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. The route continues through the downtown area of Frenchtown as county-maintained 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 developmentalong the road increases before it reaches the Dvoor's 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 & 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
Route 12 was legislated as a new route in the 1927 New Jersey state highway renumbering that was to run from Frenchtown east to Route 28 in Raritan borough, running along its current alignment to Flemington and following present-day US 202 to Raritan. By the 1940s, US 202 and Route 29 were designated along the alignment of Route 12 between Flemington and Raritan, cutting the route's eastern terminus back to Flemington. In the 1953 New Jersey state highway renumbering, Route 12 was officially legislated to run from Frenchtown to US 202 and Route 69 in Flemington. There are currently plans to modify or eliminate the three traffic circles along Route 12 in Flemington in order to ease traffic congestion along the route. The Main Street Circle was converted to a modern roundabout in 2010 while plans are still being thought out for the other two traffic circles.
