Smart Motorway gantry sign
A gantry structure spanning the motorway with electronic signals, blue signs showing an exit and the following text.
Exit destinations: A1081 St Albans.
Forward destinations: (M1, M40, M4); Watford, Heathrow [Airport symbol] M25 .
Junction number: 22.
The A1081 is the former southernmost section of the A6, which used to begin in Barnet. This bit of the M25, between junctions 22 and 23, is built on top of it, which is why there's a gap in the A1081 between those two junctions.