How are speed limits calculated?
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I had a relative who worked in a city taffic department. He described experiments they did. For example, they build a variety of different speed bumps and hired stunt drivers to go over them at different speed in both cars and motorcycles. Oh, and hook-and-ladder firetrucks too. He taught me that for many speed bumps, they get worse and worse until you hit a certain speed, and then they start getting better again. This isn’t a recommendation, of course.
I can’t remember all of his stories, but I got the sense it was more about experiments and experience than about calculations. That was a long time ago, so it may be different now.
Ideally the way it works is that traffic engineers conduct a speed study where they take into account a bunch of different factors including measuring the average speed of motorists on the roadway (if it already exists) and then come up with a recommendation that’s used to set the speed limit.
What happens a lot of the time is that some retiree decides that the cars are going too fast down a given road and starts calling up their local government to complain about it. Old people are vastly more likely to turn up and vote in local elections and so they tend to get their way when it comes to city/county level issues like speed limits.
They’re not. Many laws require an arbitrary number. Speed limits are an example. Another example is the driving age of 16 in most North American jurisdictions. The legislators all know that there is nothing magical that happens when a person turns 16 that makes them capable of learning to drive. They know that plenty of 14-year-olds would make fine drivers, and plenty of 30-year-olds should never be driving. But the fact is that the line has to be drawn somewhere, and it has to be the same for everyone (in a democracy–in another type of state it can be different, e.g. in Saudi Arabia until very recently, men could drive at 16 and women could drive at no age), and 16 is what got picked.
In Canada the driving age had been challenged as unconstitutional because it is so arbitrary, and laws should not be arbitrary. The arbitrariness was upheld by the court as a reasonable and necessary limit on the right to be free from arbitrary restrictions. Why? See above. Basically, the line must be drawn somewhere.
The exact same applies to speed limits.
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.
They are not and they are based on old cars properties… If you go in history of the speed limits in your country, you will see nothing has changed about speed in the past 50+ years. Meanwhile we have seatbelts, airbags, deformation zones… Speed limits should be higher or none these days.
Where I live they’re supposed to be 85% of the average speed when measured. So if I measure a bunch of cars in an area and that average is 70ish, the speed limit should be 60. That doesn’t always work in practice, though. More often than not they simply assign a stretch of road with those of like characteristics (residential, straight vs. curved, etc.) and assign a speed limit the same as other similar roads.
Something interesting that I also recently learned from a traffic engineer friend, in my state they can’t lower the speed limit in a construction zone by anymore than 10mph. On a highway near me the normal speed limit is 70mph. Currently they’re doing construction that has traffic reduced to a single lane with Jersey barriers on each side. So, the construction workers have to work alongside tractor trailers doing 60 through a 10 foot wide cattle chute. Scary.
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