A study released yesterday by the Virginia Department of Transportation raised the possibility of reducing car-pool restrictions on Shirley Highway from four persons to three, but only if Congress finances a $15 million expansion of the road to include a third express lane.

The study, presented yesterday to the Commonwealth Transportation Board, said that reducing the required number of car-pool occupants without building the additional lane would cause "heavy congestion and seriously harm the HOV {high-occupancy vehicle} concept."

The study comes in response to a proposal by Rep. Stan Parris (R-Va.) that the car-pool limit be lowered to allow more drivers to take advantage of the restricted lanes on Shirley Highway. Parris has been a vocal critic of the car-pool restrictions and has frequently locked horns with state officials over the issue.

In a statement yesterday, Parris called upon the state to "immediately open up the restricted lanes for HOV-3 vehicles." He added, "I am prepared to take whatever action is necessary to accomplish this goal."

Parris attached an amendment this year to the federal highway spending bill that successfully defeated the state's attempt to extend evening car-pool restrictions by half an hour to 6:30 p.m.

The car-pool restrictions apply to the reversible lanes in the middle of I-395 between the Potomac River and Springfield and to the express lanes of I-95 south to the Occoquan River.

State officials object sharply to Parris' latest proposal. "The key to the success of the car-pool lanes is the time savings," said Virginia Transportation Secretary Vivian Watts. "It would be irresponsible to go ahead with HOV-3 knowing that you would be jeopardizing the success" of the car pool lanes.

State officials said the shoulders of the existing reversible lanes could be strengthened to accommodate a third lane, which they said would cost about $15 million to build. Commonwealth Transportation Board member Joseph Giuffre urged Congress to grant special financing to build the additional lane "if Congress wants to reduce the HOV-4 to HOV-3," according to a statement released yesterday by the department.

Members of the Commonwealth Transportation Board discussed the car-pool study yesterday at their monthly meeting, which was held in the Tidewater community of Kilmarnock, Va.

The study acknowledged that the existing car-pool lanes were not yet functioning at capacity, even during the busiest hour of the day. But it also said that reducing the restrictions to three people "will encourage car pooling along the corridor and more vehicles will be added to the facility faster in the future."

Plans to extend the car-pool lanes farther south could also increase demand, the study noted. The state plans to build reversible lanes between the Beltway and Quantico, a distance of about 19 miles.