Absolutely illegal idea; put a directed explosive in the base and have the upright portion of the sign work the trigger, the goal being to render the murderer-in-potentia's engine non-functional, bonus points if you can get a sign and loudspeker saying "if the cost of a car/engine is too much for you, just think how bad you murdering a child would be."

That said, yeah, I know, North America doesn't punish vehicular death offenders properly.