Harry J. Pizza, a retired federal printing officer who lives near Rock Creek Park, was a bit perplexed by the National Park Service's controversial plans for restricting car traffic on a central roadway through the park.

So Pizza, 61, dropped in last week for a chat with park service officials. "I'm completely in accord with the idea," he said. But he added, "Instead of closing the road, couldn't they build bicycle trails alongside of it?"

"We did look at that," replied Michael Donnelly, regional planning coordinator for the park service. "It's very expensive--both dollarwise and from an environmental degradation standpoint."

Pizza was the first of about 75 visitors at a 14-hour "open house," held by park service officials in a basement room of the Rock Creek Nature Center on Glover Road NW in the park. Officials said the informal session was designed to answer questions and clear up misunderstandings about the plans, which have been debated since the late 1970s.

Last month, the park service announced three proposals for limiting car travel on parts of Beach Drive, a two-lane, north-south road that winds along the banks of Rock Creek.

Last week's session drew both critics and supporters for the proposals. Rick Morgan, a bicyclist, praised the proposals, saying, "It's very dangerous to ride a bicycle on Beach Drive during rush hour."

Harold Gray, executive secretary of the National Capital Area Transportation Federation, opposed the plans. "One of the great values of the park is to see it through the windshield," he said.

One proposal would restrict rush-hour traffic on Beach Drive to a single lane between Joyce Road and Broad Branch Road, starting in May. One lane would be set aside for bicyclists and joggers. The other would be open to southbound cars only from 7 a.m. to 9:30 a.m. and to northbound cars from 4 p.m. to 6:30 p.m. Both lanes would be open to cars at other times.

Another proposal, intended to go into effect in 1985, would permanently bar commuters and other through traffic from this section of Beach Drive by closing off the road at Boulder Bridge. Sightseers, including elderly and handicapped visitors, would still be permitted to drive on Beach Drive, but would have to turn around at the bridge.

The third scheme, which was instituted as an experiment last summer, would ban cars on weekends and holidays from two sections of Beach Drive--between Picnic Grove 10 above Sherrill Drive and Wise Road, and between West Beach Drive and the District line.

Park service officials said they expect to announce a decision on these proposals in May after reviewing comments from local government agencies and the public. The deadline for submitting comments is April 15.