PeterE wrote:Conceptually, are they any more dangerous than allowing overtaking on a normal two-lane road with a dashed centre line?
I don't know, but one thing that occurs is that if the road is wide enough for three lanes, then it's probably wide enough for four cars abreast (two going each way). If it's marked as two lanes, with just a single dashed centre line, then four abreast might well happen. If you're overtaking towards someone but able to keep on your side of the centre line, they might decide to overtake too if they can also keep on their side of the line. Four abreast like that is, I'd have thought, far less likely if the road is marked as three lanes as in the link you posted. Intuitively, it seems like there would probably be be more risk in four abreast than three, but I don't have any evidence one way or the other.
As I recall, this stretch
of the A303 used to be marked as just two lanes and four abreast wasn't uncommon. It's now the boringly [*] sensible alternating overtaking direction layout that Ancient described - more restrictive than your example.
[*] Well, boring apart from the almighty dash to all get ahead of each other every time your side of the road goes from two lanes down to one.