Virginia highway officials, acknowledging they are having difficulty enforcing car pool restrictions on portions of Shirley Highway, announced yesterday that a lane restricted to car pools will be opened Monday afternoon on southbound I-95 from Springfield to the Occoquan River.

The new lane is a companion to a northbound I-95 car pool lane that opened on the same stretch last winter and has cut as much as 15 minutes off the morning drive from Prince William County into Washington.

Officials said yesterday they consider the lanes a success, although police have had difficulties enforcing the requirement that vehicles in the lane have at least four riders. When troopers stop violators along the section south of Springfield, traffic in all lanes frequently becomes snarled, they said.

The new southbound high-occupancy vehicle (HOV-4) lane will be in effect between 3:30 and 6 p.m. in the far left lane. The section of the interstate will be expanded from three lanes to four by opening the right shoulder of the road to traffic between 3:30 and 7 p.m.

Several emergency pull-offs have been added to that section of the highway's shoulder.

Starting Monday, car pools and commuter buses will be able to use express lanes for the first time over the 19 miles of highway between the District and the Fairfax-Prince William county line during both morning and evening rush hours.

"We feel this will be a step toward improving the commute for people in southeast Fairfax, Prince William, and Stafford counties," said David Gehr, Northern Virginia district engineer for the Virginia Department of Highways and Transportation. "This is not a final solution; it is not a total solution. We recognize that."

Gehr said using the shoulder as a fourth lane is an interim solution to traffic problems on I-95 until permanent express lanes, which already exist in the median of I-95 between the District and Springfield, can be built to Triangle in Prince William County.

A similar temporary solution between 6 and 9 a.m. on the northbound lanes of the interstate has drawn mixed results, said Lt. J.W. Petefish of the Virginia State Police.

Although the new lane has lessened congestion in both the express and regular lanes, Petefish said, the heavy volume of traffic and the lack of a shoulder during rush hour makes it hard for troopers to ticket motorists in violation of the HOV restrictions.

Some motorists also have been confused by the new road laws, with some illegally using the shoulder during non-rush hours.

Although the highway department has built several emergency pull-off areas in both directions of the highway, Gehr said, when cars do break down on the shoulder it can create backups.

Gehr credited a group of Prince William County residents known as the "Forgotten Commuters" for spearheading the interim project. In 1984, founders Carl Peterson and John Powell, infuriated by delays during their commute to Tysons Corner, sent Gov. Charles S. Robb a plan for a temporary lane, a version of which was eventually approved by state officials.

"These commuters are no longer forgotten," Gehr said. "We are trying to address their problems within the resources available."

The stretch of I-95 south of Washington is the busiest highway in Virginia and among the busiest in the nation, said highway department spokeswoman Marianne V. Pastor. About 150,000 vehicles a day use the section of road just south of the Potomac River.

Rep. Stan Parris (R-Va.) said a survey conducted recently by his reelection campaign organization found transportation to be a chief concern among two-thirds of the people polled in his district. "You can debate all you want about Afghanistan or tax reform," Parris said. " . . . . In Northern Virginia, people told us that transportation was their principal concern."

The two sets of interim lanes cost about $4.4 million, Gehr said. The permanent expansion of the road is scheduled to begin in the winter of 1987 and be completed in four phases by the spring of 1992, at a cost of $111 million.