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: A4 [Hollow square diversion symbol] [Hollow triangle diversion symbol] ; Langley; Colnbrook; B470 ; Datchet.
Forward destinations: SOUTH WALES; Reading, Slough M4 .
Junction number: 5.
If you came this way looking for The WEST - which has been on signs from the start of the M4, and also on the M25 - you'll find it has now abruptly disappeared. It seems that the eastern edge of Slough is where the West of England begins.