Motorway gantry sign
A gantry spanning the motorway with blue overhead signs showing the following text.
Forward destinations: SOUTH WALES; Cardiff, Newport M4 [Hollow square diversion symbol] .
Exit destinations: M48 Chepstow & Services.
Junction number: 21.
Change Route: M48 Chepstow
There's a rather glorious view of the Severn Bridge's white towers as you come down this hill, and if you'd like to see them close up, you can branch off here for the M48.