The Highway Trust Fund is staring down an insolvency crisis due to diminishing gas tax revenues. Central to this development are increasing fuel efficiency standards of gas powered vehicles and the roll-out of alternative fuel vehicles (AFVs), which are not subject to the gas tax in most states. However, there are more cars on the roads now than ever, and many of them are powered by alternative fuels like natural gas, propane, and electricity. Are drivers of those vehicles paying their fair share for maintaining roads and bridges? Some states have enacted flat registration and licensing fees to address these issues.