Over here (Finland), tax revenue from fuel and vehicle taxes is 7 G€ per years, government expenditure on roads is in the order of 1 G€ per year. There is no relationship of vehicle/gas taxes and road maintenance; taxes are just general revenue.

And I think it's just the same in most countries: taxes are just taxes, general revenue; not earmarked money for some particular purpose.

(What baffles me is that the gas tax in U.S. is so low; that contributes to the situation where vehicles are often very large and consume ridiculously high amounts of fuel).

I had a big long post written about the why and how of the gas tax in the USA, and the economics of consumer preference, the history of American car design, and the implications of the historic price of fuel on car design, penetration of auto use, and the effect of all that on urban planning.

It was too long, and I fear it might be boring. So, to summarize: due to historic accident, Americans like big cars. Moreover, no matter what cars they might prefer, ownership of a car is an economic necessity for most Americans-again, historic accident. Those Americans are also not very well off, at least by northern-Europe standards, so money for gas is a big line-item in the family budget. A regressive tax increase on fuel would be clearly felt && very unwelcome, even though many if not most could see the logic behind it.

Agreed, but what I don't understand is why poor people (who have trouble with fuel price) still buy so big cars.

That's only true if our goal is to minimize consumption, which I'm not sure is advisable for the majority of roads, especially once we switch to electrical vehicles. A subsidized transportation network is pretty effective at encouraging commerce.

That being said, I do think there are localized areas where we want to decrease usage. So the future probably involves a combination of tax-free electrical vehicles and substantially more toll roads/congestion-charged zones.