So just... let them go. It's easy to do.

Escalation usually doesn't improve things, including for the person displaying the universal finger (or throwing the lug nut).

You've got a choice: You can keep plodding along in your truck while you escalate and retaliate and get all grumpy and stuff, or just keep plodding along in your truck without any of that noise.

The errant, instigating driver won't really learn anything either way.

You misunderstand. There is no grumpiness involving in throwing a lug nut at some unhinged asshole's car. On the contrary, it's quite cathartic.