I am assuming, that if you break the law of the road in a State, even one you are not resident of, you are still liable for prosecution?

If you do not have diplomatic immunity, you are still bound to follow the road rules of that State.

Each State is its own country (making the Governor a sort of King in each).

The States only agreed to work together - however, that has slowly turned into a merging a Federalized body.

If the States make a return to their independence, you would have a border patrol at every State line, checking for vehicle compliance, and, compliance with any law they have that is different to your State of residence.

You would need a passport check.

This would be like traveling in Europe, before the EU Federalized it.

Go from France to Germany - passport stamped.

Go from Arizona to New Mexico - passport stamped..

Continental travel would be slower, and, you would need to familiarize yourself with the road rules of that state