The tax would presumably be proportional to (or at least a function of) the amount of use. If the only time a person relied on the road was when they needed an emergency service, then they would pay very little tax indeed. If someone decided to start a delivery business, which saw thousands of cars going to and from peoples houses every day, then they would pay a much larger tax. This tax would presumably be passed onto their customers, making their customers pay a higher tax than the rest of the public.