SOMERSET — Expect a detour if you’re going to be traveling along the Riverside Avenue portion of Route 138 after dark during the next two weeks.

MassHighway said yesterday the section of the highway that will pass under the road leading to the new Brightman Street Bridge will be closed from 9 p.m. to 5 a.m. beginning Tuesday and running through July 18.

The road will be opened for the weekend of July 12 and 13.

During the overnight closure, traffic will be detoured along Route 6 (also known as GAR Highway), Brayton Avenue and Read Street.

MassHighway spokesman Adam Hurtubise said the road is being closed because the overpass piers have been erected and workers must now install the beams that will cross Riverside, permitting construction of the ramp to the bridge, which is scheduled to be completed in 2012.

The closure is not related to the repaving of Route 138, which will take until the end of next year and is expected to begin next week or soon after.

gemery@projo.com