eli5: How do they decide what the speed limit is for a road?

Question

Basically the title. I’m curious what they do to decide. surely they don’t just stick a random number on a sign and call it a day.

In: 16

Answer 1

They gradually increase the speed limit until a certain number of fatalities per 10k TH (traffic hours) are reached. Then they reduce it by 5mph. Basically the same way they determine lethal doses of medicine.