No, this is not going to be a link to a video of one of those terrifying ridge tops in China or crumbling walkways in South America, it is in fact about something I saw last week in Buckinghamshire.
As can be seen from my map below, a small river, the Colne Brook, flows from east to west and under the M25, then bends to the left and flows southwards parallel to the motorway.
My photos below were taken at the point where the river emerges on the west side at TQ04068309, after passing beneath eight lanes, ten if you count the hard shoulders.
You can see there is a concrete ledge on the left side of the tunnel. However, there is only about 2ft 6in (75cm) headroom! The ledge is also extremely filthy, the river has probably been over it on occasions. It is not clear whether the sign "FP To Uxbridge" with its arrow was painted officially or tongue-in-cheek. I suspect it is probably the latter, for if you look at the map the green dotted line of the footpath stops short of the motorway.
There is a trod way (currently blocked by cut branches from some very recent path clearing) down to the ledge, perhaps it is merely made by animals such as foxes or muntjac deer.