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Eastbound, heading into London on the A4. Up above us the M4 runs seemingly forever.

A curve in the road really shows off the scale of the structure. All the same, it's showing its age these days - it is regularly closed for maintenance at night.

The eastbound ironing board - the slip from the M4 to A4 is right beside us and about to give us our third lane back, and beyond that, the A4 to M4 slip is climbing up to the top level.

A little further on, and the A4 to M4 sliproad swerves across to join the last half mile of motorway. It seems quite improbable that it can all safely be supported here, though I assume it works as it's all still up there.

A quick about turn, and here we are climbing up the westbound A4 to M4 sliproad - halfway up an ironing board leg, if you will. It's really quite claustrophobic, made worse by the traffic. And this is only early evening on a Saturday.

A little further up - the light is for the M4 to A4 sliproad dropping down between us and the motorway, and beyond it, some sign gantries on the M4.

A look to the side at a confusing array of ascending and descending ramps.

Finally we reach the top and - yikes! - this is how much merge space there is. The speed limit up here is, rather sensibly, 40mph. Not that it makes the slightest difference in this traffic.

And we're off! I'm willing to believe this is a motorway - it's only slightly narrower and less motorway-like than parts of the Leeds Inner Ring Road, for example. But that is called the A58(M), whereas this claims to be the M4 London to South Wales Motorway.

Some dramatic evening clouds make for a pleasant exit from London.

Lane control signals - managing to look even more spindly and temporary than the road itself. Where does the ladder on the left hand side take you?

And suddenly, it all ends! One moment you're on a narrow elevated road, and suddenly it just widens out to three lanes with a generous hard shoulder and you're in the countryside. How strange!