The National Park Service is proposing major traffic restrictions in Rock Creek Park to reduce heavy commuter automobile use of the historic park's narrow, winding roads north of the National Zoo.

The changes are proposed in a master plan for the popular bike route through the park and will be aired at public hearings in December or January.

All of the proposals are designed to reinforce the District of Columbia's transportation policy of discouraging commuter use of cars -- the primary source of air pollution here -- and encouraging use of mass transit and bicycles.

Commutes comprise about two-thirds of the 21,000 cars a day using Beach Drive at its busiest spot, near Piney Branch Road. During rush hours, commuter cars constitute virtually all of upper Rock Creek Park traffic, according to Park Service surveys based on interviews with motorists.

The thousands of bicyclists, joggers, picnickers and sightseers who use Rock Creek Park each week would benefit most from the proposals.

The plan is divided into eight "alternatives" that range from minor changes in park roads and paths to a permanent ban on auto taffic along the narrowest and most scenic sections of Beach Drive, primarily between Broad Branch and Joyce roads.

But a permanent ban on cars may have little chance of approval by the Park Service.

When irate Maryland commuters earlier this year accused the Park Service of having a secret plan to close Beach Drive, Regional Director Manus J. Fish -- called at home by a Post reporter -- said he had no plans to close the road permanently.

Fish said this week that he was unaware of the alternatives being developed for the Rock Creek master plan at the time the reporter called.

"At the time I thought it would be impossible to close any roads," said Fish. But he insisted all options are officially open "and no decisions will be made until after the public hearings."

Other alternatives to be considered call for rush-hour closing of northern and southern entrances to Beach Drive or midsections of Beach Drive. This would leave park roads open during weekday hours when tourists and sightseers use the roads, but would make it inconvenient, if not impossible, for motorists to use Beach Drive as a major north-south commuter route.

Commuters could use Metro's Silver Spring line, soon to be extended to Rockville and Wheaton-Glenmont, the Park Service study notes, or some of the major auto commuting routes on which the District is not planning to restrict traffic -- Wisconsin and Georgia avenues and 16th Street.

These streets already carry much of the Northwest corridor's daily rush-hour traffic. Beach Drive, although its single inbound lane may be jammed with cars, carries only 5 percent of the Northwest traffic.

The District plans to restirct commuter traffic soon along two major Northwest commuter roads, Connecticut Avenue and Reno Road, and recently limited commuter traffic on 13th Street NW by making it a two-way local street instead of a one-way six-lane commuter route. The city also plans next month an experimental closing of the inner traffic lanes on Logan Circle, at 13th Street and Rhode Island Avenue NW.

Another Park Service alternative is permanent closing of one of the two lane of Beach Drive between Broad Branch and Wise roads, making it into a bike path.

The single auto lane could either be permanently one-directional (the environmental assessment of the master plan recommends northbound) or could accommodate twice-a-day commuter traffic by being reversible. The latter proposal would have little impact on most car commuters since Beach Drive now has only one lane of traffic in each direction.

The most expensive of the proposed alternatives would be to complete the existing bike trail, begun in 1968 as the Washington area's first major bike trail.

Because of tight budgets, the Park Service left five and a half miles of the trail unifinished -- near the Montgomery County line and in the narrow central section between Broad Branch and Joyce roads. These sections, where motorists, bicyclists and joggers share the roadway, have caused the most problems.

Constructing five and a half miles of trail, bridges and elevated boardwalk for these sections, and improving the present trail -- only two- to three-feet wide in places, would cost an estimated $1.1 million.

One of the least expensive alternatives, at about $200,000, calls for continuing bicycle-car use of Beach Drive, but with strict police enforcement of speed limits and traffic laws that give bicyclists and motorists equal rights to the road.

U.S. Park Police surveys show that 98 percent of motorists using Beach Drive regularly exceed the park's 25 mile-an-hour speed limit. Enforcing or lowering the speed limit would permit bicyclists to keep up with the traffic flow. This alternative also includes some bike trail improvements.

Another alternative calls for a weekend ban on cars from most upper sections of Beach Drive. For at least 10 years cars have been banned from the central section of Beach Drive on Sundays between 8 a.m. and 6 p.m. Few motorists have complained about that ban, and it has encouraged thousands of cyclists, joggers and hikers who use Rock Creek Park each weekend.

A celebration was held last week at a Georgetown bike shop, Tow Path Cycles, to honor local bicyclists who helped create the Rock Creek Park bike route by convincing the Park Service here to build its first bike path, to close Beach drive on Sundays and in 1971 to experiment with closing a lane of the four-lane Rock Creek Parkway for bicycles.

Extending the Sunday ban to Saturday is not expected to generate much opposition, the 80-page Park Service study notes, since it was tried several years ago without public complaint. However, the study says this would not solve the weekday conflict between commuter cars and bicycles or reduce heavy auto traffic in the park, which are primary problems.

Some of the less-costly alternatives -- or combinations of alternatives -- could be implemented by the Park Service within the next year, even with a severely restricted operating budget.

There is little money, however, for major construction projects such as building 5.5 miles of new bike trail. Congress has cut back capital budget funds for the Park Service here, forcing the elimination this year of 19 park construction projects from its six-year capital budget. Cut projects range from new restrooms at Lafayette Square to a new course at Oxon Cove.

The Park Service is expected to announce within the next few weeks when the hearings will be held.