Route 12 begins at the Uhlerstown–Frenchtown Bridge over the Delaware River in Frenchtown at the Pennsylvania / New Jersey border, where it connects to
Pennsylvania Route 32. The bridge and short portion of its eastern approach is maintained by the
Delaware River Joint Toll Bridge Commission. two-lane undivided Bridge Street. Here, it intersects Trenton Avenue which is also the northern terminus of Route 29 before turning north onto Race Street and intersecting the southern terminus of CR 513. Route 12 heads east on Kingwood Avenue and passes through residential areas. The route crosses into Kingwood Township, where it becomes maintained by the
New Jersey Department of Transportation with two eastbound lanes and one westbound lane. As Route 12 climbs into the
Hunterdon Plateau, it heads through a mix of woods and farms with intermittent residences, narrowing to one lane in each direction at the top of the plateau before crossing CR 519 in
Baptistown. Route 12 continues southeast through rural areas, crossing the
Lockatong Creek before intersecting CR 615, where it becomes the Samuel Hill Highway. A short distance past this intersection, the route briefly passes through the southern corner of
Franklin Township before entering Delaware Township. Here, the road crosses
Wickecheoke Creek before coming to an intersection with CR 579. Past this intersection, Route 12 enters Raritan Township, where it continues through rural areas with some residences. The residential development along the road increases before it reaches the Dvoor's Circle (a
traffic circle) with CR 523. Past the traffic circle, the route crosses a small stream and enters Flemington. Here, Route 12 passes businesses and heads south of a
park and ride lot serving
Trans-Bridge Lines buses before crossing the
Black River and Western Railroad and coming to the Main Street Circle, a modern-designed
roundabout, where it intersects CR 611 and Reaville Avenue. From here, the route continues east a short distance as a four-lane
divided highway before ending at US 202 and Route 31 at the Flemington Circle. ==History==