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Have you even evar driven a van?
Blind spots do happen, it's rare and usually down to awkward acutely angled junctions and that sort of thing, but there are occasions when to see properly in a car you'd have to turn your head to see through the back window or a rear passenger window, which you can't do in a van.
Friendly sign is friendly. Van driver just wants me to take care, like I do him/her.
I approve.
Since when can't van drivers see what's next to them? Does the driver have a neck brace?