Bill D., the dippable mirror (which didn't reach here until long after it's invention as I recall) came to my mind as an invention last night, as yet again I was followed by an idiot with his headlights at the wrong angle. (I assume male - may be wrong. But most drivers are male.)

it wasn't so much the brilliance of the device, but the necessity for it that exercised my mind. Rear headlight glare may be caused by the occasional person forgetting to dip, but seems far more often to be more long term problems. Such as drivers who have been abroad not readjusting the aim of the headlights (I live in Kent), drivers who have not adjusted for a load raising the front of the car, drivers with a failed headlight running on full beam to compensate, drivers who have chosen to use fog lights in clear conditions (and where there has not been fog elsewhere on their journey) and drivers of large SUVs whose lights are too high off the ground (doesn't happen with lorries).

When following another vehicle, it should be easy to spot if one's lights are aimed wrongly and do something about it.

The mirrors don't approach the basic fault - and do not solve the problem of those drivers mentioned above when their glare is coming towards you.