Also, the problem with roads is the fact that they are a monopoly, and as such the prices would become unreasonable, sooner or later, everywhere.
And the vote with your pocketbook does not really work also because people are naturally reluctant to give up any cash that they feel they don't absolutely have to, especially for something as remote as national defense. The might of the American army would become history in a flash, and like it or not a lot of what makes America great comes from the fact that it is the greatest military power in the entire history. The effects would not become visible only overseas, but back home and very nasty and very soon. Like any great enterprise, the army can not be simply wound down. The conservation costs alone for all the equipment are staggering. If you would cut the funding tomorrow in half, the entire thing would just implode.