The District of Columbia begins the long-awaited renovation of the Southeast/Southwest Freeway next month, a five-year, $50 million project that will create detours for thousands of motorists and change commuting patterns throughout much of the metropolitan area.

"Everybody crossing one of the main bridges into the city, and maybe even as far away as Chain Bridge, is going to feel the impact of this project," said George Schoene, the city's director of traffic engineering. "That's the magnitude."

Nearly 150,000 motorists a day use the Southeast/Southwest Freeway, which spans the southern portion of the city between the Potomac and Anacostia rivers. The 20-year-old highway is the major artery into the center of Washington from Alexandria, southeastern Fairfax County, the southern half of Prince George's County and southeastern Washington.

The project, which is expected to be completed in 1991, involves resurfacing nearly three miles of road, rebuilding deteriorated concrete joints and renovating the 33 ramps leading on and off the roadway. Other improvements include replacing the highway's rickety steel guardrail with safer concrete barriers, improving lighting and erecting better signs.

"Put all that together with 150,000 vehicles a day, and you'll have a sense of the challenge that's facing us," Schoene said.

The project is supposed to be completed in six phases, the first of which will affect the section between the 14th Street bridge and Ninth Street. The District has awarded a $5.4 million contract to the Fort Myer Construction Corp. of Washington for the first phase.

To avoid mass confusion, D.C. officials have launched a local public relations blitz and printed brochures. The message is simple: Prepare for detours and delays, and, if possible, leave the car at home.

"We really need to get motorists out of their cars, whether it's into Metro or into a car pool," Schoene said.

District transportation officials have approached Metro about adding subway cars on the Blue Line and are working with their counterparts in Maryland and Virginia to spread the word about the project, he said.

The Southeast/Southwest Freeway was one of the last major arterials built in the District. It connects the Potomac River bridges at 14th Street SW with the Sousa Bridge across the Anacostia River.

The road originally was supposed to link up with the Center Leg Freeway, which was never built but which would have traversed the city and joined with the Capital Beltway to the north. Neighborhood opposition defeated that project in the 1960s, and all that remains is the Center Leg Tunnel.

Time, weather and steadily increasing traffic volumes have taken their toll since the Southeast/Southwest Freeway opened in the mid-1960s. In particular, the road is renowned among motorists for its bone-rattling washboard surface.

District officials blame that problem on deteriorating joints between the concrete slabs that make up the roadbed. Spaced at 12-foot intervals, the joints consist of gaps that are supposed to allow for the expansion and contraction that accompanies changing weather.

Many have crumbled over the years, causing "that symptomatic thump-thump," said Gary Burch, the District's chief of design and engineering. Burch said the damaged joints will be filled with asphalt and then the road will be resurfaced.

"It's generally not a very difficult engineering problem," he said. "The difficulty is that there are so many autos and people who will continue to use it, and that complicates the way we have to do it during construction."

The city's traffic engineering department has worked out an elaborate system of detours for motorists who continue to use the freeway. During the second and third weekends in August, for example, automobiles traveling in westbound lanes of the freeway will be detoured between Seventh and Ninth streets SW and directed down Maine Avenue to 14th Street. Trucks will be detoured to 14th Street via 12th and C streets SW.

Schoene said, however, that the detours will accommodate only about half the hourly traffic volume on the road. "We really need to tell people, 'Go somewhere else,' " he said.

District officials said the detours in August will be confined mostly to weekends. In September, work will begin and continue every day on westbound lanes between Ninth and 14th streets SW, forcing the closure of one of three lanes.

But Schoene said the worst of the delays will not begin until spring, when resurfacing begins on eastbound lanes between the 14th Street bridge and Ninth Street SW.

That undertaking will mean delays not just for motorists using the freeway, but will cause backups on the bridge as commuters stream toward downtown D.C. in the mornings, officials said.

"Each stage is going to get worse," Schoene said.