Construction will begin on the Joshua Chamberlain Bridge next week.
The Maine Department of Transportation will begin its work on the bridge connecting Bangor and Brewer on Monday.
That will require reducing the bridge to one lane of one-way traffic into the fall, allowing traffic to head westbound from Brewer to Bangor.
Bangor traffic traveling to Brewer will be detoured to the Penobscot Bridge which carries Route 15 (State Street) over the Penobscot River.
Just one sidewalk will be open during construction.
The $5 million project includes replacing bridge joints, making structural repairs and resurfacing the road. Originally, the project was estimated to cost $1.5 million, but subsequent inspections found more work was needed.
The Bangor Police Department on Friday morning urged drivers to seek alternative routes and to leave themselves extra time if they are traveling between the two river cities.