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

Generally, you get the default speed limit and you design the roads to make that safe — you get minima for the width, radius of the curves and probably more things.

There are also rules for setting a specific speed limit. For a given road, when not explicitly constrained, people will naturally drive at some speed (or, rather, a distribution of speeds, because different people are different). You set the speed limit to the given quantile of that distribution.

Interestingly, in some places (I’ve seen this is Zürich), roads are designed to make that natural speed lower, by making them narrower and adding some curves (by narrowing them down _on alternating sides_).