks on I-66."