For two small stretches – in Clifton at the interchange with Route 19, and in Paterson as it crosses over the Passaic River – CR 509 splits into separate northbound and southbound alignments.

At the Union County line, the CR 509 designation ends and Raritan Road continues into Scotch Plains.

[3] The signed mainline portion of CR 509 begins at a complex intersection in the town of Westfield, seven miles (11 km) west of Elizabeth.

State Route 28 (South Avenue) and Union County Route 610 (North Avenue) come from Scotch Plains to the west on either side of the train tracks running along New Jersey Transit's Raritan Valley Line and criss-cross at a traffic circle a few hundred feet from the Westfield Train Station.

(East Broad Street continues straight as Nomahegan Drive for less than a mile before coming to a dead end).

It crosses back under the Garden State Parkway before moving into Passaic County, through the cities of Clifton and Paterson, where it ends at CR 504.

The two segments of CR 509 were previously connected through southern Union County via Raritan Road, Martine Avenue, West Broad Street, and Route 28.