Biopolitical anticipated this post with his comment -- in fact, my post on the gas tax started off on the topic of road privatization but then, well, veered off course.
I like the idea of road privatization on the surface. The road system is similar to the mail system in that when it was instituted, there wasn't really a private company that had the necessary capital to take on such a project themselves. Now, however, we have FedEx and other shipping companies that do have that capital for shipping, and which would undoubtedly have branched into mail delivery were it not for the government monopoly. Why not the same for roads?
There are a few considerations here: