Paying for roads

Paying for roads is always problematic. So how can we do this? My first instinct is to make sure that gas taxes are dedicated to road use, not put in the general fund first. And why hasn't the gas tax revenue scaled with population and traffic growth?

UPDATE: I spoke with the reporter who wrote the article. While revenue has gone up based on the amount of gas consumed, with it being just a flat rate per gallon, it hasn't scaled with inflation.