So long as those roads are used for shipping mail and connect states to each other, facilitating interstate commerce (to make it regular), they're perfectly Constitutional...
Gotta love the cop out since the vast, vast majority of the traffic on the interstate highways have no commercial purpose and unless you're using them purely for commercial activity then you are being a hypocrite for using them.
But I love the notion that the nanny staters always go to when the issue of being overtaxed or the government overly intrusive arises... "because you think government doesn't have the power to do (some big idea), good luck without your police, firemen, etc!" It's so demagogic... police and firemen are local issues, not federal ones. Ditto for the vast, vast majority of public roads (most belong to states/counties/towns).
Huh? How am I a nanny stater? Because I disagree with the absurd notion of original intent? The very fact that other founding fathers wrote the anti-federalist papers that had views of the constitution that were in direct opposition to the writers of the federalist papers should be enough to show that the concept is absolute bunk. When someone like you tries to claim the "original intent" of something you always use a specially hand-picked group of people that agree with you and ignore the dozens of others that held an opposing view on the subject. It's also funny in that you know absolutely nothing about my political views and have incorrectly plugged me as some sort of tax and spend liberal which is ludicrous and wrong. I'm not a left winger nor have I voted for a Democrat in my life. But I'm sure your pea-sized brain is unable to cope with the notion that someone might have a more nuanced political philosophy than your "zOMG YOU DISAGREE WITH ME THUS YOU HAVE TO BE A LIBRUL!!!!11ELEVENTYONE" view of the world.