Workers last night began a critical but disruptive phase of the Woodrow Wilson Bridge construction project, and project managers renewed their warning to weekend drivers: Stay away.

A forecast that includes heat, humidity and occasional thunderstorms will not stop the work, construction managers decided last night, saying they had built some flexibility into the 57-hour work schedule.

The weekend-long project involves paving a new segment of the Capital Beltway on the Alexandria side of what will become the new Wilson Bridge.

The plan included a widespread diversion of traffic from Interstate 95 as it approaches the Beltway from the south. The area is one of Washington's most heavily traveled gateways and on normal days provokes abundant frustration among drivers.

Backups on the northbound lanes of I-95 below the Beltway could mean delays of 60 to 90 minutes. Project managers think the worst of it may be this afternoon, though the lane closures and detours are not scheduled to end until 5 a.m. Monday.

The plan called for diverted traffic to flow west around the Beltway and north to Interstate 395 and Interstate 295.

Early this morning, with the diversion scheme in its early hours, it seemed to be working relatively well. A survey of VDOT traffic cameras showed traffic flowing with little delay on the Beltway and on I-95 south of the Beltway. A two-car accident on the Beltway's outer loop near Telegraph Road delayed traffic for a time.

"This is going to have major regional spillover effects," said John Undeland, a spokesman for the bridge project. "It's going to be a shock to the transportation system. Do yourself a favor and steer clear."

Not everyone is so eager to keep people out of the area.

Along the Richmond Highway in Alexandria, past the construction equipment, the Ourisman Dodge Suzuki dealership has posted a sign that reads, "We're Open."

For the past two years, car buyers have had to search for the dealership's entrance because of construction near the bridge.

This weekend, the dealership will offer an extra incentive to car buyers.

"Anybody who's brave enough to come in despite the Wilson Bridge is going to get an extra $300 for their trade-in," said Jim Boone, general sales manager.

The Beltway's outer loop between Route 1 and the Potomac River will be restricted to one lane. Several detours will be put in place to reroute outer loop traffic around that stretch, which is nearest to the construction zone.

Drivers northbound on I-95 have two primary choices as they approach the Beltway.

Through travelers will be advised to loop around Washington using the western half of the Capital Beltway by taking I-495 west.

Those headed for the District or Prince George's County will be advised to take I-395 and, if appropriate, I-295.

The ramps from Route 1 to the outer loop also will be closed.

Police and motorist assistance patrols in Virginia, Maryland and the District are stepping up their roadway monitoring.

Some of the patrols will be able to dispense water and gasoline. They will be able to remove disabled vehicles to keep traffic flowing.

Kellie Boulware, speaking for the Maryland State Highway Administration, said officials expect more traffic coming over the American Legion Bridge.

"We're preparing for a severe backup on the Beltway, but we're hoping that motorists will try to avoid the construction area altogether," Boulware said.

For those heading east and southeast of Washington, or all the way around, Route 301 was recommended as a possible route.

Radio ads in the Outer Banks of North Carolina, Virginia Beach and Richmond are suggesting that travelers go around Washington using Route 301 to Baltimore and I-95, Undeland said.

For those heading west, bridge officials suggested the possibility of taking Route 17 from just north of Fredericksburg and, if necessary, Interstate 81 farther west.

Staff writers Steven Ginsberg, Jenalia Moreno, Martin Weil and Lindsay Ryan contributed to this report.