Smart Motorway gantry sign
A gantry structure spanning the motorway with electronic signals, blue signs showing the left lane exiting and three others continuing ahead, and the following text.
Exit destinations: A41 ; Aylesbury; Hemel; Hempstead ; Watford.
Forward destinations: (M40, M4), Rickmansworth; Heathrow [Airport symbol] Gatwick [Airport symbol] M25 .
Junction number: 20.
Just before we reach junction 20 we cross a long viaduct over the valley of the River Gade. Local residents were concerned about the sight of the bridge crossing their pleasant meadows, so engineers changed the angle of the viaduct supports to stand parallel to the line of the valley, making them and the structure much less intrusive as you gaze along the course of the Gade.