[Dylan16807]

>If everyone were driving slower, everyone would be safer

But that's always true. All the way down to 10mph speed limits.

The job of speed limits is not maximum safety at the cost of everything else.

In practice, people can tell the design speed of a road and mostly follow that, no matter what the signs say. Defecting or not doesn't matter very much.

[TeMPOraL]

> In practice, people can tell the design speed of a road and mostly follow that, no matter what the signs say. Defecting or not doesn't matter very much.

It does, because what people can tell is only the design of the stretch of the road they're at. They have no awareness of how it interacts with rest of the roads in the area. By ignoring the rules they make it significantly harder to optimize traffic flow globally. And even if the limits were wrong, it's still better to have bad rules that are actually followed, because then people responsible can actually observe they're bad and change them for the better.

[ubernostrum]

By definition, the design speed of a segment of road comes from taking into account the entire segment. If a segment contains sharp curves, for example, then the entire segment can be designed around that fact.

[Robin_Message]

> In practice, people can tell the design speed of a road

I think there have been quite a few studies showing most people think they are above average at driving. I don't think people do this at all. I think they are overconfident, hurrying apes, with no real conception of stopping distance or kinetic energy, aiming to just miss each other with these multi-ton projectiles. It's sadly not surprising how many people are killed on the roads.

[Pyxl101]

It's not about some particular kind of skill-based estimate of the road's design speed. It's much simpler than that: people drive at the fastest speed at which they feel safe.

If people are driving on a large separated highway with big lanes, plenty of light, and it's straight with no curves, and no traffic for miles, then they might feel safe going 80 or 100 mph if their vehicle can handle it. People on a small, narrow, windy street with pedestrians around will drive another speed. The point is that people make these judgment calls primarily based on instinct, rather than based on signs.

Imagine that you were walking down a path on foot, and one sign said the walking path was supposed to be 1 mph, and another said 2 mph, and another said 3 mph. Would those signs mean much to you, or would you trust your two feet, sense of balance, and situational awareness to find the right speed? People do the same in cars. Dylan16807 is right.

Most people do, anyway. A small subset of people, I believe it's about 5%, follow the sign regardless. These people create traffic problems since they move at a different speed than the rest of traffic. I read a good article about how traffic planners are re-evaluating the idea that slow speed limits are a good thing or beneficial for safety. The conclusion was that slower speed limits do not actually benefit safety; if the posted speed limit is out of alignment with the road, and whether people feel safe, it creates more problems than it helps. The takeaway from the research was that if traffic planners want people to drive more slowly, then they need to create smaller, narrower roads.

[TeMPOraL]

> Would those signs mean much to you, or would you trust your two feet, sense of balance, and situational awareness to find the right speed? People do the same in cars.

Except people don't have speedometers, and cars do. They are there to be used.

> A small subset of people, I believe it's about 5%, follow the sign regardless. These people create traffic problems since they move at a different speed than the rest of traffic. I read a good article about how traffic planners are re-evaluating the idea that slow speed limits are a good thing or beneficial for safety. (...) The takeaway from the research was that if traffic planners want people to drive more slowly, then they need to create smaller, narrower roads.

Society is fixed, biology (or in this case: external conditions) is mutable. It's a sad but I guess unavoidable outcome - since people under given conditions behave predictably like morons, it makes no sense to ask them to be responsible, so let's redesign the roads instead.

[terryf]

> It's sadly not surprising how many people are killed on the roads.

In 2013, US, total number of traffic deaths was 32719. Total number of miles driven: 2,946,000,000,000.

Miles driven per one death: 90,039,426

It's not even one-in-a-million chance of dying. It's one-in-90-million chance. That is really very good odds. Driving is really quite safe and getting safer all the time. Cars manufactured today have safety systems and braking ability, traction control and other safety measures that are basically incomparable to cars made 20-30 years ago. The speed limits are still the same for everyone though. The whole point of a car is to quickly get from one location to another.

[TeMPOraL]

> It's not even one-in-a-million chance of dying. It's one-in-90-million chance. That is really very good odds.

For you individually. Multiply that by the amount of drivers and the average amount of miles driven, and you get multiple deaths per day - almost one hundred a day, actually, in 2013.

The individual odds make drivers feel safe - and thus behave like idiots - and the result is tens of thousands of unnecessary deaths annually.

[sokoloff]

> It's not even one-in-a-million chance of dying. It's one-in-90-million chance.

How many trips are only 1 mile in length?

[ubernostrum]

Being able to sense the design speed of a road is completely unrelated to someone's estimation of their driving skills.

The design speed of a road is, in fact, something you're probably only partly consciously aware of as you're driving on it; it's something you get not from a speed-limit sign but from cues like the way curves are built, the length of merging ramps and exit lanes, or how far ahead of something a warning sign is posted. Whether you think you're an above-average driver has nothing whatsoever to do with the fact that you will, consciously or not, learn to pick up on those cues and you will adjust your driving to it unless you're constantly watching your speedometer (and even if you do, if you drive significantly below the design speed, there will be times when you're uncomfortable doing so, though you might not realize why it's making you uncomfortable).