Note the way to do this is to follow engineering rules.
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.
But you're saying the speed limit (being too high or low) makes the road dangerous. Aren't the people driving their cars too fast making the road dangerous?
A great many people when they see a open straight road with little obstacles or pedestrians will go 45-50 MPH. A great many people when they see a 15 MPH sign will go 20 MPH. Pair that low speed limit with a "fast" road and you will end with many people going 45 MPH and many people going 20 MPH. This variance in speed, with some people going much slower than others can be more dangerous than if most went the same speed - e.g. if the limit were 45 MPH.
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.
> You are correct that people driving too fast make the road dangerous, but so does people driving too slow.
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 don't think I've ever heard someone argue that the 85th percentile rule is actually a good invention - it's a disaster that codifies the behaviour of speeding drivers.
I agree that merely lowering speeds without changing the design speed is a bad move, though.
The problem is that people are naturally very bad drivers and are especially bad at judging what safe speed is.
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.
People naturally choose speeds according to how safe they feel not how safe it is for everyone else.
If you have a residential area road which is straight and wide enough to go fast people will choose higher speed than on a narrow road.
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).
I live in BC, where speed cameras are banned by law. Right next door is Alberta.
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.
I'd go further and say automated average speed cameras are the most effective I've seen. Point speed cameras just get marked on a map and cause sudden braking and acceleration to dodge them - this can be effective at particular danger spots, but I always feel the average speed cameras in the UK are far more effective at changing driver habits in general.
Yeah, but those probably cost more and don't make for a nice environment outside of cars. Traffic calming can be super cheap and it makes for a super pleasant environment for everyone.
Traffic calming is usually more expensive than speed cameras. A speed bump isn't expensive but people are usually talking about stuff like bump outs and raised intersections and protected bicycle lanes and wide sidewalks when they are talking about traffic calming. Those are six figures per intersection. Cameras are low five figures.
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.