The road begins at the southern apex of a triangular one-way system in Caernarfon on the A487. It heads south, signposted for Beddgelert. As it leaves Caernarfon it passes the site of the Roman legionary fort Segontium and later crosses the Afon Seiont and the course of the Caernarfon-to-Llanberis railway. After 1.5 miles it crosses a roundabout at Caeathro. The road as far as Waunfawr is reputed to be a Roman road and is fairly straight. After Waunfawr the road joins the Gwyrfai Valley, a glaciated U-shaped valley along the west side of Snowdon. At various places along the valley it crosses and recrosses the Welsh Highland Railway. The road passes along the east shore of Llyn Cwellyn (Caernarfon's water supply), before climbing to a summit just south of Rhyd Ddu.