You build a private road, on your own private property, and you can go as fast as you like, for as long as you like. Good examples of that is KCIR (Kansas City International Raceway), a drag strip that's in suburban Kansas City, or Heartland Park, a race track in Topeka, KS. I've taken my bike to Heartland Park for track days, riding it on the city streets and interstates, and I followed the laws required to ride the bike on public roads. However, once on the track, I had to follow track rules, which included things that made my bike illegal to ride on the street (like covering or deactiving all the lights and turn signals, and removing the mirrors). I was then free to ride as fast as I like, in fact, I was timed at over 127MPH on the straight there. And that was OK.
And I've got friends who've driven their cars to KCIR, and once on the track, managed speeds well in excess of the 45MPH speed limit of Noland Road in Kansas City, which is the street you need to drive down to get to KCIR.
Mark
And I can name plenty of other things you can't do in the privacy of your own property, including paying for sex and doing drugs. I know many libertarians argue that those things should be legal. But my point is that we don't live in the society run by libertarian rules.