This is an issue that hasn't received a lot of public scrutiny: gasoline taxes are essentially highway user fees, and federal highway funds depend on people driving a certain number of miles on those highways each year.

It's a problem now, but nothing compared with what will happen when alternative-fuel vehicles start to comprise a significant percentage of cars on the road. After all, if you drive an electric car, or a hydrogen fuel-cell car, or even a car or truck fueled with red diesel, you're not paying into the highway-construction trust fund.

Assuming that gasoline-powered vehicles will be supplanted over time by other technologies, what's the fairest way to assess the user fee? Toll roads? A federal automobile licensing fee? Increased revenues from the general fund?

At what point to do you end gasoline taxes because they discriminate against people who own internal-combustion engines? Or do you keep them in place as a sin tax, in order to herd people into newer, cleaner technologies?