Are variable speed limits enforceable under law?

With reference to the email about the variable speed signs on the M42, I have always understood these to be legally enforceable. However whilst obeying them I seem to get passed by a lot of vehicles clearly going much faster. Is my understanding wrong or are there a lot of speeding tickets being sent out?

Asked on 18 May 2013 by CP, Arnside, Cumbria

Answered by Honest John
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.
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