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: M1 The NORTH; Luton [Airport symbol] .
Forward destinations: (M40, M4), Aylesbury (A41); Heathrow [Airport symbol] M25 .
Junction number: 21.
Change Route: M1 The NORTH
Here we are, the exit for the M1, the main road north from London. This junction is enormously busy and is built on four levels, but you can only turn to and from the M1 northwards - there's no access to or from the south.