There is one reason to compare it to the cost of gasoline, which is predicated on an inverse relationship between the cost of fuel and the amount people are willing to spend on it. While there are many quibbles and outright logical flaws in the reasoning behind the gas tax, this one seems relatively sound:
1) Gas tax is a certain percentage of cost of fuel, collected as fuel is purchased. Fuel use approximately correlates with wear and tear on roads.
2) Price of gas increases dramatically (roughly 4x) with no corresponding increase in taxes (since it's a fixed rate, not a fixed percentage). This causes a short term decline in usage, reducing (slightly) wear and tear on roads. but then...
3) Drivers acquire more efficient vehicles to offset the price of fuel. These vehicles aren't any smaller or lighter, so they cause the same wear and tear on the roads, but they deliver less fuel-tax revenue to pay for that road use.
I do appreciate the inflation method for calculating fuel taxes, though. I had a conversation with some friends recently about the comparative pointlessness of a five cent deposit on a can of soda or beer now, versus in the mid 1980's, when if you returned ten cans you could use the deposit money to buy a full can of soda.