Your libertarian fantasy would be fine if it wasn't for the fact that you 2 tonne steel box can kill /other/ people when you drive excessively fast and crash.
I'm not a libertarian, I just think the current speed limits should be increased to the speed people actually drive at, and then enforced. Speed differential is a big component of what makes driving dangerous, rather than just speed itself. 120km/h-130km/h is not excessively fast on a highway when everyone else is going that speed. If you don't want to take the risk of going on fast roads, that's fine, just take country roads or slower city roads, pretty much the only risk you have is crashing on your own.