Virginia Gov. Charles S. Robb paid a quarter yesterday as the first toll on the new $56.7 million Dulles Toll Road, then drove on in the rain in a borrowed sedan to officially open the highway.

The 13-mile road between the Beltway and Rte. 28 in Loudoun County "should ease, ever so slightly, the traffic congestion in Northern Virginia," Robb told about 75 persons from a portable grandstand erected beside a toll booth.

The time was 3:40 p.m. The barely finished toll road, the first in Northern Virginia in 50 years, did not actually open to the public until 7 p.m., after the evening rush hour.

Its first full test by commuters will come this morning when the four-lane highway becomes a major new route for more than 40,000 motorists in one of Virginia's most traffic-clogged corridors.

State Highway Commissioner Harold C. King, who hailed the road as a "milestone" of cooperation among local, state and federal officials, said the new highway is safe and sufficiently complete to be used, despite its appearance.

Although state highway officials and contractors have worked overtime on the project for several months, its still has unpaved road shoulders, roughly paved ramps and some temporary plywood toll booths, looking like outhouses, standing beside entrance and exit ramps.

The major toll plaza near the Beltway will be completed by Nov. 1, King said, as will the ramps and a one-mile section of the toll road near Reston that is still roughly paved. The gravel road shoulders will be paved by mid-December and the landscaping and a sound-suppressing wall completed by next May.

Robb praised Fairfax and Loudoun county officials, the state General Assembly and the Federal Aviation Administration for "its far-sightedness" in allowing room for a parallel highway to be built when the access highway to Dulles International Airport was constructed in 1961.

The airport access road, which commuters used illegally for 20 years and legally in the last year with special FAA stickers, is closed as of today to all except airport traffic. While FAA police may allow a day's grace period for errant motorists, FAA officials have announced there will be strict enforcement of the airport-traffic-only rule and $25 tickets will be given to violators.

The new toll road, like the parallel airport access highway, feeds into I-66, which was opened last year inside the Beltway and which has itself dramatically changed traffic patterns. Together the highways, although complicated with rush-hour restrictions and now toll booths, offer commuting motorists a smooth, traffic-light-free route into the nation's capital from Loudoun County.

"Remember, despite what The Washington Post and Reston folks think, this road goes to Loudoun County," James F. Brownell, chairman of the Loudoun County Board of Supervisors told the crowd on the wet, windswept toll road yesterday.

Brownell was referring to full-page advertisements hailing the advent of the "Reston Expressway" that appeared in the newspaper the last two days.

Michael Was, general manager of the Reston Land Corp., which bought the ads, said yesterday the purpose was not to claim the roll road for Reston, but to get companies "to consider Reston and that Reston access will be dramatically improved with the opening of the road."

Highway officials have told motorists using the new toll road to carry lots of change, since exact change -- ranging from 25 to 85 cents -- is required when the toll booth attendants are not on duty. Automatic toll machines on entrance and exit ramps will be in operation between 9:30 p.m. and 5:30 a.m.

Robb hinted that many motorists may grumble about the stops and the tolls. "If it's like other toll roads, it will not be fully appreciated by travelers," he said.

But they should force themselves to remember "how important this day is . . . for residents of Loudoun and western Fairfax County. . . ."