A470

The A470 is the north-south highway running the length of Wales. Mostly a quiet rural drive, it's suddenly transformed between Cardiff and Merthyr Tydfil, becoming a major and unique expressway route.

Think of the busiest roads in Wales and you will inevitably think of the M4 and the A55. You might also bring to mind the A465 Heads of the Valleys Road, and the A449 down the Usk valley. Often overlooked, though, is the twenty or so miles between Cardiff and Merthyr, a section of the A470 that carries as much traffic as many lengths of the Welsh M4 and considerably more - two or three times more, in fact - than the A55.

This section is, by any meaningful measure, an urban motorway. The length from the M4 to the A472 at Abercynon was built during the late 1960s and early 1970s and is a fast-paced, grade-separated route through a continuously built-up steep-sided valley. It comes complete with full hard shoulders and split-level running along hillsides.

If you enjoy ingenious 1960s highways threaded through town centres and difficult terrain, there's really nothing in the UK to match it; it's like a version of the M8 through Glasgow with better scenery. The ambition of the project was considerable; it originally came with an interchange at Coryton, but when the M4 arrived, that was replaced with Coryton Interchange - a three-level stacked roundabout interchange, yes, but one notable for having what is believed to be the UK's largest roundabout. The size of the roundabout, and the number of parallel lanes it carries, is scarcely believable until you've driven around it.

The "Cardiff-Merthyr Road" built in the late 1960s ran from the M4 to Abercynon, where the towns and villages spreading north from Cardiff along the valley of the Taff begin to thin out. It was extended in the 80s to reach the A4060 eastern bypass of Merthyr at Pentrebach. It was then extended further in the mid 1990s, passing around the west side of the town, to bypass the A470 through to a point north of Cefn Coed. That new length pushed the fast and very scenic dual carriageway further north, but it's not quite the same road - no longer at all urban, and with a series of flat roundabouts instead of those motorway-style junctions.

The chances that the expressway A470 will ever extend further north are very slim; north of Merthyr Tydfil right through to its northern extremity at Llandudno, the rest of the A470 is a much quieter route. But it will eventually connect to another modern expressway at Cefn Coed, where the final phase of the A465 dualling project will give this almost-motorway a northern terminus on Wales' new east-west route between Swansea and Raglan.