Beginning Monday evening, thousands of Northern Virginia commuters will face new, tougher restrictions on their use of a four-mile stretch of the Shirley Highway rush-hour car pool lanes between Alexandria and Springfield.

All southbound drivers may still enter the permanent, reversible car pool lanes from a ramp between Duke Street and Edsall Road. But vehicles that do not meet the car pool requirement of containing three or more persons will be forced to leave the lanes at Route 644 (Old Keene Mill Road) in Springfield.

Virginia transportation officials hope to discourage most drivers from getting in the car pool lanes in Alexandria and to reduce the heavy congestion that now occurs in the lanes near the Capital Beltway during the evening rush hour.

The policy change, recently approved by the state's transportation board, is another example of how officials are constantly tinkering with the rush-hour car pool lanes in an effort to get the most use out of them.

Currently, vehicles with at least three persons are allowed to use the car pool lanes on Shirley Highway from the 14th Street bridge in the District to just outside the Beltway. Drivers may continue on temporary car pool lanes to the Occoquan River.

Because of the heavy southbound traffic congestion that occurs evenings on the Shirley Highway between the Beltway interchange and Springfield, officials decided in the mid-1970s to permit drivers of vehicles with fewer than three persons to enter the car pool lanes midway between Duke Street and Edsall Road.

Those drivers have been permitted to continue on the permanent car pool lanes until the temporary lanes begin about a mile south of Route 644. At that point, those drivers must shift over to the regular lanes.

Gradually, however, as traffic in the region has grown, the car pool lanes between Alexandria and Springfield have become as clogged as the regular lanes, prompting the policy change.

More than half of the average of 3,214 vehicles in the car pool lanes between Alexandria and Springfield have fewer than three persons in them during the evening rush hour and most of those drivers continue south to the point where the temporary car pool lanes begin, officials said.

When those drivers merge from the permanent car pool lanes to the temporary ones, a bottleneck occurs because the drivers are trying to move out of the car pool lanes into the regular lanes, officials said.

The speed of traffic in the car pool lanes between Alexandria and Springfield has slowed to an average of 26 miles an hour during the evening rush hour, compared to 55 mph in the permanent car pool lanes between the District line and Alexandria.

Mary Anne Reynolds, a spokeswoman for the Virginia Department of Transportation, acknowledged that the change will add to the congestion already experienced in the regular lanes near the Beltway. But state officials have to make sure, she said, that the car pool lanes are free-flowing to encourage their use.