Managers of the Woodrow Wilson Bridge project are warning once again of the potential for historic traffic jams while workers realign a second section of the Capital Beltway this weekend.

A similar call went out in mid-July, but fears of monumental backups proved to be unfounded. In fact, highways around the bridge looked more like the free and open roads that commuters dream about than the parking lots officials anticipated.

But project managers cautioned that this time could be the real thing -- especially if everyone takes a blase attitude after last month's success -- and are again warning of 15-mile backups, 90-minute delays and the worst traffic tie-ups in the project's 11 years of construction.

Bridge officials advised drivers to take the western half of the Beltway, Route 301 through Maryland or Interstate 395 through the District to avoid bridge traffic.

Even if 75 percent of drivers avoid the area, "we're still looking at 10- to 15-mile backups," said project spokesman John Undeland. "That's what the projections show."

The inner loop of the Beltway will be reduced to one lane from 8 p.m. tomorrow to 5 a.m. Monday. Severe weather could delay work, and a final decision on whether to go ahead will be made at 6 p.m. tomorrow.

Undeland said traffic problems are more likely this time because there is more work to do and because it's harder to divert traffic in Maryland than it was in Virginia, where the lane closures were last time. In that case, drivers were detoured onto the western half of the Beltway and I-395 at Springfield.

Undeland also said there is considerably more traffic on the inner loop, where this weekend's work will take place. On a typical Saturday in August, there are about 81,000 vehicles on the inner loop, compared with about 72,000 on the outer loop.

To make their assumptions about long delays, bridge officials simply weigh traffic against lane capacity. For instance, the inner loop normally can carry 6,240 cars an hour at 60 mph. That's enough to handle traffic during most times except morning and evening peaks.

But drivers don't go 60 on a single lane through a major construction zone. Bridge officials estimate that traffic might go about 45 mph, meaning 1,930 cars would get through each hour.

By 7 a.m. Saturday, traffic projections show, there probably will be 2,397 cars trying to get through the work zone. That traffic volume will probably peak at 5,396 by 2 p.m., according to the projections, setting the stage for a massive tie-up.

The number of vehicles and length of backups could continue to expand well beyond the peak as more cars get stuck in the work zone.

Bridge officials hope their warnings will keep most people away and prevent that scenario, but even a quarter of normal traffic could pile cars for up to 15 miles, and an accident or other incident could make it even worse, they said.

For all of Washington's everyday struggles with traffic, drivers have gotten pretty good at staying away from major events when they are told to. In addition to clearing the highways last month, Washingtonians have in recent years stayed off the roads during Ronald Reagan's funeral, NATO's 50th anniversary, protests of the World Bank and other major events.

"If people are informed ahead of time and take a breath, it does work remarkably well," said Bob Marbourg, veteran traffic reporter for radio station WTOP.

Marbourg cautioned that drivers shouldn't think that just because things went well at the bridge last time, there won't be anything to worry about this weekend. "Everybody needs to take seriously the potential delays and do everything they can to plan a route that takes them away from the area for the weekend," he said.

During the work period, the inner loop will be reduced to one lane from just before the Interstate 295 interchange in Maryland to the Route 1 interchange in Virginia.

Ramps from Interstate 295 south and Route 1 to the inner loop will be closed, as will ramps from the inner loop to Route 1 and Church Street (to Mount Vernon). The Church Street ramp will remain closed for the next three years. There will be posted detours for all routes.

The work being done this weekend will connect the inner loop to a newly paved stretch of road on the Virginia shore. Project officials said it is necessary to close Beltway lanes in Maryland so they can be blocked off gradually and give drivers time to merge.

Workers will lay 1,500 tons of asphalt that will rise up to 24 inches. They need all weekend to do that because only about two to three inches of asphalt can be laid at a time, and it has to cool before another layer can be added.

The highway switch is necessary to build an overpass at South Washington Street in Alexandria.

The work is part of a new, 12-lane, $2.43 billion Woodrow Wilson Bridge that will replace an aging structure that carries nearly 200,000 vehicles a day. It will be replaced by two six-lane spans, the first of which is scheduled to open in the spring.

The Beltway at the bridge will be widened to 12 lanes, and several interchanges will be reconfigured to handle the expanded highway.

Work on the outer loop finished nearly 24 hours earlier than expected last month, largely because surveyors discovered that they didn't need as much asphalt as they thought and because everything went smoothly. Bridge officials said they can't plan for such luck this weekend and are allowing extra time for what they call the "oops factor."

A construction worker lays asphalt on the existing Wilson Bridge in July.