I think I saw the most dangerous single piece of overtaking I've ever seen yesterday.
The road is a part of the A9 in the highlands. It's single carriageway and there's a slight rise, then a brow (blind, but not very sharply blind) after the brow there's a left hand curve.
It's possible to overtake safely on the rise and pull in before the brow.
On this occasion I'm following a queue of four vehicles, HGV at the front, car, van and another HGV. There four are so close together that I've decided that I can't possibly overtake because there's nowhere to pull back in. The front HGV is doing 40mph, so we're all doing 40mph. As we start on the rise a white van moves to overtake me. No problem; there's a nice return gap for him in front of me.
I'm stunned as the white van decides to stay out and starts to pass the group in front of me. The white van stays out over the brow and around the left hand bend and eventually passes the 4. If anything had come the other way there would have been a very big crash. Fortunately nothing did come the other way. Pure luck.
Immediately after this the front HGV pulls into a layby. I consider trying to gather the data and a witness to report dangerous driving, but I'd have to chose between the front HGV driver as witness and chasing the white van to get his details. I do neither.
There are so many obvious faults to create this whole scenario, I hardly know where to begin.

The HGV speed limit didn't need to be 40mph and the front HGV could easily and safely have been doing 56mph.

The 40mph HGV was causing traffic behind to bunch and I suspect that meant that the white van couldn't see a way out of the path he'd started on. If there had been a gap to pull back in he might have taken it.

Obviously traffic shouldn't bunch so tightly that no return gaps are available to overtaking traffic.

When the bunched traffic saw the white van getting into trouble one of them should have opened a return gap.

The white van driver should have known much better than to start to overtake without an identified return gap. But we've never taught drivers to overtake, and a few words about planning a return gap in the Highway Code are nowhere near enough.
Rule 138

A double white line might have helped, but it's possible to cautiously overtake a 15mph tractor near the brow, so a double white line system would sometimes be inconvenient.
My conclusion is that the original causal factor for the incident is bonkers HGV40 speed enforcement. Without that the front HGV wouldn't have been doing 40mph - just 2 years ago 40mph HGV on this road were as rare as hens' teeth - they were rightly travelling at their top limited speed of 56mph. If the front HGV hadn't been doing 40mph, the following traffic wouldn't have been so bunched. And if the front HGV had been doing 56mph, it's quite possible the white van wouldn't have decided to overtake at all.
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