Depending on the severity it ranges from vandalism to criminal mischief and interfering with transportation to reckless endangerment. Regardless I won't feel bad if someone who is actively threatening the lives and safety of other gets their property damaged a bit, also I'm being hyperbolic not actually advocating it incase that wasn't clear to some dolts.

Only a good guy with a brick can stop a bad guy with a car.

Obviously, the legal technicalities are 99% of the reason why I'm not actually doing it (edit--throwing tomatoes, not bricks!) The other 1% is the off chance that it goes in an open window, hits someone, and causes a fatal accident.

Slapping a sticker on a car that says "I'm a shit driver" would be safer for the driver, but clearly not for the vigilante.

I've spent a lot of time thinking about this and would love for there to be a safe projectile that doesn't cause permanent damage for this task, but I just don't know it.