A newly built Route 1 bridge that spans the Capital Beltway in Alexandria is scheduled to open this weekend, a major step in fixing one of the region's most confusing and congested interchanges.

Drivers are advised to avoid the area Saturday afternoon and all day Sunday as workers close ramps and lanes to switch traffic to the bridge, which is slightly east of the current roadway.

The biggest impact will be on drivers heading south on Route 1 from Alexandria. Southbound traffic will not be able to cross the Beltway for much of Sunday. Drivers will be detoured onto the inner loop of the Beltway to the Telegraph Road exit, where they will get back on the Beltway and take the outer loop to the Route 1 South exit.

Also Sunday, northbound Route 1 traffic over the Beltway will be reduced to a single lane.

The $147 million reconstruction of the Route 1 interchange is a key part of the Woodrow Wilson Bridge project. The interchange bridge that will open this weekend is one of two three-lane spans that will carry Route 1 traffic across the widened Beltway. The second bridge will be built partly in the footprint of the current four-lane Route 1 bridge, which will be demolished this year.

"We can't build it until the old bridge is out of the way," said John Undeland, spokesman for the bridge project.

Once completed next year, the bridges will expand Route 1 from four to six lanes. The entire interchange makeover is expected to be finished by 2009, according to project officials.

The twin-span Wilson Bridge over the Potomac River is mostly complete, with one of the bridges carrying traffic and the other scheduled to open in summer 2008.

The $2.44 billion project also includes four interchange upgrades, two each on the Maryland and Virginia shores.

On the Maryland side, the Interstate 295 interchange is about three-quarters done and the Route 210 interchange is a little more than half finished.

In Virginia, work on a revamped Telegraph Road interchange, which will be the final piece of the project, has only recently gotten underway.

The Route 1 interchange project is expected to bring substantial relief to the 115,000 daily drivers. The old interchange has long been problematic and confusing, sometimes sending drivers on an unwanted -- and often lengthy -- detour across the Wilson Bridge. And the volume of traffic had overwhelmed the interchange long ago.

The old interchange also had dangerous merges, said Richard Baier, transportation director for Alexandria. "The new lanes will help traffic flow that much better," he said.

Since the Route 1 project began in 2004, temporary ramps have been used to direct traffic through the interchange. Those ramps, which are not lighted at night and require drivers to go excessively slow, have brought their own challenges, Baier said.

The switchover will begin Saturday after 2 p.m., when outer loop traffic exiting to Route 1 north will be directed onto a temporary ramp that will bring traffic across the new bridge.

On Sunday, beginning at 4 a.m., southbound traffic on Route 1 will be diverted at the Beltway. The work could continue to 10 p.m.

The work was originally scheduled for Friday night and Saturday, but project planners pushed it back a day at the request of Alexandria officials so it wouldn't conflict with the city's St. Patrick's Day parade.