Smart Motorway gantry sign
A gantry structure spanning the motorway with electronic signals, blue signs showing an exit and the following text.
Exit destinations: A408 Heathrow [Airport symbol] (T2 & T3); Uxbridge .
Forward destinations: The WEST, Reading; Heathrow [Airport symbol] (T5) M4 (M25) .
Junction number: 4.
A left turn here, and onto the spur, will get you to Terminals 2 and 3. Terminal 4 was at the last junction. Terminal 5 is at the next one. And there's no exit for Terminal 1, because it was demolished, but it lives on in our hearts.