|Via:||Porthyrhyd, Nantgaredig, Brechfa|
|Distance:||23.8 miles (38.3 km)|
|Meets:||A476, B4317, A48, B4300, A40, B4337|
|Route outline (key)|
The B4310 is a long rural B-road in Carmarthenshire.
The road starts on the A476 in Tumble and heads northwestwards to Cwm-Mawr, where it meets the B4317 before crossing the Gwendraeth Fawr. The road then zigzags through Drefach before reaching a T-junction. This is the original northern end of the B4310; it was extended north in 1935 but this required a long multiplex along the A48. That road has since been diverted onto a bypass, so the B4310 is the only classified road here. We TOTSO left and continue to Porthyrhyd, which marks the end of the former multiplex.
When the A48 was bypassed, the road west of Porthyrhyd was renumbered as a spur of the B4310. It continues west via Llanddarog to end on the current route of the A48. All movements are possible at the junction owing to a gap in the central reservation.
Returning to Porthyrhyd, the mainline of the B4310 TOTSOs right to meet the A48 at a simple GSJ. We pass the National Botanic Garden before reaching the B4300 which is crossed via a long staggered crossroads/short multiplex. The road crosses the River Towy at an old, narrow bridge immediately afterwards before going through Nantgaredig, where it crosses the A40.
The road now climbs up into the hills. It goes through Horeb, one of several Biblically named places in Wales, before bending very sharply right to go through Brechfa. The road descends into the valley of the Cothi River, which it has been running parallel to since Nantgaredig, following it upstream for a few miles to Abergorlech. It then climbs out of the valley to end on the B4337 in Llansawel, which is in the valley of two of the Cothi's tributaries.