Just because there is Constitutional authority for something doesn't mean we have carte blanche to shovel money down a black hole trying to keep a bloated bureaucracy competitive with the free market. Did the federal government have anything to do with roads before the Interstate Highway System? I'm asking, I truly don't know. But it seems logical to me that it was mostly a state responsibility. In which case, some states may choose to exert more control over infrastructure building than others, with different degrees of public vs private ownership. And the best systems will be seen and emulated by others.
Anyway it seems pretty clear to me that "establish post office and post roads" means that federal government has the authority to make sure the mail gets delivered, nothing more. Think about it; the men who wrote that clause didn't even know what a locomotive was let alone air mail or e-mail. If they thought the federal government should have absolute authority over all roads, they would have explicitly phrased it that way.