Follow up / related question: as I understand it, this was used to force the legal drinking age in states up to 21. Was there much resistance to this, particularly from a state's rights perspective?

As a follow-up​ question, how prevalent was Libertarian type thinking at the time, and was there any opposition to this law from a specifically Libertarian perspective?

Socially there was great debate over the route of new Interstate Highways. With the U.S. Numbered Routes in the 1920s, the routes were built in the counties who could afford to contribute. For instance, Bedford County, Tennessee was bypassed when US 41 was built because they were too poor to pay for the construction. Likewise, the routes of the roads were debated based on practicality. A man built a garage in Hebardsville, Henderson County, Kentucky because the original plan was to run US 60 through there. The route was changed to eliminate a river crossing, and the garage had virtually no customers.

Likewise, the Interstate Highway system was ran through areas that would benefit from the increased traffic and justify the costs. Many interstates simply supplanted existing numbered routes. I-24 runs with US-41 through Southern Kentucky then I-75 runs with 41 from Chattanooga to Tampa where 41 goes east on the Tamiami Trail. It's a rare example of a North South road being signed differently.

Remember, US-91 was in the West and US-1 is in the East. The Interstates are reversed to prevent confusion. Odd numbers run North and south, evens east and west. There are some exceptions though, like 24, which runs diagonally. Also, if the second digit is a 1 or 0 it's a main route. 2,3,4 and so on are arterial. A three numbered route is a loop is the first number is even and reconnects to the parent road. A first odd number is a spur that doented reconnect. Think about I-264 in Louisville.

There was great social upheaval due to some routes being planned to cut out certain population centers. US-31, I think it's 31, ran through Huntsville, AL. It received no interstate coverage. Once an interstate was built, the town exploded. Interstates were very valuable to a region, and there were plenty of Congresional battles to route them in certain places. Kentucky was completely cut of a major East/West interstate for instance, which led to the construction of the Parkways system which is now being redesignated as part of the interstates.

Generally, the biggest battles were over the routing of roads, but in some places US-Routes are still used. 31 is a major long haul route, for instance. Generally, the decommissioning of US-66 was kind of the end of the US Numbered Routes System

The main fight in Congress was over financing. The motorist and trucking lobbies objected to raising the gas and other excise taxes, and elements in the Eisenhower Administration were still toying with the idea of all-toll financing, which was unlikely to build a complete national network. The breakthrough on financing came from Hale Boggs (D-La.) and George Fallon (D.-Md.) in spring 1956, after three years of discussion.

I’ve never heard of any organized opposition that was part of congressional debate on environmental or social grounds, though commentators like Lewis Mumford, Christopher Tunnard, and others were philosophically concerned with how additional high-speed highways might affect big cities. But the Interstate system was primarily thought of as serving intercity traffic, and the need to accommodate increased volumes of big trucks and to improve highway safety was foremost in the minds of most. Superhighways were already being built all over the country, usually as improvements to existing US highways or as toll-financed turnpikes.

General routes for the network, including urban segments, had been set out in the 1955 Yellow Book but those weren’t specific enough to concern (or delight) specific landowners. The “freeway revolt” of the late 1960s began with concerns about harm to specific inner-city neighborhoods or parks, and eventually became part of more general 1970s concerns about the environment and about who makes decisions regarding disruptive public works projects. In a few places, objections and legal actions from businesses that would be bypassed were said to delay opening of the new highways temporarily, but I’ve never researched those cases to see what legal claims were made and whether the highways were, in fact, delayed.

I like Earl Swift’s The Big Roads as the most readable history of the Interstates, but Mark Rose's Interstate: Express Highway Politics 1939-1989 is the more scholarly source on the congressional machinations. A more recent book by Joseph DiMento and Cliff Ellis, Changing Lanes: Visions and Histories of Urban Freeways, recounts the history of how traffic engineers and city planners wrangled with this new force reshaping the city.

I'd also be interested in knowing what kind of resistance the people who had businesses along the older highways put up considering the interstate system basically put them out of business. Did they forsee this or did they think the interstate system wouldnt be a problem?