I don't know about US, but in EU you can buy mini-cars that have 4 wheels, but are equipped with 50-250cc engines, and do not classify as cars legally, so you don't even need a car driving licence - a moped driving licence is usually enough. They also don't have to conform to "car" safety standards(they usually do have seatbelts though), and yeah, they are not allowed on the motorways in most EU countries, just like mopeds aren't.

https://www.youtube.com/watch?v=HW2nI7VdS_U

https://www.youtube.com/watch?v=qdw6mgtTwt8