Driving between South Wales and the West Country? The M49 is for you.
Some little motorways have been described as nothing more than glorified sliproads. More than anywhere else, that is true of the M49, which runs for just five miles between the M4 and the M5 near Avonmouth, west of Bristol, with no intermediate junctions. It provides a direct route for traffic going between the South West (and Bristol) and Wales, avoiding Almondsbury Interchange. And it does absolutely nothing else. In fact, because it only links the M4 and M5, it is the only motorway in the UK that you can only reach by motorway - there is no way on from ordinary roads.
The junction at the northern end of the M49 has sliproads to provide access in all directions, but the reason for that isn't clear. There's no real reason to allow traffic on or off the M49 from the M4 to the east - anybody doing that is going round two sides of a triangle. The extra sliproads are perhaps to provide a way to avoid the bridge tolls, or perhaps to function as a bypass of the M5 in emergencies, or even to provide access to the weighbridge in the middle of the roundabout there. However, the most likely reason is that there has always been an idea that a junction might one day be built on the M49, and if that happens there will need to be full access at its northern end.
Plans for that midway junction were resurrected in 2015, to support expansion of the industrial area next to the motorway at Avonmouth. It was then built, but without a connection to the surrounding network - so at the time of writing it is a complete motorway junction that remains closed because it leads nowhere. We have a blog post all about it from 2022.
Motorway numbering conventions suggest that the number M49 is inappropriate: since the road lies south of the M4 and west of the M5, then (depending which version of the numbering system you prefer) it should actually have a number beginning with either a 3 or a 5. It happens that the numbers M39 and M59 are both available. Just saying.