West of its interchange with Interstate 81 (I-81) in Strasburg, SR 55 runs concurrently with the easternmost portion of US 48 as part of Corridor H. East of Front Royal, the state highway serves as the local complement of I-66 as it passes through the towns of Marshall, The Plains, and Haymarket.

SR 55 begins at the West Virginia state line at the highway's summit of Great North Mountain.

SR 55 heads northeast as two-lane undivided Wardensville Pike through George Washington National Forest and descends the mountain to the hamlet of Star Tannery, where the highway leaves Frederick County by crossing Cedar Creek.

SR 55 passes through the Shenandoah County communities of Lebanon Church and Clary on its way to a diamond interchange with I-81, which serves as US 48's eastern terminus.

[2][6] Just north of Signal Knob, the northernmost peak of Massanutten Mountain, SR 55 passes to the south side of the B-Line.

The four-lane undivided highway crosses the South Fork and passes through a pair of right-angle curves onto 14th Street and then Royal Avenue.

East of US 522, SR 55 reduces to two lanes and leaves the town of Front Royal as John Marshall Highway.