How are speed limits calculated?
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Ultimately they’re not calculated. They’re decided on by a person or persons making their best guess, who are making trade-offs between safety and traffic flow.
How those initial guesses are derived are based on a number of factors: trial and error, past experience, similarity to existing roads, size of the road/number of lanes, length of the road, number and type of intersections, min/max/average traffic, municipal zoning, pedestrian traffic, condition of the road, proximity to certain risk factors (schools, shopping strips etc), and more. Documented processes, guidelines and best practices exist for these things, and they are different for each government – [example](https://roadsafety.transport.nsw.gov.au/downloads/nsw_sza.pdf).
Then of course the speed limit can be changed based on statistics gathered over time – number/severity/type of incidents, data from traffic counters etc.
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