Variable message sign
A gantry structure spanning the road with electronic signals over each lane and a large MS4 message sign on the left.
A 40mph speed limit is shown over all lanes with the message "report of pedestrians".
Somewhere here the M25 squeezes through the arches of a brick railway viaduct, which usually has some impressively large graffiti on it. For many years it famously read "GIVE PEAS A CHANCE", but at the time of writing says "HELTA SKELTA". The viaduct was part of the Great Central Railway and now carries Chiltern trains towards High Wycombe and Aylesbury.