NH 18 begins as a spur of I-93 (the Franconia Notch Parkway) at the height of land in Franconia Notch. (U.S. Route 3 is overlapped with I-93 on the Parkway.) NH 18 proceeds to the northwest, intersecting NH 141 and NH 142, near its interchange with I-93. In downtown Franconia, NH 18 joins with NH 116 and meets the eastern end of NH 117 before leaving town and crossing into Bethlehem. 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). The highway crosses into Littleton, splitting with NH 116 in the center of town. Just west of downtown, US 302 splits from NH 18 as well. (This intersection is signed as the northern terminus of NH 10, although this designation officially ends much further south.) Northwest of town, NH 18 joins NH 135 near another interchange with I-93. The two routes have a direct interchange with I-93 near the Connecticut River at exit 44 (I-93's last exit in New Hampshire). NH 135 splits off NH 18 before it crosses the river into Vermont (along with I-93 to its west) and becomes VT 18.