Route 109 begins at Route 16 in Milford, just east of Route 85's southern terminus. The route proceeds to the east, meeting Interstate 495 (I-495) at exit 19 before crossing into Norfolk County and the town of Medway.
Route 109 commences at Route 16 in Milford, situated to the east of Route 85's southern endpoint. The route proceeds eastward, intersecting Interstate 495 (I-495) at exit 19 before entering Norfolk County and the town of Medway.