If you couldn't see something the size of a camera van it clearly wasn't safe to overtake!
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This is an assumption. Having successfully navigated trucks up and down the country with only a few minor incidents that comes with the job, my awareness and anticipation I would say is pretty good.
The van was behind a copse in a layby. I knew the straight bit of road was upcoming and a quick glance with a clear view of the right hand side of the road and any oncoming traffic of which there was none, and I put my foot down knowing this straight bit of road was not long, so I was keen to get round it asap. And I don't have x ray vision.
in Germany, trucks and larger vehicles are encouraged to pull over to the right. We used to (and for some inexplicable reason probably to do with self entitlement), drivers of slower vehicles indicated left to allow others to pass when it was safe - but people rarely bother anymore.
As for the almost blanket speed limit of 40 for trucks on the roads you mentioned and across Scotland, the 40mph limit causes frustration for car and truck drivers alike when (and you mentioned the A9), it would be perfectly possible to have the 56 limit for trucks to alleviate tail backs and reduce crazy overtaking stunts on many stretches. Some of these limits are simply a revenue scam.
Yes, I agree with FA's mention of the A roads through villages who benefit from 20mph limits and fully endorse the change but the 20 limits in a lot of situations are simply non-sensical and a lot of them in the rural locations I see around D&G, mainly ignored completely.
I always say drive according to conditions and the environment but unfortunately, feel compelled to legislate for stupid.