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 begins at Route 16 in Milford, then proceeds to the east, intersecting with I-495 at exit 19 before entering Norfolk County and the town of Medway.