|Length:||31.8 mi (51.2 km)|
| A487 road
The road crosses the Preseli Hills and for almost all of its route winds through farmland.
The northern two-thirds of the A478 was for some centuries a drovers' road transporting goods and livestock from north Pembrokeshire and Ireland (Cardigan was then a port) before the coming of the railways in the mid-19th century. The road was turnpiked probably in the 18th century. There have been only minor and local deviations in the route, principally to improve river crossings and eliminate dangerous bends. Short sections of the original road, little more than a country lane, can still be seen, as can some early stone bridges.
One mile south of Cardigan the road bridges the River Piliau and crosses the county boundary into Pembrokeshire at Glanpwllafon. It then runs through Pen-y-bryn village and Bridell hamlet, bridges the River Plysgog and continues to Rhoshill hamlet, after which it is crossed by the B4332 Eglwyswrw-Cenarth road. The road then climbs the northern slopes of the eastern end of the Preseli Hills through Blaenffos village, bridging the River Nevern (near its source) at Riverlea, to Crymych village.
Still at an altitude of over 200 metres (660 ft), the A478 passes through Pentre Galar hamlet, crosses the county boundary to Carmarthenshire and passes a viewpoint near the summit of Carn Wen, where it reaches its highest altitude of 260 metres (850 ft). The scars of extensive quarrying at Carn Wen (also known as Garnwen) are clearly visible from the road; quarrying for aggregate resumed there in 2013.
Dropping steeply to Glandy Cross hamlet and the village of Efailwen, location of the first of the Rebecca Riots, the road re-crosses the county boundary to Pembrokeshire and passes through Llandissilio and Clynderwen with Clynderwen railway station. Until around 2001 the road re-crossed the county boundary to Carmarthenshire between Clynderwen and the A40, but after local government boundary changes it stays wholly in Pembrokeshire.
The A478 crosses the A40 road at Penblewin roundabout and passes through Narberth town centre, where it crosses the B4314. After Narberth, the road passes through Camp Hill and Templeton, then crosses the A4115 close to Templeton Airfield, formerly RAF Templeton. After passing Folly Farm Adventure Park and Zoo and through Begelly village it crosses the A477 road, then a junction with the B4316 (to Saundersfoot). After several more hamlets (Pentlepoir, Wooden, Moreton, Twycross and New Hedges) it reconnects with the B4316, reaching the coast near Tenby harbour.
The majority of traffic is local, agricultural and, in season, holiday traffic. North of the A40 there is more HGV traffic owing to restrictions preventing large vehicles from exiting Fishguard to the north; this traffic must leave Fishguard to the south and use the A40/A478 to access the A487 for Cardigan and points north. In 2012 Pembrokeshire County Council said the A478 at New Hedges, near Tenby, carried an average of 9,900 vehicles a day.
- "Driving directions to Norton/A478". Google Maps. Google. Retrieved 27 July 2014.
- OS Landranger Series: Cardigan & Mynydd Preseli Map 145 (2007)
- "Pembrokeshire County Council Correspondence: 09.11.12". Retrieved 1 May 2014.