Route 31 is a state highway in New Jersey, United States. It runs 48.93 mi (78.75 km) from U.S. Route 1 Business /US 206 in Trenton, Mercer County, north to an intersection with US 46 in Buttzville in White Township, Warren County. Along the way, Route 31 heads through the communities of Flemington, Clinton, and Washington. Most of the highway is state-maintained; however, the section within the city limits of Trenton is maintained by the city. Much of Route 31 is a two-lane highway that passes through farmland, woodland, and mountainous areas. Two portions of the route—from Trenton to Pennington and from Ringoes to Clinton—consist of more development.

U.S. Route 1 Business is a four-lane surface road that provides an alternate route to the Trenton Freeway (US 1) northeast of Trenton in Mercer County, New Jersey. The route is 2.73 miles (4.39 km) long and runs between US 1 in Trenton and Lawrence Township. On the border of Trenton and Lawrence Township, US 1 Bus. intersects the northbound direction of US 206 at the Brunswick Circle. The route was once part of a longer U.S. Route 1 Alternate, which continued southwest through downtown Trenton and into Morrisville, Pennsylvania.