Drivers on Interstate 66 near Manassas should expect nighttime delays of up to an hour for the next two months as half the highway is closed while workers widen it, Virginia transportation officials said.

Starting tonight, four miles of the westbound lanes of I-66 will be closed from 9 p.m. until 5 a.m. Monday through Thursday nights and from 9 p.m. Saturdays to 9 a.m. Sundays. There will be no Friday night closures to accommodate the legions of people who travel west on I-66 for weekend trips.

Motorists heading west will be detoured to Exit 47B, where they will take Sudley Road (Route 234) to Lee Highway (Route 29), which they will follow until it meets I-66 at Exit 43.

Transportation officials said local and detouring traffic will slow those roads, and they anticipate delays of 45 to 60 minutes. There aren't many alternatives for local drivers, but long-distance travelers could use Routes 15, 17 or 50 if they want to avoid the area.

The westbound detour will last at least two weeks, and then the detour will be shifted to the eastbound side for at least the next two weeks. The eastbound detour is the reverse of the westbound one.

Officials with the Virginia Department of Transportation said the closings will alternate roughly every two weeks for the next two months to make it easier for construction crews to work.

Officials also said the delays and detours are necessary so workers can remove four miles of old pavement, four lanes wide, and add several inches of new pavement to a widened road. The aggressive closing plan allows VDOT to finish the work in 60 days instead of 90, officials said.

"The original plan was to close 66 to one lane and funnel everyone through the construction project," said VDOT spokesman Ryan Hall. "But it would have been the same delays to the motoring public," while increasing danger to drivers and workers.

The work is part of a 3.9-mile project that will widen I-66 from two to four lanes in each direction between Sudley Road and the Route 234 bypass. Motorists will have three regular lanes and one high-occupancy vehicle lane in each direction. Construction on the $37.1 million widening began last August and is expected to be completed in October 2006.

The stretch outside Manassas is the site of one of the most regular jams in the Washington region. The westbound lanes of the highway, which is the primary commuter route between the western suburbs and job centers in Dulles, Tysons Corner and the District, narrow quickly there from four to three to two lanes. That virtually guarantees a lengthy backup most days, including weekends.

The project is expected to relieve some of that jam by extending the expanded highway west of Manassas, where many drivers exit. But officials acknowledge that jams will remain because the state doesn't have enough money to widen I-66 all the way to Gainesville, which has grown by tens of thousands of residents in recent years. State plans call for construction to begin on that part of the highway in 2008.

Eastbound closures will last from 8 p.m. to 4 a.m. Monday through Thursday nights and from 9 p.m. Fridays and Saturdays until 9 a.m. Saturdays and Sundays. There will be no eastbound closings Sunday nights so that weekend travelers can return.

VDOT officials said they will alert the public a week before closures switch from one side of the highway to the other.

Officials said no closings will take place from Friday night through Monday night over Labor Day and Columbus Day weekends, during events at the Nissan Pavilion, or from Sept. 18 to 25 for the Presidents Cup golf tournament in Gainesville.

Prince William County officials said they would be monitoring the area.