On roads around the world, how are safe speed limits decided and what factors go into making those decisions?
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It will vary in different places around the world. Straight, flat roads with little to no risk of foreign objects (ie. gravel/rocks) fouling the surface would get a higher speed limit than a road with lots of tight corners and frequent rock faces dropping rocks and gravel on the road as they erode.
Weather conditions may also determine speed limits, especially in stretches of road that see dramatic changes in altitude. If ice/rain are common, the speed limit will be reduced accordingly.
Lots of factors including:
Road design – for example is it narrow with bends or is it a separated carriageway
Junctions – grade separates (I.e. flyovers) or does traffic directly join the carriageway
Visibility
Volume of traffic
Accident rate
Urban or rural
Hazards
And more
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).
It’s supposed to be based on visibility and stopping distance. For example, if someone randomly pulls out of a driveway in front of you, you should be driving at a speed slow enough to stop before you hit them, starting from the time you see them. And it works the other way around, the person pulling out of the driveway should have enough of a time buffer to safely pull out, depending on how far they can see you from.
For highways the visibility is usually much better, which results in higher speed limits, but there’s still curves that may limit visibility. And at the extreme end, the curvature of the earth limits visibility.
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