Smart Motorway gantry sign
A gantry structure spanning the motorway with electronic signals, blue signs showing an exit and the following text.
Exit destinations: A405 (M1 South), St Albans; London (NW & C) .
Forward destinations: (M11), Hatfield (A1(M)); Stansted [Airport symbol] Dartford Crossing M25 .
Junction number: 21A.
Change Route: A405 (M1)
This exit comes hot on the heels of junction 21, and is where the M25 meets its predecessor, the A405 North Orbital Road, a sort of 1930s attempt at a London orbital. Today its ambitions are much more modest. You can use it to get from Watford to St Albans, or you can use it to hop off the M25 for a minute to find cheaper petrol than at the services.