The rebuilding of the Whitehurst Freeway, a plan that first was discussed more than 20 years ago, will begin next month, fouling up traffic for thousands of drivers from the Capital Beltway to downtown Washington over the next 2 1/2 years.

Unlike any city project in recent years, the Whitehurst renovation will affect more than just the drivers of the estimated 50,000 vehicles that use the elevated freeway along the Georgetown waterfront each day. Inbound and outbound traffic will be slowed in various places throughout the region, usual commuting routes altered and some waterfront businesses disrupted.

The problem with this project, according to District officials, is that the four-lane freeway will be reduced to two lanes, and all traffic will move in one direction. Drivers will be able to travel west, or outbound, only between 2:30 and 9 p.m. weekdays. At other times, the two lanes will be open for cars traveling east, or inbound.

The renovation also will require the closing of the Whitehurst ramps to and from the Key Bridge. That, plus the change to a one-way road, will undoubtedly result in the diversion of traffic to other crowded roads such as M Street in Georgetown, the George Washington Memorial Parkway in Virginia, the Theodore Roosevelt Bridge and Rock Creek Parkway.

Traffic on the Capital Beltway also will be affected. Officials are asking Maryland drivers who normally come into Washington from Beltway exits before the American Legion Bridge to continue across the bridge into Virginia and take the George Washington Parkway, especially during non-rush hour periods.

"Until motorists learn new routes, they're going to get caught at the changeover," said George W. Schoene, the city's chief traffic engineer.

The exact day of the changeover is uncertain. Some initial work will begin Nov. 5, Schoene said, but the one-way traffic pattern will begin in early December, probably after the resurfacing and widening of the Roosevelt Bridge is completed. At some point, the Roosevelt Bridge may be configured to four lanes in the rush-hour direction.

There was some doubt whether the project to rebuild the half-mile bypass around Georgetown would proceed at all. The city's plan to fix the structure was challenged by several residents and organizations, some of whom preferred replacing the freeway with a grand boulevard similar to Lake Shore Drive in Chicago. Despite last-minute pleas, District officials said yesterday they will start the work next month.

When the Whitehurst project is finished, the rebuilt roadway will have two lanes in each direction, as it does now, but also will be widened eight feet to include shoulders on both sides for emergencies. The project will cost $35 million to $40 million, of which 80 percent is federal money.

The westbound ramp now used especially by Virginia-bound drivers to enter the Key Bridge will be closed during construction, affecting about 10,000 vehicles a day. It will be replaced by a new, third lane coming off the Whitehurst that takes drivers to the signal at M Street and Canal Road NW to make the right turn to the bridge. Until then, officials are asking drivers to use the Roosevelt or Memorial bridges.

The ramp leading from the Key Bridge to the Whitehurst, also used by about 10,000 vehicles a day, will be closed for the first seven months of construction. In the meantime, drivers will be asked to take the George Washington Parkway to the Roosevelt, Memorial or 14th Street bridges.

To spruce up its appearance, the Whitehurst will be painted in two tones of gray. New, brighter lights will be installed above and below, along lower K Street, and the supporting columns will be redone in a faux, or false, stone. K Street along the waterfront will be upgraded after the Whitehurst project.

Perhaps best of all, "There will be no problem with potholes for many years," said Gary Burch, the District's chief engineer. The Whitehurst is notorious for its many potholes.

Nothing has been easy for the Whitehurst, named for a former District highway official and built in 1949 as a bypass of Georgetown connecting Key Bridge, Canal Road and M Street with downtown Washington.

The city and residents living near the freeway have been talking about tearing it down since the 1960s, but it wasn't until 1982 that the current plan emerged.

Opposition to keeping the Whitehurst centered mostly on aesthetics.

An alternative proposal by Washington architect Joseph Passoneau got the most attention; that called for replacing the Whitehurst with a six- to eight-lane boulevard. He said the plan would cost slightly less than the city's and would serve traffic better.

District officials found Passoneau's plan more expensive, unsafe and inefficient in moving through traffic around Georgetown. The National Capital Planning Commission, the planning agency for the federal government, sealed the project's fate in January when it approved the city's plan by one vote.

Getting approval was hard enough, but District officials encountered more trouble with the narrowness of the freeway. For crews to rebuild two lanes at a time, the median will have to be torn out and two feet to four feet of the other two lanes will be needed. That will make the two open lanes so constricting and unsafe that officials decided they could be used only in one direction.

Eastbound traffic will have the longer use of the freeway because traffic coming from Northwest and Maryland has fewer alternate routes than westbound traffic. For example, when the Whitehurst is shut, eastbound drivers probably would flood onto M Street in Georgetown, which is already congested. Westbound drivers can take several routes around the Whitehurst.