The Virginia Department of Highways and Transportation is proposing to extend the popular Shirley Highway bus and carpool lanes an additional 19.1 miles south on I-95, between Springfield and the Stafford County line near the town of Triangle and the Quantico Marine base.

Studies for the $80 million project were approved last week by the Transportation Planning Board of the Metropolitan Washington Council of Governments.

If the board ultimately approves the project, the two reversible bus and carpool lanes could be completed by 1985, according to estimates by the highway department. The extended lanes would be constructed in the I-95 median and would be open only to buses, van pools and carpools with four or more persons. The existing lanes on Shirley Highway extend 12 miles from the Potomac River to just south of Springfield.

The project would include building ramps to the new bus lanes at major intersections south of Springfield and to the existing bus lanes at the Beltway. At present, there are no entrances for carpools on the I-95 bus lanes from the Beltway. The Beltway ramps would cost an estimated $10 million.

I-95 already is Virginia's most heavily traveled road, carrying more than 110,000 cars a day, including almost 4,000 carpools and nearly 600 buses.

Planning board officials estimate the existing bus lanes have taken more than 30,000 commuter cars off Northern Virginia roads.

The additional bus lanes have been endorsed by federal and local officials except in the District of Columbia, which could eventually oppose the road project as it opposed I-66 because they could bring more traffic into the city.

District officials are not expected to make a decision until after Virginia completes the detailed environmental impact statement and engineering studies approved by COG's planning board.

The planning board also is expected to approve the additional bus lanes. "They certainly are consistent with our policy of supporting use of vans, carpools and buses as a means of conserving gasoline" and helping reduce Washington area air pollution, said Albert Grant, COG's director of transportation planning.

Virginia highway officials predict the additional bus lanes could reduce traffic on I-95 by nearly 4,000 cars an hour during rush hour. The existing bus lanes have what appears to be the best enforcement record of similar highways in the nation, they said, and State Police enforcement on the new sections would be the same as for the existing bus lanes.

Overall traffic on I-95 had been estimated to continue to grow by an additional 50,000 cars a day by the year 2005, according to highway planners. But officials say these estimates will be revised because rising gasoline prices and gas shortages are expected to slow the growth of automobile traffic. p

Building the bus lanes in the I-95 median would require purchasing about 10 acres of right-of-way at interchanges and constructing sound barriers, retaining walls and glare shields for motorists.

In additions to connecting the Beltway with existing bus lanes, the project would also connect the proposed bus lanes to the Franconia-Springfield Metro-subway station, now expected to open in 1986.

South of Springfield, ramps also would be built at Rte. 617 (blacklick Road), where the present bus lanes end, Rte. 642 (Lorton Road), Rte. 639 (Horner Road), just north of the Rte. 234 interchange on I-95 and just north of Rte. 619 near Quantico, which would be the southern end of the bus lanes.