Simply put: if you need (in the most extreme sense of that word) to transition more of the US vehicle fleet to diesel, you'll figure it out.

Whether that's brutal enforcement of HOV lanes, cranking up efficiency regulations to the point where's it's diesel or a hybrid, or something I haven't thought of, is all moot.

Personally, I think we're headed for fuel cells, once we figure out how to produce, handle and distribute hydrogen. Massive-scale solar-powered electrolysis is surely going to be on the agenda at some point.