Route 99 begins where North Washington Street becomes New Rutherford Avenue on the north side of the Charlestown Bridge. This occurs at the intersection with Chelsea Street, situated above the northbound tunnel of U.S. Route 1. Route 99 travels as a limited-access highway through Charlestown along New Rutherford Avenue, intersecting with ramps for U.S. Route 1 and passing under the Gilmore Bridge that connects Charlestown to Cambridge.
Route 99 commences where North Washington Street converges with New Rutherford Avenue on the northern side of the Charlestown Bridge. This occurs at the intersection with Chelsea Street, situated above the northbound tunnel of U.S. Route 1. Route 99 traverses as a limited-access highway through Charlestown along New Rutherford Avenue, intersecting with ramps for U.S. Route 1 and passing under the Gilmore Bridge that connects Charlestown to Cambridge.