Traffic restrictions on Reno Road and 34th Street NW designed to sharply reduce the heavy flow of morning rush-hour commuter traffic on the hilly, twisting arterial were ordered yesterday by the D.C. Department of Transportation, to take effect June 29.

One of the two southbound lanes now used by cars headed toward downtown Washington will be closed to traffic, and the curbside space will be made available for neighborhood parking.

In adopting the restrictions on morning traffic, transportation director Thomas M. Downs rejected a proposal to apply similar restraints on homebound traffic in the evening rush hour. He said safety problems for children attending elementary schools along the route are greater in the morning when the rush hour coincides with the start of the school day.

Reno, Road and 34th Street, together with a short section of 41st Street, form a continuous, meandering route roughly midway between and parallel to Wisconsin and Connecticut avenues. The Reno Road corridor carries about 3,000 cars, about 65 percent of them belonging to Maryland commuters, between 7 and 9 p.m. each weekday from Western Avenue -- the District's northwest boundary with Maryland -- southward to Massachusetts Avenue NW. From there, the traffic funnels into downtown Washington.

The action is the latest in a series of moves by the transportation department to discourage commuter traffic along arterial streets in residential neighborhoods. Previous similar actions were taken on 13th Street NW and Independence Avenue SE.

Such moves have come under sharp attack from the American Automobile Association, which devoted much of its winter edition to criticism of what it called "DOT's antiautomobile policies."

Down's decision was a partial victory for residents in the Reno Road corridor who had campaigned intensively for about five years to ban most commuter traffic. A formal proposal that went beyond Downs' decision of yesterday was made last November by his predecessor.

"It [Downs' action] sounds like a step in the right direction, and I hope it's just a first step," said Esther Foer, public relations chairman of the Reno Road Corridor Coalition, which mustered the campaign. "It's not everything we could hope for."

Downs described the restrictions as experimental, and said they would remain in effect for six months. After that, he said he would consider whether to modify them, either by loosening or tightening the restraints.

Downs said he set the June 29 effective date because, by that time, Wisconsin Avenue will be repaved at two Metro subway station locations where traffic is now disrupted. All surface work related to the subway on Connecticut Avenue recently was completed.

During the last three years, Downs said, traffic volumes have declined substantially on Wisconsin Avenue, Connecticut Avenue and Reno Road, apparently the result of high gasoline costs, more ride-sharing and increased transit usage.

In an effort to discourage Marylanders from using Reno Road, Downs ordered a prohibition on left turns at all times Western Avenue into 41st Street. He also ordered a prohibition on left turns during the morning rush hour from Western onto 42nd Street. This will affect motorists who now enter the District on Connecticut Avenue at Chevy Chase Circle and then move laterally to Reno Road, seeking a faster drive downtown.

Several other left-turn bans will be imposed at intersections on Reno Road, but there will be no restrictions, as were proposed, on entering Reno Road from side streets.

Also, Downs said, the curbside lane reserved for buses on Wisconsin Avenue between Cathedral Avenue and Calvert Street will be opened to all traffic.