Smart Motorway gantry sign
A gantry structure spanning the motorway with electronic signals, blue signs showing an exit and the following text.
Exit destinations: A404 Amersham; Chorleywood .
Forward destinations: (M1, M11), Watford (A41); Luton [Airport symbol] Stansted [Airport symbol] M25 .
Junction number: 18.
This junction is squashed into quite a small space. Exit here and you'll be thrown around some tight corners on a two-way sliproad to avoid disturbing too many of the big houses set in secluded gardens.