Speed Limits... I didn't realise

I was reading last Thursday’s (27th Nov.) Commercial Motor on Friday and came across a letter on the legal Q&A page (Page 34 if you have a copy).
A chap from Bury St. Eds. has written in, explaining that he’d just been fined £60 with three points for speeding on a section of the A38(M).
The particular section of motorway was signed as having a 60mph speed limit, as opposed to the national speed limit, and he was apparently clocked doing 46.
According to the lawyer who answered his question, as the speed limit for cars was reduced, then the speed limit for all other classes of vehicle is also reduced by the same amount. At least i think that’s what he’s trying to say, because to me it looks as though he’s got all his limits and speeds in a bit of a pickle.
Is this correct? If so I’d never realised it. I’d thought that if, for example, i was on a dual carriageway and there was a 50mph limit, i was also legally allowed to do 50 (I drive an artic btw). What this fella is trying to say is that if the speed limit is signed as 50mph, this means that the limit for cars is reduced 20mph from 70, so LGV’s speed limit is also reduced 20mph from 50 to 30! I find this hard to believe, especially as i’d travelled on a section of the A206 on Friday which is a dual carriageway signed at 40mph, did this mean i had a legal limit of 20mph??!
If anyone has a copy of the magazine, take a look. I’ve read it several times now and i’m sure that’s what he’s trying to say, but as I’ve said, the speeds that the lawyer quotes seem to be a bit hit and miss. Anybody shed any light?

Simon

(Also posted on the professional drivers forum)

Roger Breaker:
I was reading last Thursday’s (27th Nov.) Commercial Motor on Friday and came across a letter on the legal Q&A page (Page 34 if you have a copy).
Is this correct?

No it’s wrong! I read it as well and had to reread it several times!!

A motorway is a motorway whether its was built as one or it is a motorway classified A road such as the Aston Expressway. National speed limits apply up to any other signed speed limit.

I always thought that on such a road if the signed limit was 60 then the only vehicles that limit affected was those that would normally be allowed to exceed 60, i.e. cars. I’m not going to change the way I drive. It would just be a simple case of referring the court to the highway code which is very clear on this subject.