I like the ingenuity of your idea here - my only thought was if RFID readers could detect/register tag information quickly enough to make this work. You could

I like the ingenuity of your idea here - my only thought was if RFID readers could detect/register tag information quickly enough to make this work. You could generalize your idea to a more normal case of 'mix balls of {METAL/MATERIAL} into pavement and equip cars with sensors' Clever.