|Length:||10.7 miles (17.2 km)|
|Meets:||A470, A4054, A4059, A4223, B4276, A4059|
|Former Number(s):||A4059, A4224|
|Old route now:||A4059|
|Route outline (key)|
The B4275 runs along the right bank of the River Cynon for some distance. It has a strong relationship with the A4059 which runs largely on the other bank.
The road starts at a south-facing GSJ on the A470 and A4054 south east of Abercynon and runs north to cross the Taff and the Cynon before doubling back on itself to run in the narrow space between the river and railway line for a short distance. It bears right under the railway line to run through the centre of town, where there is a brief one-way loop where westbound traffic has to go round three (unsigned) sides of a rectangle. After the two directions rejoin the road bears right at the top of the hill.
On the edge of Abercynon is a spur (the first of several) leading to the A4059. The road then continues upstream, going through Penrhiwceiber to reach Mountain Ash. There is a one-way loop in the centre of town, with northbound traffic using the deliberately narrowed Oxford Street and southbound traffic going one block east past the station. The road then meets a set of traffic lights from where there is a further spur across the river to the A4059.
Leaving Mountain Ash the road runs adjacent to the railway line for a short distance before going through Abercwmboi. On the far side of the village there is another spur to the A4059 which by now has crossed the river.
The B4275 runs through the suburbs of Aberdare where it cannons off the A4059 at a roundabout. There is then a short multiplex along the A4233 through the centre of town; most of this is another one-way loop. At the western end of the High Street there is a roundabout where the A4233 bears off right to reach the A4059 and the B4275 continues ahead.
Most of the B4275 mentioned above is actually the pre-bypass route of the A4059.
In 1922 the B4275 avoided the centre of Abercynon. It started to the north of the town on the usual A-road and headed south across the river to join the current route of the B4275; this section is now the road's southernmost spur. The B4275 then followed its current route as far as Mountain Ash where it ended on the A4059 in the centre of town, adjacent to the bridge over the river.
In 1935 the B4275 was extended north. There was a short multiplex over the river before the road continued along the left bank via Cwmbach to end in Aberdare.
Later, the A4059 was upgraded in the Cynon Valley. It took over the former B4275 from Mountain Ash to Cwmbach, before following its own route into Aberdare. The old road was renumbered A4224 and it has been suggested that that road also took on the B4275 south of Mountain Ash. However, this did not last long. Certainly the B4275 was in existence between Abercynon and Mountain Ash by 1975 and when the A4224 was abolished shortly afterwards it received that road's route, giving it a spur over the river in Mountain Ash.
When the Aberdare bypass was built later still, the B4275 was extended along a further section of pre-bypass A4059.