|Location Map ( geo)|
|Distance:||7.2 miles (11.6 km)|
|Meets:||A4093, A4119, A4058|
|Former Number(s):||A4058, A4093, A4119, B4271, B4279|
|Route outline (key)|
The B4278 is a local route that runs through Glamorgan, mostly following the alignment of former A-roads that have been rerouted since the late 1980s.
It starts on the A4093 just east of Hendreforgan, and heads east before turning north on the western side of Tonyrefail. The route crosses the A4119 at Penygraig and meets the A4058 on the south bank of the Rhondda Fawr close to Dinas Rhondda railway station.
After concurring with the major road for some 350 m upstream and crossing the river, the B4278 sets off anew from the roundabout by Tonypandy railway station (actually in Trealaw), heading now downstream (eastward) along the north bank of the Rhondda Fawr for 3 km to Porth, where it goes over the Rhondda Fach and under the A4233 without a junction. It continues for a further kilometre or so by twin, parallel routes which bring it to junctions with the A4058 east of town.
From Trealaw a branch heads northward for nearly 2 km to Llywnypia, where it meets the A4058 close to Ysbyty Cwm Rhondda.
The original course of the B4278 was from the B4271 (upgraded to A4119 a few years later) in Penygraig to the A4058 in Porth. By the end of the 1920s, it had been extended round through Trebanog to end on the A4119 in Tonyrefail. This latter section was renumbered as an extension of the B4277 in 1935, and is now partly numbered A4223 and partly unclassified again.
Around 1988, following the rerouting of the A4119 onto a new relief road, the B4278 was extended along the old road to Williamstown, the old road ahead becoming an extension of the A4093. Following the construction of the A4093 Tonyrefail Western Bypass in the mid 1990s, the B4278 was extended to its current southern point.
The remaining northern section is the old route of the A4058 before it was rerouted further south, including a section of the B4278's original course.