HGVs speed is limited by law, car drivers by a lack of ability.
The A9 (Killer Road!!) is currently covered by average speed cameras from Dunblane to Inverness with a trial speed limit of 50mph for HGVs, clearly signposted every few miles in both text and picture form.
This does not stop Joe Dawdle in his Kia tootling along at 48, doing his upright citizen bit to keep those devils in lorries in their place having failed or refusing to recognise the signage and not knowing that the national limit for cars, 60mph, still applies.
When these keepers of the queen's speed are abed or at home flagellating themselves everything works just fine.
Humans are the problem, not machines.