If that really is the reason then that’s poo, another example of not actually addressing the issue.
If people were driving dangerously trying to overtake when inappropriate etc. then that should be dealt with, a good old dose of calm-the-f-down would maintain the original safety level for drivers and improve it for other road users, and at what cost? A few seconds on a journey, well big deal…
I think the bigger problem is having one catch-all limit for a very varied range of roads and infrastructure. Increasing the speed of trucks addresses one problem – dangerous overtaking – but creates other ones. An A road like the A9 is a fairly newly build road – pretty straight, long gentle curves, side road have a slip road on and off, its backed up by a parallel bike /pedestrian path and so on. The A76 near me is the same class of road but its narrow, has no space for pedestrian other than on the carriageway, it has crossroads in dips between blind summits, theres no run on/off slip roads so vehicles need to all but stop to join and leave and so on.
I think we need a two tier national speed limit (for cars as well) – one for roads of a given standard in terms of the safety of junction design, room for other road users and so on and another for roads that don’t meet that standard to give more protection to non motorised traffic sharing that space.