The Montgomery County Council, taking the final steps to allow increased development in Silver Spring, adopted landmark legislation yesterday requiring measures to get commuters out of single- occupant cars and into mass transit and car pools.

Most existing employers and all new ones in the redeveloping Silver Spring downtown will be required to devise ways to reduce traffic.

The council's designation of the suburb's central business area as a transportation management district, approved on a 6-to-0 vote, creates the first such district in the Washington area and one of an estimated dozen nationwide.

The district is at the heart of County Executive Sidney Kramer's ambitious and controversial plan to redevelop Silver Spring.

The plan, formally enacted yesterday as the council met in an all-day session to put the finishing touches on 70 pages of complicated legislation, lifts development limits in Silver Spring to allow major retail and residential projects similar to those that transformed Bethesda and Tysons Corner. But it applies transportation management in an effort to avoid the massive traffic jams that accompanied the renaissance of those two urban areas.

Robert S. McGarry, Montgomery's transportation director, sees the transportation management district as the prototype solution to the area's increasing traffic congestion. Similar legislation has been discussed for Bethesda and is considered a possibility for Wheaton and Friendship Heights.

The local future for these districts, which originated on the West Coast as a way to reduce traffic and improve air quality, largely depends upon how successful county officials are in meeting goals that they acknowledge are ambitious and that their critics call unrealistic.

The plan calls for increasing by September 1990 transit ridership from 20 to 25 percent of the Silver Spring work force and average car occupancy from 1.2 to 1.3 people a vehicle. Citizen groups that strenuously opposed Kramer's redevelopment plans, as well as county and planning department officials, expressed doubt that the goals are achievable, but there was general agreement to make the effort.

Kramer has requested a budget of $1.1 million to run the program. If it is approved by the council, four employes would open an office in Silver Spring to negotiate employe commuting plans, develop incentives for transit use and work with an advisory board.

Since 1982, Montgomery County has used an ordinance requiring provision of adequate public facilities to get some developers to undertake traffic reduction methods. In May, Alexandria passed a law requiring developers of large projects to reduce by as much as 30 percent the rush-hour traffic their new buildings would generate. But the Silver Spring legislation is unique in this area in that it applies to existing employers in a geographic area.

"It is the first time we would be saying -- and saying with the force of law -- to existing employers, 'Hey, we need your help,' " said Planning Board Chairman Norman L. Christeller.

The legislation, which goes into effect immediately, requires employers of at least 25 workers to file plans with county transportation officials outlining methods they would use to reduce rush-hour traffic. Employers may change work hours, encourage workers to use car pools by providing preferential parking, subsidize transit fares or take other steps. Failure to file a plan can result in a fine of up to $50. An estimated 350 companies in Silver Spring employing three-quarters of the 25,000 employes who now work there will be affected, McGarry said.

There are stiffer requirements, buttressed by possible sanctions, for developers, who must enter into binding agreements with the county on how they will reduce rush-hour traffic. The agreements would set forth penalties for violations, and the county could add a $1,000 fine.

"If they don't sign on," said McGarry, "they won't be building in Silver Spring."

McGarry, who preaches the virtues of car-pooling and transit use with the force of the Army Corps of Engineers general he once was, concedes that it will not be easy to achieve the goals. In addition to incentives such as the sale of reduced transit fares, the county proposes disincentives. The key, McGarry said, is restricting the amount of long-term parking.

While the emphasis now is on voluntary participation, the county can shift to currently undefined "mandatory mitigation measures" to be imposed on employers if the goals are not met.