Smart Motorway gantry sign
A gantry structure spanning the motorway with electronic signals, blue signs showing an exit and the following text.
Exit destinations: A412 Rickmansworth; Maple Cross .
Forward destinations: (M1, M11), Amersham (A404); Watford, Stansted [Airport symbol] M25 .
Junction number: 17.
There's been talk for years about providing another service area on the western side of the M25, and the latest idea seems to be attaching a new service area to this relatively lightly used junction. But this is the green belt, and the local residents tend to like their leafy surroundings to remain leafy, so don't hold your breath.