I think adjacent states need to have parity of state gas taxes so that there is no motivation to cross state lines to game the system. Instead of raising the gas tax, they should institute the per-mile fee for the incremental amount. Once that system is in place, plug-ins can pay their fair share. Of course, the open question is what is the "fair share" for a PHEV that could use a wildly different ratio of electricity and gas depending on the user's habits. One would have to allow access to the vehicle's data to do an accurate and "fair" calculation.