The first new highway lane between the District and Virginia in 20 years opened last night on the Theodore Roosevelt Bridge.

For the rest of this week, the new fourth lane on the bridge, which crosses the Potomac River and connects with the George Washington Memorial Parkway, Interstate 66 and Route 50 (Arlington Boulevard), will be open only during the evening rush hour. Next week, the lane is expected to be open all day.

The 26-year-old bridge is the preferred route for thousands of commuters, especially from Northern Virginia, who have been slowed more than usual by eight months of construction.

Nearly 100,000 vehicles a day travel over the Roosevelt Bridge, and the count is expected to increase during the next year as drivers who normally use the Whitehurst Freeway seek other routes while the Whitehurst undergoes major reconstruction.

By next Monday, D.C. public works spokeswoman Tara Hamilton said, officials plan to open the fourth lane all day. Crews are finishing work on the median of the bridge, which is why drivers will continue to be restricted by orange barrels until next week.

Originally, District officials planned to add the fourth lane in the inbound, or eastward, direction to relieve the severe backups that occur during the morning rush hour. Traffic frequently creeps at 15 miles an hour in the morning -- or stops altogether -- as about 6,500 vehicles an hour crowd onto the bridge. The evening rush hour averages 5,000 vehicles an hour.

But the Whitehurst project, which begins next month, forced a change in plans. During reconstruction, the elevated freeway along the Georgetown waterfront will be reduced from four lanes to two lanes over the next 15 months, and all traffic will move in one direction. Outbound, or westward, traffic will be able to use the Whitehurst only from 2:30 to 9 p.m. weekdays. At other times, the two lanes will be open inbound, or eastward.

With additional outbound traffic using the Roosevelt and Memorial bridges, officials decided to configure the Roosevelt so the fourth lane would be used by outbound traffic. The fourth lane will revert to inbound traffic when the Whitehurst project is completed.

Outbound drivers reach the Roosevelt Bridge from ramps off Constitution Avenue and the E Street Expressway.

The District may get permission from federal officials by next summer to use a movable concrete median barrier on the Roosevelt Bridge that will allow workers to move the barrier each day so the fourth lane is in the rush-hour direction, officials said.

The $2.9 million Roosevelt Bridge project involved restriping the existing six lanes, instead of actually widening it to include the new lane.

The lanes were narrowed from 12 feet to 11 feet, and the median was reduced from six feet to two feet and replaced with a barrier.

District officials were prodded by Rep. Frank R. Wolf (R-Va.) to add an inbound lane, but initially balked because they did not want to increase traffic in the downtown area.