Man of Honour
What would you think was going on? Would you do anything other than reduce your speed? Do you know the circumstances in which a 20mph limit would be imposed?
Lowest I've seen is a 40.
I slow and move over to lane 1, behind a truck and follow if possible.
Too many drivers in the outside lane see this as a chance to continue speeding, overtake cars and get to the front, following each other too closely.
I used to be one of them but now I don't mind being last if it means I get home in one piece.
And @DRZ said, start checking the sat nav for an exit and alternative route.
If I saw a 20mph limit on a motorway I'd be slowing down as safely as possible, eyes peeled and tbh looking for the nearest exit as something horrendous is likely to happen
tisOh come on Wales isn't that bad