If Austrian fuel taxes were all used in order to build and maintain roads, we would have the most luxurious road system on Earth. Most developed countries use fuel taxes as a piggy bank for the social budget and whatever other fancy strikes spendthrift politicians on a whim. Its true, road taxes in most countries are anachronistic and the volume-based system should be changed towards an energy content-based system. Some fuels such as methane are more voluminous but also much better for the surroundings. But first, stop the robbery.
Per-gallon taxes are a 20th Century idea. When the range of fuel efficiency in the automotive fleet was narrower, this mindset was defensible, but innovation has led to expansive automotive diversity.