I-66 could channel about 700 additional cars an hour across the Theodore Roosevelt Bridge into the District in the morning and evening peaks, Washington 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 across the river, 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 crowded bridges, however. Every extra car using the Roosevelt Bridge will mean one fewer somewhere else. In particular, city officials feel 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 its roads 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 that 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 that 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 400 of the new vehicles will enter the E Street Expressway, Cross' projections show, and 200 will go onto Constitution Avenue. About 160 are expected to use the Potomac Parkway.

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 4,650. 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 the Potomac Parkway and other routes.

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," Cross said.

Cross is less sure what bridges will be helped by the new 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 some time for the situation to stabilize, as drivers experiment and settle on the new path of least resistance for their particular trips.