It is a local road serving Franconia, Bethlehem, and Littleton, New Hampshire, which I-93 bypasses.

NH 18 has two interchanges with I-93 in Bethlehem, joining U.S. Route 302 westbound at the second one (US 302 east provides access to downtown).

(This intersection is signed as the northern terminus of NH 10, although this designation officially ends much further south.)

Route 18 was transferred to state highway systems in Vermont, New Hampshire and Maine.

As a result, Route 18 was truncated on the overlapped roadways and later extended along other roads to Franconia.