The most disruptive phase of the project to rebuild the bridge south of the Harbor Tunnel toll plaza is expected to begin Saturday and continue for two weeks.
The four-lane Interstate 895 bridge will be narrowed to one lane in each direction as crews repair the steel structure and replace the concrete surface.
Maryland Transportation Authority officials are asking motorists to use alternate routes, especially during rush hours, to avoid traffic snarls. A state traffic analysis predicted that backups could stretch up to four miles at times during the two-week period.
The bridge averages about 71,100 vehicles a day. During the peak of the morning and evening commutes, it handles about 5,600 vehicles an hour.
After the two-week phase, from mid-August on, "we're not going to have continuous, single-lane closures," said John Sales, an agency spokesman.
The project is expected to be completed in October.
During the final 45 days of work, traffic in both directions will shift to two outside lanes — each narrowed by about a foot — to allow construction in the center portion of the bridge.
The span, called the K-Truss Bridge, opened in 1957. The name is derived from the shape of the bridge's steel superstructure. About 18 months ago, state engineers concluded after tests of the concrete deck that the bridge needed a major overhaul.
Officials timed construction so disruptions would occur in the window between Sailabration in June and the Baltimore Grand Prix on Labor Day weekend.
The nearly $3.9 million project is being funded by toll revenue.