|Distance:||12.9 miles (20.8 km)|
|Meets:||A484, B4306, B4317, B4308, A484, A4214|
|Route outline (key)|
The B4309 starts on the southern edge of Cwmffrwd, a village a couple of miles south of Carmarthen itself. The road heads southwards and soon meets the B4306. Over the next few miles it descends to Pontantwn where it crosses the Gwendraeth Fach.
It then climbs again to a disused quarry then runs through Meinciau and descends through Pontyates to cross the Gwendraeth Fawr, a smaller stream than the Gwendraeth Fach but with a wider valley, as suggested by its name.
The road then crosses a railway level crossing with manual gates, although the line has obviously been out-of-use for years. Immediately afterwards the B4317 joins from the right and multiplexes along the B4309 for a short distance. The B4309 continues to climb out of the valley and reaches a summit by skirting the top of a hill. It then descends into the village of Five Roads, where it does indeed meet one road on the left and two on the right, before continuing on along a small valley into Llanelli.
In the suburb of Furnace the road meets the B4308 at a mini-roundabout before reaching a T-junction and the A484. This is the original southern end of the B4309 but it has since been extended following a short multiplex west along the A484. It regains its number heading southeast along Greenway Street and Coleshill Terrace to end on the A4214 Church Street. In reality this southern section is one-way northbound so can only be accessed in the other direction.