system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Who or what determines a road's speed limit?
gpt
Who sets it, and puts up the roadsigns? Typically whatever governmental body is responsible for the roads in that area. In the U.S. most highways are managed at the state level, and local roads at the county/city level.

Sometimes there are laws that determine default speed limits; other times the relevant Dept. of Transportation determines it based on conditions.

Those conditions typically are:

- How narrow the road is, and how many lanes there are.
- How many houses/businesses are along the road.
- How much the road curves or changes elevation (which effects visibility).