Extension of the Metro subway system to Franconia-Springfield could come well before 1994 if Fairfax County decides to advance Metro the $60-$70 million needed to build the line, the county's Lee District Supervisor Joe Alexander told the Springfield Chamber of Commerce.

The three-mile extension to Springfield from Alexandria's Van Dorn station -- scheduled to open in late 1990 -- "is the cheapest line to build and we could enter into an agreement with Metro to build it and get paid back later . . . when federal funds become available," said Alexander, county representative on the Metro board of directors.

County funds also may be used to start the $200 million, 35-mile Springfield Bypass, Alexander told the chamber luncheon, if the state legislature this winter raises the county's present $55 million bond limit. The county's limit was "used up" with $25 and $30 million bond issues for secondary road improvements over the past two years.

Because of the present limit on federal funding, the Metro board of directors last month was forced to approve Metrorail construction plans for only 89.5 miles of the region's 101-mile subway system.

Postponed were plans for the final 11 miles, including the Van Dorn to Franconia-Springfield extension as well as subway extensions in the District and both Montgomery and Prince George's counties.

Delayed federal funding means the Franconia-Springfield station could not open before 1994, nor the entire system be completed before 1997, a Metro spokeswoman said yesterday. If Metro guaranteed to repay the county, Alexander said, it could greatly speed up the Franconia-Springfield opening.

Alexander said Metro is committed to completing the 101-mile system and is already buying land for the station and a giant parking lot near Springfield Mall.

While county funding of a Metro extension is still just an idea, the county already is making plans to sell bonds for an early start on the Springfield Bypass, assuming the legislature increases the county bond limits in its coming winter session.

The first section of the bypass to be built, the county board has agreed, would be between Rolling Road and Beulah Street and include the connection to the Franconia-Springfield subway station. Alexander estimates it will be finished within the next three to six years.

The county this summer set aside $2.2 million and the state $1 million to start engineering designs for the highway, which will stretch like an outer beltway from Reston and Rte. 7 to Hybla Valley and Rte. 1.

David Gehr, regional administrator for the Virginia Department of Highways and Transportation, appearing with Alexander, told reporters numerous county highway projects could be expedited if state highway revenues continue to increase. State residents are apparently continuing to buy new and used cars in record numbers and traffic on Virginia roads also is at record levels, boosting state sales and gasoline taxes.

Gehr told the chamber that extension of the high-occupancy vehicle (HOV) lanes between Springfield and Triangle is expected to begin in early 1987, which will help reduce traffic congestion on Shirley Highway.