system
Given the following post, respond as if you're commenting on Reddit:
human
eli5: How do they decide what the speed limit is for a road?

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.
gpt
It depends largely on the nature and purpose of the road.  Things like width, straightness, slope, signage and surface influence how safe it is to use that road at speed.  Obviously you need to go slowly on a narrow, windy road if you don't want to accidentally leave it.  If you want a road to be fast, you need to make it straight enough that steering is easy and you need to make sure people have enough time, space and notice to react to whatever they're approaching.  The other thing is that you need to make sure the limit accounts for transitions onto other roadways or through areas of different usage.  A smooth transition between different speed zones is better than, for instance, letting a highway exit directly into a school zone.