Considering the Sage of Knoxville's recent suggestion to keep moving if surrounded in your car, Andrew Branca, a lawyer who specializes in self-defense law in the US, provides a quick analysis of how self-defense laws would be applied in the case of blocked highways and riots.
Here's a taste, but the whole thing is worth reading over at Legal Insurrection:
In short, one would apply the usual five elements of a self-defense justification to evaluate such a use of force against others, just as in any other instance of self-defense. Those elements are, of course: innocence, imminence, proportionality, avoidance, and reasonableness.
When all required elements are present, the use of force was legally justified. If any required element is missing, whatever that use of force might have been it was not lawful self-defense.
One of the challenges to legally justifying the use of force against highway blockades is the element of imminence. Do people who are merely blocking a roadway represent an imminent threat against which some defensive force might be justified?
A second challenge is the element of proportionality. That is, if the force contemplated to be used against them is one’s vehicle, this will almost always constitute deadly force–that is, force capable of causing death or grave bodily injury. Deadly force can be used in self-defense only [when] the force with which you are threatened also constitutes deadly force.