Originally Posted by rm -rf

Sailboats use radar retroreflectors to make themselves more visible to overtaking boat's radar. But these probably need to be at least a few inches wide, and maybe more, perhaps too large for a bike.

But most self driving cars use lidar (infrared, I assume) which transmits and receives a beam of light. So a normal bike reflector would make the bike very visible to the car computer. (bike reflectors are also corner cube retroreflectors, just an array of tiny ones.)

So this is passive, no batteries, and inexpensive.