The downside to fuel efficiency
If state revenue for transportation systems is largely per gallon gas-tax dependent -- and if cars become more fuel efficient, needing fewer gallons of gas to put the same amount of wear and tear on the transportation infrastructure -- or perhaps not needing any gas at all in the ideal future -- revenue goes down -- and the state goes broke.

A couple of Oregon State University professors are working on a new model where tax is levied on the number of miles driven rather than gallons of fuel burned. The system involves a computer read on a black box implanted in the car. (via)

I don't mind paying taxes (Our system isn't perfect but we in the US get a lot of bang for our buck compared to much of the world), however, there has got to be an even simpler -- less intrusive and less tech-driven solution.