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Rich2012:
On a single carriageway the national speed limit is 50mph for vehicles over 7.5t.
If I’m on a single carriageway and the sign says 50mph, do I have to travel at 40mph.

Some people keeps saying that the speed limit change only applied to the national speed limit sign

In England and Wales if the sign says 50 you can drive at 50mph in a HGV.

In Scotland the HGV limit hasn’t changed so it’s still 40mph on single carriageway roads.

It’s a national speed limit not a speed limit for roads that are signposted for the “national speed limit”.
Hope that makes sense