Effective April 1, the Shirley Highway's express lanes will no longer be restricted to car pools.

That's when the Virginia highway department has said it will begin a year-long experiment to test the feasibility of allowing cars with fewer than four riders to use the lanes during nonrush hours.

Beginning next month, the so-called HOV-4 (High Occupancy Vehicle) requirement on the express lanes will apply from 6 to 9 a.m. and from 3:30 to 6 p.m. At any other times, the lanes will be open to all drivers, a spokesman said.

The express lanes now reach from the District to the Springfield-Franconia exit just past the Beltway, and Virginia plans to extend them to Prince William County. They operate in only one direction at a time, keyed to rush hour traffic. They are now open to northbound traffic from about 10 p.m. until 11 a.m. and to southbound motorists from about noon until about 9 p.m.

The easing of the HOV requirements was mandated by Congress two years ago at the same time that it dropped the car pool requirements on I-66 from four-member carpools to three-member car pools. Both Virginia and D.C. government officials vigorously opposed the plans, saying the changes will simply clog those roads further without providing any relief for the regular commuters.

Virginia and D.C. officials also have consistently disputed the contention of area members of Congress that the highways are underused and have pointed out that reinstating the stringent car pool regulations would be extremely difficult.

A year-long test of the changes on I-66 is nearing an end.