It doesn't even have to be installed, you can project a fake speed limit for a few milliseconds and the systems will accept that. https://www.nassiben.com/phantoms
I mean, the barrier to using a fake sign or using a marker to vandalize existing signs is much lower than projecting an image from a drone. But cool idea tho.
New trend: Walk around with a DIY stop sign so that you can cross the street when drivers don't respect crosswalks.