nobody's already selling this sort of thing, are they? a little laser in a bubble on a gyroscope swivel that you'd clip on the outside of your bike handle, and it would draw a bright line on the ground at an adjustable distance from the outside of the handle (like 150cm, for german cities) to remind would-be overtakers of the necessary distance. with a bit of logic in it to adjust for handle movement, angles etc. to make sure the distance stays the same.

i'd probably buy a thing like that, but i couldn't build it.

i found a 2012 article on a rear light that would project a too-close line in either direction - but it does not appear to have gone anywhere? you can find it secondhand, but the company seems to have gone under, and it appears to have been too gimmicky and too narrow in any event.