This is a bit off topic, though I can tie it back in, but I spent an hour working on it as part of a "discussion" with a "friend" and since I couldn't find it on the internet, I thought I'd share it.
The discussion was about the gas tax as a user fee. The gasoline tax isn't really a user fee for roads. It is a user fee for using gasoline. Historically, that has been a good proxy for using roads, and if it was unfair, it was unfair to polluters and so people were OK with that. Of course now, if you drive an electric car, you use the roads for free - same as a cyclist (see, I can tie it back in).
A true user fee for roads would charge a fee per mile driven, the so-called VMT. Personally, I support a switch to the VMT, while keeping a lowered gasoline tax that supports environmental mitigation of gasoline drilling, refining and use (and perhaps a way to pay for the Iraq war, but that's a whole other issue). One great thing about the VMT is that it won't drop as cars become more efficient. It becomes a true user fee. And we can still push for more efficient cars through the remaining gas tax and CAFE standards.
So, I tried to figure out how much tax people pay on average per mile driven. This required finding the gas tax per year (pretty easy), the average gas mileage of cars per year (not so easy) and adjusting it for inflation (easy).
No one keeps track of average gas mileage of cars on the road (as far as I can tell). There are some estimates out there, but nothing definitive. So, what I used was the CAFE standards for passenger cars. Of course, there were no CAFE standards prior to 1978. For those years I used an estimate and for years not esitmated I interpolated. So, it's not perfect. But I still think this is pretty close to the tax per mile driven for the average road user by year. A way to check it would be to take the total gasoline tax collected and divide it by the total number of miles driven. I'd be glad to tweak this if anyone finds better data.
But using my numbers, the average per mile tax in 2010 dollars since the gas tax was introduced in 1932 is 11.37 1.137 cents and right now we pay 6.7 0.67 cents.
Anyway, to tie this in again, road lobbyists and supporters love to blame money spent on biking and transit as the cause for the shortfall in the Highway Trust Fund. But it's clear, looking at this chart, that the cause is the reduction in the tax per mile driven that results from inflation and greater automobile efficiency and the unwillingness of Congress to raise the tax to adjust to that. If we had a tax of 1.1 cents per mile driven (as is the historical average) there would be plenty for roads. And we wouldn't need to worry about higher CAFE standards reducing the trust fund.