It seems not. Apparently a barrister challenged the legality when the road was actually clear and there was no good reason for the lower limit, and she won her case. Since then I have not heard of anyone being done for exceeding a variable limit. Sometimes the system works very well, as it did on the M1 and M25 on 21 May. Sometimes it very obviously doesn’t and unless the Highways Agency gets it right more regularly, these limits will increasingly be ignored.