That's speculation, which isn't a good starting point for public policy. Neither is trying to force a square [solar panel pipe dream] peg into a round [economic] hole. We need to allow technology to outpace mandate, or it won't be advancement but forced, manipulated, and built on faulty assumptions. We need progress which takes time, not an end result reached tomorrow by dictate. Natural gas has already reduced pollution significantly and has more room to run. It's a pretty damn good intermediate.
For example, how dumb would we feel by forcing a heavy portion of expensive, inefficient PEV into our fleet only to watch Japan come out with a cheaper, higher efficiency hydrogen powered vehicle? Toyota, Honda, and Hyundai are coming to market with some early models.
The easiest way to get rid of a sin is to tax it. Tax pollution and let the market decide how best to avoid paying it. $5/gallon gasoline minimum price would be my preference.