A new baseball stadium in Southeast Washington would further strain some already overtaxed highways and streets in Washington on game nights, but traffic planners and engineers also say that a downtown stadium with Metro access could actually ease the evening rush.

Games played in an area with other diversions to bring out fans earlier and hold them later would keep tens of thousands of commuters off the roadways until late at night, they said.

"You have, obviously, the problem of accommodating a deluge of traffic, but there are some favorable aspects," said Robert Dunphy of the Urban Land Institute. "Spreading [traffic] out is really the game."

Traffic experts have long said that one of the simplest ways to relieve congestion is to space out the times that people use roads. There is plenty of capacity, they say, so long as drivers don't try to use the roads all at once.

Some commuters do this by leaving their homes at hours formerly restricted to insomniacs and cat burglars. Similarly, some cities have found that retail and entertainment options help keep people in town beyond the nightly rush.

A 41,000-seat downtown baseball stadium would have the same effect: Drivers who would normally join the jams that start at the 5 o'clock whistle will instead take to the highways about 10 or 11 p.m., one of the few times of day that it's not so hard to travel.

"The idea is that if you can actually find ways to spread out the commute, you can reduce the amount of impact," said Dan Tangherlini, the District's director of transportation. "By creating a big attraction like a baseball stadium, where 40,000 people will be at the baseball stadium during the peak, you've actually spread out the demand."

Tangherlini said the number of cars removed from peak traffic on game nights could be in the "tens of thousands," a significant percentage of the 150,000 to 200,000 cars that leave town during rush hour. "Even one or two percent is a noticeable portion," he said.

Still, there would undoubtedly be numerous traffic challenges on some regional roads and those around the stadium, officials said. Interstate 395, the major highway drivers probably would take through the city, and South Capitol and M streets at the stadium site figure to back up before and after games. Officials are especially worried about South Capitol, because it has become a crowded alternate route around security checkpoints that circle the Capitol.

Traffic watchers also expect that major roads leading into the city would become worse on game nights. Of particular concern is Interstate 66, the main highway leading into the city from Virginia's western suburbs, which backs up past 7 o'clock most evenings.

Walter Kulash, a traffic engineer at the consulting firm Glatting Jackson in Orlando, said that the same theory about spreading out rush-hour demand applies to ballpark traffic and that the key for the District will be whether it can draw people to the city during the hours before and after the game. "To us traffic engineers, that's as good as building new capacity," he said.

Plus, there are options. A trip from Tysons Corner, the region's biggest job center outside the District, took a manageable 40 minutes on a recent weekday evening -- and that's without wasting a minute on I-66. A quicker route from Tysons lies on the George Washington Parkway, which connects to the 14th Street bridge and onto South Capitol. Of course, the trip would take longer if thousands of fans followed the same route.

Then there is Metro. Between 33 and 50 percent of the people attending concerts and sporting events at MCI Center arrive by Metro, spokeswoman Lisa Farbstein said. When the Washington Redskins played at Robert F. Kennedy Memorial Stadium, about 25 percent arrived by Metro, she said.

For baseball, fans would exit at the Navy Yard Station's two entrances along M Street SE -- one at Half Street SE and the other at New Jersey Avenue SE.

Transit planners predict that the Green Line station would become "unmanageably crowded" by 2011 as a result of normal ridership growth, which now stands at about 3,000 passengers on an average weekday. Construction of a baseball stadium and resulting development may create crowding sooner than expected, Metro planners said. They might need to expand the station, with additional fare machines and fare gates. They also are considering plans to add a third entrance on N Street.

Tangherlini said officials are looking into whether a new downtown bus service planned for next year, known as the Circulator, could include a route to the stadium.

When the team plays at RFK Stadium while a Southeast stadium is being built, officials said, they expect game-time backups on roads around RFK, similar to what now occurs at soccer games and concerts. They said that was largely because there are few attractions around RFK to help stagger arrival and departure times and that most people who go there drive.

Officials in San Diego, where the Padres baseball team moved to a downtown stadium this year, said they have found that such a site can handle traffic relatively well, and far better than can a suburban stadium such as FedEx Field.

"We were frankly amazed at how good it has gone in the first year," said Bruce Herring, deputy city manager.

Herring said the major advantage of an urban site is the number of ways people can get to games. He said 15 to 20 percent take public transit, many walk, and drivers have a number of options in getting to the park. As for whether a downtown stadium has had any effect on regular rush-hour traffic, Herring said, "It helps it a little bit. It has not made it any worse, that's for sure."

Staff writer Lyndsey Layton contributed to this report.