Could people start wearing small transmitters that nearby autonomous vehicles can read?
Another idea would be to just throw them right in front of an autonomous vehicle at full speed. Let's test those brakes!
(sorry, but the original idea is just so ridiculous that I am incapable of responding in a non-sarcastic way to this...)
[1] https://www.theverge.com/circuitbreaker/2017/9/25/16362296/g...
Lanes are marked, usually. So why not embed something like an UV/IR-only color pigment in the lanes? Invisible so it doesn't affect human drivers but can easily be picked up by cameras.