Currently lorry drivers are required to stick to 40 mph on single-carriageway roads where the speed limit for other road users is higher. This is dangerous on at least two levels. Firstly it causes frustration in drivers that are held up behind the slow-moving lorry, which can result in less than safe overtaking manoeuvres, but, possibly more significantly, travelling at such slow speeds demonstrably causes a lowering of attention, and can be very soporiphic. With nothing to grab their attention, drivers will often start to concentrate on other things than the road ahead, and may even start to fall asleep, a theory which is backed up by the significant number of accidents involving HGVs unsuccessfully negotiating roundabouts and junctions. By allowing HGVs to travel at the normal speed limit for every road, they will cause less dangerous driving around them, and be less dangerous themselves. Let's face it, they have better brakes than other road users, and the drivers have to undergo more stringent driving tests, so should be perfectly capable of judging what speed to go round corners safely.
I would actually go further and suggest that no vehicle should be allowed on a road unless it is capable of diving at the speed limit. This would also combat those ridiculous cases of one HGV taking five miles to overtake another HGV because they are both limited to 58 mph, give or take half an mph.