Montgomery County Executive Charles W. Gilchrist yesterday proposed a $5.5 million mass transit program over the next two years to encourage commuters to use public transportation rather than drive on heavily traveled I-270, Rte. 29 and other congested roads in the northern part of the county.

The program would increase the number of rush-hour express buses on I-270 from 14 to 19 per hour, and those on Rte. 29 from four to five. It would pay residents and businesses a subsidy of up to $200 a month for establishing van pools on I-270 or Rte. 29, increase the staff of the car and van pool program in Silver Spring and Bethesda and build two new commuter parking lots. If the program is funded by the County Council, some aspects could start in September.

The mass transit package comes in the midst of a county development boom that has fueled the construction of thousands of houses and office buildings. The rapid pace of development has outstripped county and state construction of new roads and road improvements, causing unprecedented traffic jams in fast-growing areas of the county such as Germantown, Gaithersburg, White Oak and Cloverly.

At the same time, use of mass transit has risen. About 12,019 more people in Montgomery County ride Metrorail, Metrobus and the county Ride On buses per weekday than used those services last year, according to Robert J. McGarry, the county's transportation director. During rush hours, ridership on county I-270 express buses has increased by 11 people per day since March, he said, while ridership on Rte. 29 express buses has shot up 120 a day.

To make Gilchrist's plan work, the council would have to appropriate about $4.6 million in new money, and several council members said yesterday they were inclined favorably toward the proposal.

"This is the kind of thing we need to decongest our roads," said council member Neal Potter, chairman of the transportation committee.

Barbara Singer, 42, who drives on I-270 every day from her home in Gaithersburg to her job at a Chevy Chase advertising agency, said yesterday she hopes that measures will be implemented.

"I-270 is basically a parking lot in the morning," she said. "At 7:30 at night it's worse. By the time you get to work you feel like you've already been to work and back. People are crazy trying to decide which is the best lane to get in. You feel like you've been in a fight and you can't decide whether you've won or not."

A large chunk of the new money, about $2.6 million, is targeted for the two new commuter parking lots proposed for next year -- the White Oak park and ride and the Glenmont park and ride. The 450-car White Oak lot will be built at Rte. 29 and New Hampshire Avenue. Commuters will be able to park free and take an express bus to the Silver Spring Metro station, Gilchrist said.

The 300-car Glenmont lot will be built on the west side of Georgia Avenue, across from the site of the Glenmont Metro station. Until the Metro station opens, commuters can park there and get an express bus to the Shady Grove Metro station.

Gilchrist proposed about $665,000 to study future transporation projects. Those include Metrorail passes allowing unlimited travel for a monthly fee, an east-west mass transit route between Silver Spring and Bethesda, an exclusive bus lane for congested Rte. 355, and the design of a system for faster passenger pickups and dropoffs by 200 buses serving the Bethesda and Silver Spring Metro stations.