Should the Gas Tax Change from Flat to a Percentage?

Michigan lawmakers are reviewing the gasoline tax which is currently levied as a flat tax and making it a percentage or variable tax. The theory is that at a flat tax, based on the amount of gallons sold, we are reducing the revenue received from the tax as taxpayers reduce the amount they travel for economic reasons. So, by going to a tax based on the price of gasoline, it is viewed that the revenue will increase. 

While this sounds like it makes sense at first, I was thinking about down the line when the prices and costs increase.  With the percentage tax, not only will the market price of gasoline increase, but so will the tax.  So, when prices are high, the revenue will be high, and vice versa.  However, why do we need more tax when prices are high?  Will roads be falling apart faster?  I would suggest exactly the opposite.  As prices increase, citizens will drive less, conserve more, and roads will last longer.  And, as prices fall, there will be less tax, and consumers will conserve less and drive more due to the low prices. 

It seems, on the surface, that the flat per gallon system works alright, but possibly we need to analyze the possibility of changing the rate instead of the system.  With the flat rates, revenues would seem to be more predictable and stable.