Speed limits are a whole fiasco.
In the US, the original doctrine for speed limits is that they are supposed to be DESCRIPTIVE OF THE ROAD, and primarily for safety. The "speed sign" is supposed to inform you how fast THE ROAD is...how fast you can safely drive the car on it. It was NOT originally supposed to tell you how fast you actually could or should drive -- that's up to the driver. And it wasn't supposed to be a statutory limit of how fast you are permitted to drive. It was supposed to convey real, consistent, objective information to the drivers about the ROAD ahead. They weren't considered "speed limits", they were just "road speed signs".
The standard way to set these "road speed" signs, at least in the US, is/was to observe actual drivers using the road, and then set the "speed sign" to the 85th percentile of the speed of actual road users. The whole idea is that based on the narrowness or curviness etc. of the road, you post a sign to inform drivers "you probably want to go about 40mph here, based on what most drivers do. If you have a big truck, better go slower, if you have a sports car, you'll be fine at 50, but 85% of drivers go 40 here". This is still officially the method for setting speed signs in many road design manuals and practices in the US. When they use this 85th percentile, they usually invoke the term "design speed" of the road as opposed to the "statutory speed limit". But you have no idea if a given "speed limit" sign is a "design speed" sign or a "statutory speed limit" sign, and that's the whole problem.
Speed signs like this still exist on the German Autobahn. Everyone knows there's no "speed limit" in the US sense. But you might be confused to still see the "speed signs" on the autobahn. These are speed signs of the original type. Those are informing you about the road, for your information, but you won't get a ticket for going faster unless you do something else stupid. But they are ACCURATE, and you can trust them, and better pay attention to them. They weren't set by politicians. It's YOUR responsibility to drive your vehicle appropriately, and the road speed signs are a tool to help you do that.
In actual practice, widespread auto mobility has been practically a humanitarian crisis. Millions of people are seriously injured every year in the US alone (a staggering 300 billion dollars of hardware are smashed every year too) and it was never better, it used to be even worse. The idea that we can trust drivers to drive responsibly, just didn't work out, especially not in the US, where anyone who can fog a mirror gets a drivers' license, there is usually no practical alternative to driving, and we never take away licenses basically ever. Maybe it works a little better in Germany, where getting a license means you have to take real driver training, multiple tiers of licenses, take 6 hours of first aid training (seriously), official yearly eye exam, 0.05% BAC is an instant license suspension, 0.08% is instant license revocation, and causing an accident above 0.03% (seriously) can result in jail time, special endorsements required for transmission type and vehicle type, etc. But in the US, we talk big about personal responsibility, but nobody wants to actually hold drivers responsible for their behavior (nowadays that would probably be considered racist too) so we get the lowest common denominator solutions as always, punishing everyone. Texas and Montana were the last states to have roads without statutory speed limits.
Attempts to limit car speeds are the earliest and most effective attempts to stop the automobility bloodbath. Statutory speed limits are perfectly legitimate for populated areas. 90% of pedestrian collisions under 20mph are survivable. In the earliest days of automobiles, some cities didn't let cars enter at all unless accompanied by a guide walking, the logic being that cars going walking speed was the only safe way to mix cars with pedestrians (still basically true). Cincinnati had a citywide 20mph speed limit and even tried to pass a law requiring 25mph speed governors as early as 1925. There are plenty of reasons to allow cars into environments where they need to drive slower than they could technically negotiate an empty road.
The problem is the failure to maintain a clear distinction between "road-speed indicators" of the original type, and statutory speed limits. In the US, this has devolved to where all the signs say SPEED LIMIT, you have to assume it's a statutory speed limit I guess, but you really have no idea. When you implement the statutory speed limit, you lose the information about the road design. Assuming you obey the limit, you are stuck going just as fast in a Miata as a loaded Amazon truck. And since the reason for the speed limit varies -- sometimes safety, sometimes noise, sometimes congestion management, increasingly rarely A RELIABLE INDICATOR OF THE ROAD CONDITION like they should be, but you'll never know which -- the speed limit can no longer be trusted to tell you anything about the road at all. There are plenty of rural roads with 55mph speed limits will definitely kill you if you try to go that fast. But more often, the speed limit is set deliberately low for noise or safety reasons, so people learn to ignore them, then when they come across a road that has a REALISTIC speed limit, they look at the 30mph sign, shrug it off and assume they will be fine going 40, and then end up over the mountain.
I guess in the ideal world, roads would be marked with both...the actual design speed, and the statutory speed limit, if any. But as usual, we can't have nice things, and nobody wants to recognize the problem much less fix it.
