Take the 85th percentile rule.
If you take a neighborhood road and change it from 40mph to 25mph in an attempt to "save the children", you can easily make it more dangerous.
The 85th percentile rule figures how fast people go on a road, and sets an appropriate speed limit that people naturally follow. Attempting to set a speed limit too low or too high leads to a wide speed variance, which makes the road more dangerous.
And where roads need to be fast and move a lot of cars, separate them out from other uses.
see fig 4 on speed variance:
https://www.fhwa.dot.gov/publications/research/safety/17098/...
going faster or slower than average traffic speed shows markedly greater accident rate.
note that this can apply not only to speed limits, but things like express lanes going different speeds than other lanes
You are correct that people driving too fast make the road dangerous, but so does people driving too slow. Generally, from a safety point of view, you want the slowest speed at which almost everyone will actually drive at, as large variance in speed between drivers is dangerous. I think this is what the parent post was getting at: a speed limit too fast OR too slow will increase the number of accidents, keeping in mind that there will always be at least some drivers speeding.
It's accurate to say that people driving too fast are extra dangerous when there are slower vehicles in the road. The danger is still caused by the people driving too fast, not by those driving slowly, though. Speed kills.
I agree that merely lowering speeds without changing the design speed is a bad move, though.
We already know it's hopeless to teach them so what is left are traffic calming measures, heavy handed enforcement and technology (automatic speed limiters in cars).
Setting speed limits to speed people choose is a terrible idea.
The point of the Solomon curve is specifically that that isn't true.
This proves it doesn't work unless you only care about safety of people in cars (which Salomon curve seems to based on meaning it's meaningless for road design with the exception of highways).
Last time I was in Edmonton, known for extremely car centric design, wide roads, ample highways, etc. I was shocked by how much slower people drove, and as a result, how much safer driving was in general.
You only have to get slapped with a fine a few times before you start learning to control your speed.
Any solution needs to be wide spread.
PS plants are expensive, they need very regular maintenance.