Drivers on U.S. Highway 12 near Helena should expect lane closures, detours, and a reduced speed limit of 35 mph along a stretch of Prospect Avenue beginning Monday, May 4, and running through Wednesday, August 19, 2026. The construction project spans milepost 45.7 to 46.1 on US-12 B, with work affecting both eastbound and westbound lanes at several key intersections.

The most significant impacts will be felt at the 18th Street and Carter Drive intersections. While work proceeds through the 18th Street intersection, left turn lanes in both directions onto 18th Street will be closed. The Montana Department of Transportation advises that drivers heading to businesses north of Prospect Avenue use Carter Street or Gibbons Street as alternatives, while those needing to reach businesses south of Prospect Avenue use North Street to access 18th Street. When work shifts to the Carter Drive intersection, the eastbound left turn lane will be closed, and drivers can use 18th Street to reach businesses north of Prospect Avenue.

The project is expected to run Monday through the third week of August, with left turn lanes intermittently closed as construction operations move east along the corridor. Motorists are encouraged to allow extra travel time during the construction period, particularly during peak morning and evening hours. Real-time traffic information is available at 511mt.net.