An array of prominent wingnuts believe that the Tenth Amendment dictates that the federal government shouldn't be meddling in a variety of "unconstitutional" programs -- including the federal highway system.
Once again, we have to say: Step away from the Constitution, wingnuts, before you hurt your walnut brains.
Article I, Section 8. The powers of Congress include: "To establish Post Offices and Post Roads." The U.S. Postal Service uses the highway system. Additionally, the highway system is used to transport soldiers -- required for "the common Defence." In fact, the full title of the highway system is "The Dwight D. Eisenhower National System of Interstate and Defense Highways" because it was partially inspired by the way the Nazis used the Autobahn to move troops during the War.
While we're here, it might be important to remind Michele Bachmann and Rick Perry that Section 8 also includes: "The Congress shall have Power To lay and collect Taxes, Duties, Imposts and Excises, to pay the Debts and provide for the common Defence and general Welfare of the United States; but all Duties, Imposts and Excises shall be uniform throughout the United States." And: "To promote the Progress of Science and useful Arts, by securing for limited Times to Authors and Inventors the exclusive Right to their respective Writings and Discoveries."
If they're going to be literalists about the Constitution, the above passages of Article II allow the federal government to establish a national healthcare system.