I-66, the four-lane superhighway opening today in Virginia, could channel about 700 more cars an hour across the Theodore Roosevelt Bridge into the District during rush hours, city traffic engineers predict. That would worsen tie-ups on Constitution Avenue, the E Street Expressway and the Potomac Parkway.

Projections of this sort are guesses at best, and residents of Foggy Bottom and Georgetown, some of whom fought in the 1970s to block construction of the 10-mile highway, will watch in coming weeks to see precisely how the change affects noise and congestion around their homes.

I-66 could mean relief for some other crowded bridges, however. Every extra car using the Roosevelt Bridge will mean one fewer somewhere else. In particular, city officials believe I-66 may draw vehicles from the express lanes at the 14th Street bridge, which would ease a traditional choke point there.

And if restrictions limiting the road to car pools and other special traffic during peak hours succeed in luring more Virginians out of single-passenger cars, the District will enjoy an important long-term benefit: fewer cars will enter the city and clog its streets, pollute its air and destroy its pavement.

Original designs had I-66 feeding into a grid of new interstate highways in the District looping around the downtown core. But after years of civic battling, the District canceled most of those road plans in the early 1970s and diverted most of its federal funds to subway construction.

Virginia, however, pressed ahead with its end of the plan and built I-66 in from the Beltway to the river. With Rte. 50 and the George Washington Memorial Parkway also feeding the Roosevelt Bridge, that has created a classic bottleneck: speed expressways channeling traffic abruptly onto a narrow bridge and an urban grid of traffic lights, stop signs, potholes and 25-mph speed limits.

D.C. traffic officials predict the Roosevelt Bridge, already heavily traveled, will be pushed to capacity at rush hour. "Thirty inches between bumpers but moving" is how city traffic engineering director Seward Cross describes it.

Cross said about 5,710 vehicles now cross the Roosevelt Bridge into the District between 7:30 and 8:30 a.m. on working days. City engineers predict that once I-66 opens, assuming there is unrestricted access to the bridge from the George Washington Parkway, that volume will rise to 6,485 vehicles.

About 375 of the new vehicles will enter the E Street Expressway, Cross' projections show, and 235 will go onto Constitution Avenue. About 160 are expected to use other routes.

Between 4:30 and 5:30 in the afternoon, about 3,935 vehicles now use the bridge to leave the District, a number that is projected to go up to 4,670. The extra load is expected to increase congestion on roads leading to the bridge as follows: 310 on Constitution Avenue, 230 on the E Street Expressway and the remainder on Rock Creek Parkway and Ohio Drive.

Cross said traffic of this level would not create major new jams. After the actual levels are monitored, the city expects to change traffic signal timing on Constitution Avenue and may tinker with parking restrictions. But "based on these numbers, we see no need to go out and dramatically change things," he said.

Cross is less sure what bridges will be helped by the additional cars on Roosevelt Bridge. One major beneficiary, it is expected, will be car-pool lanes on Shirley Highway that feed the 14th Street Bridge. Car pools now using Key Bridge and Chain Bridge might also find the trip to work faster on I-66, he said. But their loss might be offset by non-pool cars trying to escape new tie-ups on the Roosevelt Bridge.

It will take weeks or days to stabilize, as drivers experiment and settle on the new path of least resistance for their particular trips.

Civic groups will be watching with interest. Foggy Bottom residents have been concerned for years about cars using north-south residential streets in their neighborhood and about growing traffic on local through streets and the Whitehurst Freeway. If Constitution Avenue and E Street get a new load of cars, so could Foggy Bottom's side streets.

Jenny Brake, a Foggy Bottom resident taking part in a city-sponsored study into rebuilding or replacing the Whitehurst Freeway, said it remains unclear what effect I-66 would have on her neighborhood.

But she said: "I'm not very happy about constantly catering to more automobiles. I think much, much more attention should be paid to the Metro."

In Georgetown, the concern is that the exits on I-66 that will let drivers leave the highway near downtown Rosslyn and cross Key Bridge into the District will increase Georgetown traffic.

"There's a chance to get off before they pile up on the Roosevelt Bridge. I think a lot of people will take that chance," said Don Shannon, president of the Citizens Association of Georgetown. That could mean more traffic on already heavily congested M Street and Whitehurst Freeway, he said.