The area called Imlaydale is far older than the Warren County that it exists in; it has been a farming area since 1730 or earlier, but never had more than half a dozen homes, a mill, and a small general store. The micro-hamlet of Imlaydale had a mill by 1792,and a mill on that spot operated until ca 1950. The half dozen buildings in town all date back to the mid 19th Century. The old "Clinton/Washington road" was just a series of connected dirt roads that ran generally north/south, going from hamlet to hamlet; that path predates several of the little towns along the route, including Clinton and Washington. [Oxford Furnace ca 1741 just north of Washington, High Bridge (Union Forge) just north of Clinton also was mid 18th Century ironworks, Clinton sits on an east-west route (now Rt78) that goes back to Dutch colonial era or older] When it was built, the Imlaydale bridge was really an extension of Main St in Hampton, the same route that was also Main St in Glen Gardner, which is the road that followed the Spruce Run creek north out of Clinton. This was most likely what was referred to as the Clinton/Washington road, or at least the upper part of it.
The concrete bridge at Imlaydale is half a mile downstream from Cowin's famous 1868 iron bridge at Shoddy Mills Rd, and a mile upstream from the 1910 Valley Road steel bridge.
When NJ formed it's Highway Department in 1916 they set a modern standard that all bridge roadways had to be at least 30 feet wide, so Imlaydale bridge was obsoleted even though it was only 3 years old. A bit more than a decade later the state built Route 30, a modern paved "high speed" road that skirted Clinton, Glen Gardner, Hampton, New Hampton, Imlaydale and effectively cut those towns off for many decades. Route 30 is now called Route 31, and hundreds of vehicles blast by every hour just 50 yards away, as the Imlaydale Bridge lies forgotten over the Musconetcong River.
Prior to the new concrete bridge, there had been a wooden plank bridge across the Musconetcong River behind the mill. At some point after, those plank bridges were rebuilt, and shortly thereafter replaced with two short Warren truss pony bridges, one a single span and the other a two span. These bridges did not last until 1954, although the stone pier in the middle of the river that had supported the old wood bridge and later the 2 span pony still exists. These little bridges are not listed anywhere, ever, so they may have been privately built for the convenience of farmers trying to get their wagons to the mill.