The route closely parallels I-95 along Lafayette Road for a majority of its 17-mile (27 km) stretch in the state.

travels toward I-95 and the Spaulding Turnpike made up of US 4 and NH 16 in the general direction toward Concord and the Lakes Region/White Mountains.

Roadway that had been US 1 was designated as Route 17 from Danvers to Salisbury[3] and NH 17 for a short distance in Seabrook.

The original Memorial Bridge, which formerly carried US 1 over the Piscataqua River between New Hampshire and Maine, was closed to motorists in July 2011 due to numerous safety and structural concerns.

The New Hampshire Department of Transportation operated hourly shuttle service between Portsmouth and Kittery while the new bridge was constructed.

The new bridge was officially dedicated at a ceremony on the New Hampshire side of the span just before noon on August 8, 2013.

The southern terminus of NH 1A is at the state line in Seabrook where it continues south as Route 1A.

In Portsmouth, it is known as Miller Avenue and Sagamore Avenue.New Hampshire Route 1B (NH 1B) is a 4.808-mile (7.738 km) auxiliary of US 1 serving the town of New Castle.

The road loops around over Great Island in the Piscataqua River through New Castle and then back into downtown Portsmouth.

Harbor Place defaults onto Daniel Street, which carries US 1 south into Market Square.