The reason there aren't speed bumps every few miles is the same reason it's not illegal to tell each other where the speed traps are: there is tension between public support for the goal, and public annoyance at enforcement.
This is also why it's not illegal to buy a car that can go 120 mph, even though there is not a single road in the U.S. with a speed limit that high.
> This is also why it's not illegal to buy a car that can go 120 mph
That, and I can take my 120+ mph car to the track. (Also, I can think of a couple times having a car able to go 120 mph has helped me get out of the way of some accidents a slower car would've got caught up in.)
I'd love to hear about the times going 120 mph kept you safe from an accident.
Keep in mind I'm not talking about powerful acceleration; top speed can be electronically governed. You could have a car that goes 0-60 in under 4 seconds, but is governed to 75 mph top speed by law.
This is also why it's not illegal to buy a car that can go 120 mph, even though there is not a single road in the U.S. with a speed limit that high.