Are there states where you can shoot car thieves on public roads? I'm not sure about that.
In Arizona we have the "Stand your ground law", which basically extends Castle doctrine to anywhere you can legally occupy while carrying a firearm. So, if I were threatened on a public road in Arizona I legally could shoot someone.
Does that mean that I'll walk away from the situation scott free? No, it doesn't.
Missouri's castle laws extend to anywhere you're currently residing and any vehicle. ie: if i'm camping and someone is coming into my tent, I can shoot to kill. if i'm staying in a hotel room and someone tries to break into my room, i can shoot to kill. and yes, if i'm currently sitting in my car at a traffic light and someone tries to get in... i can shoot to kill.
edit: from http://en.wikipedia.org/wiki/Castle_doctrine#States_with_a_Castle_Law
Missouri's castle law: (Extends to any building, inhabitable structure, or conveyance of any kind, whether the building, inhabitable structure, or conveyance is temporary or permanent, mobile or immobile (e.g., a camper, RV or mobile home), which has a roof over it, including a tent, and is designed to be occupied by people lodging therein at night, whether the person is residing there temporarily, permanently or visiting (e.g., a hotel or motel), and any vehicle. The defense against civil suits is absolute and includes the award of attorney's fees, court costs, and all reasonable expenses incurred by the defendant in defense of any civil action brought by a plaintiff.)
directly from the source: http://www.moga.mo.gov/statutes/C500-599/5630000031.HTM
protected against fuckers sueing me after i shoot them too.