On roads around the world, how are safe speed limits decided and what factors go into making those decisions?
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A major factor is pedestrian traffic. The odds of a pedestrian surviving a collision from an automobile has an exponential decay trend with the speed of the vehicle, or in other words the faster the car is going, the more likely the pedestrian will die or be seriously injured.
So in a lot of pedestrian areas, you will see speed limits of 35 or less. The more people you can expect to be walking around, the lower the speed will go, so you will see neighborhoods and downtown districts with 15-25 mph, and other city/town roads with 35 mph.
Country roads that are not freeways tend to be in the next tier of speeds. So you will see speeds of 40, 45, 50, 55 mph in these areas. If they are straight forward roads with few intersections, curves, hills, or turnoffs, then they will be more likely to be a 55 mph road. If there are a lot of hills, curves, or driveways, then it’ll probably be a 45 mph road.
And then you have your freeways, which tend to be 55+. If you are in a rural area away from cities, these freeways could be up to 70 mph (or 75 in some states). As you approach cities or interchanges, freeways will drop their speed closer to 55 mph. If you take a freeway into a hazardous area (i.e. a mountain freeway) the speed might further drop to 45 mph.
On ramps, sharp turns, traffic circles, and other incidental road junctions might have yellow speed limit signs that have a temporary drop in speed for safety (i.e. the speed limit might be 45 normally, but if a sharp turn might cause a vehicle to overturn at that speed, then there will be a yellow sign recommending 35 mph at that turn).
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