To quibble in a minor way:

there would be no treaties in place allowing the use of the US road system

Approximately 80 per cent¹ of interstate truck tonnage on the "Pacific corridor" is currently carried by trucks registered in Mexico, under the existing NAFTA treaty.

¹ Published industry opinions, neither confirmed nor refuted by official (government) sources.

California would have no difficulty entering into that treaty on the same basis as Mexico - and/or Canada(?).

John