You got the question I was responding to backward, it was said that there is absolutely no rational reason why cars should be able to drive above 150km/hr.

"Come up with some reasons why people wouldn't want to drive fast" is trite, boring, I would have ignored it.

Edit to clarify: "Why not" in my reply isn't "Why not drive fast", it's "You say there is no rational reason, why not? Here's one".

Why do you assume what's rational and what's not? There are plenty of situations where arriving sooner is more rational and important than saving gas.

(leaving aside legality or safety)