As has been pointed out already, one of the biggest challenges our current system of paying for our roads is that vehicles are increasingly more efficient. As a result, they use less gas and pay less towards the roads while having the same impact on them. It is impossible to understate the vital importance of a well maintained, functioning road system; it's not only how we get to work/school/play, but it's also what keeps goods flowing into stores and food to the shelves of our supermarkets. It's not a glamorous thing, but we let it fall apart to our own great peril.
What I know is that we will need to shift our funding paradigm away from a gas tax and into something else. As I elided to above, an individual's "use" of the roads doesn't amount only to the amount that person drives on those roads in a given year. Indeed, the person who lives in a city and doesn't own a car still get's "use" out of the roads every time he or she walks into a store and buys something or FedEx drops off the shoes she ordered two days ago from Amazon.
Bad roads not only make it unpleasant to drive, but they also make it less efficient. As I've started paying attention to my energy usage more, I've noticed that, while on the freeway, the recently paved sections require less energy to drive across.
What's my point? Well, we need the roads and we need them to be in good shape. The way we've paid for them in the past isn't working now and it will continue to work less in the future so we need to make some changes. Given that we all benefit from the roads I'm okay with the notion of taxing everyone for them equally regardless of how much one drives but I recognize that's not at all a popular notion. The idea of a per mile with weight modifier tax is fine with me if it gets the job done and we can figure out how to do it without spending all the money we raise trying to raise it.