Smart Motorway gantry sign
A gantry structure spanning the motorway with electronic signals, blue signs showing the left two lanes exiting and the other three continuing ahead, and the following text.
Left-hand two lanes destinations: M25 Watford (M1); Oxford (M40); Gatwick [Airport symbol] (M3); Heathrow [Airport symbol]; (T4, T5 & Cargo) .
Right-hand three lanes destinations: Central London; Heathrow [Airport symbol] (T2 & T3) M4 .
Junction number: 4B.
Change Route: M25 Watford & Gatwick
Four lanes open out to five as we approach Thorney Interchange, a four-level junction where we cross the M25. You can stay straight on for London, or exit here for... well, the rest of the UK.