At Stoutsburg, CR 518 crosses the Keith line and continues into Montgomery Township, Somerset County, passing farms, woods, and residential subdivisions. The road crosses CR 601, at which point it turns to the east and continues to an intersection with US 206/CR 533 a short distance north of Princeton Airport in a commercial area. East of US 206/CR 533, the route enters Rocky Hill, where the name becomes Washington Street, and passes wooded residential areas, intersecting CR 605 in the center of the community. From this point, CR 518 enters woodland and crosses the Millstone River, where it enters Franklin Township and becomes Georgetown-Franklin Turnpike. Immediately following the Millstone River, the road crosses the Delaware and Raritan Canal and reaches the CR 603 intersection. The route ascends Ten Mile Run Mountain and passes through more forests to the north of a quarry, climbing a steep hill, and curves to the northeast, flattening out into a mix of farms, woods, and homes. CR 518 reaches its eastern terminus at an intersection with Route 27 (though which the border between Franklin Township, Somerset County and South Brunswick, Middlesex County runs).