Traffic on the new Dulles Toll Road moved smoothly yesterday despite earlier predictions of congestion and confusion on the first full day of operation for the $56.7 million highway.

Hundreds of commuters apparently avoided the road and illegally used the parallel limited-access airport highway, which is restricted to airport traffic.

After watching violators yesterday morning and giving some of them warnings, Federal Aviation Administration police officials announced that beginning Thursday, they will start arresting motorists who illegally use the Dulles Access Highway.

The crackdown on illegal use of the 23-year-old access highway has long been threatened by FAA police, but never begun because of political pressure to give commuters a break in the traffic-clogged Route 7 corridor.

"I thought I'd just test the system," one motorist told Police Lt. William B. Morse yesterday morning. That driver and others were given warnings and their names and license numbers were recorded.

"I told him 'You come back and test the system in another day or two,' " Morse said, as he watched his officers pull over dozens of other motorists.

The rewards of getting on the airport highway are great. Not only can motorists avoid paying a toll of up to 85 cents on the new Dulles Toll Road, but they can ignore I-66 rush-hour restrictions, which permit only three-person carpools on I-66 inside the Capital Beltway.

Airport traffic merging onto I-66 from the Dulles Access Highway is excepted from the high-occupancy-vehicle (HOV) restrictions.

"We've had a lot of scofflaws out there today," said William Halligan, head of FAA public safety for the federally owned Dulles and National Airports.

FAA police say their crackdown will be aided by their computer record of the more than 30,000 cars permitted to use the airport highway on a temporary basis during the past year with special decals.

Some parts of the new highway are unfinished. Some entrance ramps and a one-mile stretch near Reston are only roughly paved, and there are some unpaved road shoulders and some temporary plywood toll booths. But the highway's first full day of use was as "smooth as can be," despite a few minor mechanical glitches, said Philip DuMars, toll road manager.

Officials were not certain how many vehicles used the highway yesterday but traffic-counting equipment will be installed today.

They said the traffic flow seemed good and that a few new signs, plus better-timed traffic lights on routes feeding into the highway, will help eliminate what little confusion and congestion there was.