Old roads and construction (then called Georgetown) to
New Brunswick. The road also served as a highly used connector route for the Jersey and Castenoga wagon lines, which served both of the aforementioned communities. The route from Trenton to Pennington was part of the
Pennington Turnpike, another privately maintained pre-designation highway. The turnpike was chartered on March 17, 1854, by the General Assembly along an alignment of the old Pennington Road. The turnpike road was to be constructed no further than in breadth, and to be made out of stone, plank or gravel. The turnpike cost one cent for every carriage or sled pulled by horses or mules (and one cent more for every animal), five mills for horse and rider and every dozen of pigs and sheep. It cost two cents to move a dozen cattle, mules or horses. Fines were levied as high as $10 (equivalent to $ in ) to anyone who vandalized any structure along the turnpike. If the company did not construct their highway in six years, the state would then take over and make it a public highway. On November 1, 1898, the turnpike, made mostly of stone, was taken over by the state at a cost of $31,661.87 (equivalent to $ in ). Repairs were slated to be completed on the road by October 31, 1899. Also on March 17, 1854, the state legislature made the charter official for the
Hopewell and Ewing Turnpike, which used the portion of Route 31 from Pennington to the current-day intersection with CR 654. The turnpike was taken over by the state in 1897 and the privately maintained company was dissolved. The formation of current-day Route 31 began around 1916, when a portion of the highway, along the community of Hampton was taken over by the state and was repaired to state standards for $300 (equivalent to $ in ). In 1923, the state of New Jersey passed the Good Roads Act, which greatly expanded the state highway system in New Jersey. This act took over pre-existing highways and rebuilt them to a standard mandated by the state. Construction began on Route 31 in 1926, when the State Highway Department began work at the southern terminus in Trenton. A total of 11 bridges were constructed along the highway during this time, signifying the route's construction northward. The oldest ones, dating back to 1926, were located in Ewing and Oxford Township. The one in Ewing was along the first portion of the highway to be constructed. By 1929, the construction had reached Lawrence Township, where another long stretch of highway was built. During 1930, the construction had reached Clinton, Glen Gardener, and Washington. Construction of the highway progressed northbound, with sections of the Mercer-Hunterdon-Warren route reaching Hunterdon County in 1931. The Hunterdon County portion up to the Musconetcong River was constructed for six months, with completion in October 1931. Construction of the highway northward from the Musconetcong River started in October of the same year. The mainline construction reached the northern terminus in Buttzville in 1932. The route, although the construction had reached its northern end, was not complete due to bypasses around Clinton and Pennington were proposed to be built. These bypasses were never constructed, and during 1934, more portions of the highway in Hunterdon County were finished. The route was completed in 1935, providing an uninterrupted route from the capital city of Trenton to northern Warren County.
Designation What is today Route 31 was defined in the
1927 New Jersey state highway renumbering as Route 30, a highway that began in Trenton and ended in Buttzville. In the
1953 renumbering, Route 30 was renumbered as Route 69, as the number conflicted with
US 30 in southern New Jersey. The bypass of Ringoes, shared with US 202, opened in the 1960s. Due to the
sexual meaning of the number and resulting
sign theft, Route 69 was changed on June 5, 1967, to Route 31 beginning with sign swapping by state crews. The justification for the removal was that people were stealing signs as a trophy for dorm rooms and clubhouses because of their interest in the number 69. A
limited-access freeway was once planned to serve traffic along the Route 31 (then Route 69) corridor. Plans came forth in the 1960s for a highway to run along Route 69 from the in-construction
US 1 freeway (then designated
Route 174) to the US 202 freeway in Ringoes. The estimated cost to build the highway according to the proposal estimated to $18 million (equivalent to $ in ), with construction completing in 1975. There were plans to extend the freeway further in both directions, including northward to
I-80 in
Hope Township and southward to
I-195. Each of these two extensions were brought forth in the 1960s, and were later shelved due to lack of support. In the early 1970s, a extension of Route 31 was proposed from Oxford, connecting the highway with the proposed
Route 23 freeway to
I-84 near
Port Jervis, New York. This northern freeway extension of Route 31 was to be known as Route F-31 and the Foothills Freeway, and was to run through the
Kittatinny Mountains, passing through
Stokes State Forest and
High Point State Park. The proposal gave an estimated price tag of about $105 million (equivalent to $ in ). The northern freeway faced high opposition from residents living in Warren and Sussex counties, who feared environmental damage. A group called Opponents of the Foothills Freeway formed to prevent the road from being built. Along with the growing opposition, environmental and financial issues arose, causing the New Jersey Department of Transportation to cancel the Route 31 from being built mid-1970s. Following the completion of New Jersey's portion of
I-287 in 1993, more out-of-state tractor trailer drivers began to use the alignment of Route 31 as a toll-free alternative to the
New Jersey Turnpike. On July 16, 1999, oversize trucks were banned from using Route 31 as part of a larger statewide ban imposed on New Jersey roads not a part of the
National Highway System by Governor
Christine Todd Whitman. The legislation had a significant effect on trucks that followed Route 31 as it forced them to use I-287 and the New Jersey Turnpike instead. In the mid-2000s, the portion of Route 31 was widened during extensive construction between Flemington and Clinton that widened the road to four lanes.
The South Branch Parkway from
OpenStreetMap|alt=A view from aerial of the community of Flemington with Route 12 in pink, County Route 523 in tan and U.S. 202/NJ 31 in green In 2005, the
New Jersey Department of Transportation (NJDOT) introduced a case study for Route 31 for its "New Jersey: Future in Transportation" project. Over time, the residents and business owners of Raritan Township and Flemington had growing concerns over the congestion along the highway and US 202. After trying to use vacant land to the east of the current alignment, NJDOT put forth the Flemington Bypass, trying to relieve congestion on Route 31 along the commercial district and to provide access to the industrial land between the two highways and the
South Branch of the Raritan River. The project was never completed and the proposed site remains undeveloped. NJDOT is now looking at a scaled-down bypass, proposed as the South Branch Parkway, to connect Route 31 to US 202 in Flemington. The parkway was to be built as a two-lane alternative to Route 31 defining the border of the suburban and rural portions of Flemington and Raritan Township. Along with the new parkway, a brand-new local street network was proposed to help support the new alignment. These would minimize the pressure of traffic on Route 31 and include better roads for bicycles and pedestrians. More direct routes to different portions of the community would be provided to maintain the community's character. The proposed parkway was to also help upgrade the sensitive history along the river. ==Major intersections==