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turchin comments on Open thread, Jul. 18 - Jul. 24, 2016 - Less Wrong
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Why car safety is not advertise as its main quality?
Tesla suffered its first fatal accident in self-driving mode after driving 130 million ml, while the average mileage between fatal accidents in the US is 90 million ml. This is presented as evidence of the safety of Tesla.
However, the safety of cars of different classes of security has 1000 times difference.
Kia Rio has one fatal accident at about 10 million ml, and Subaru Legacy has less than one per billion km (in fact zero).
The latest data on the risks of different car models is here: http://www.iihs.org/iihs/topics/driver-death-rates
I did some calculations based on presented data and typical car driving 20 000 ml a year assumption.
Dodge Caravan has the risk of a fatal accident on a 1 to 10 billion miles. (I saw it in similar sheet before.)
These cars are 3-5 times more expensive than the Kia, and due to the greater mass, strength and quality provides great security.
I think there's a major selection effect when safety comes into play; that is, there is a sizable fraction of drivers who do prioritize safety, they buy the cars that are reputed to be safest, and then those cars appear even safer in the statistics. (For example, there are some engineering differences about the Subaru compared to other cars, but the differences between Subaru drivers and the drivers of other cars are probably larger.)
But roughly only a half of accidents could be blamed on each car driver, so even safest driver would get only 50 per cent reduction in accident rate. Other reckless drivers could rear-end him or even t-bonned.
Many serious accidents are single car crashes (more than half here). And a lot of collisions that aren't officially your fault you can still avoid if you pay attention.
I think that if we include all driving habits, single car crashes, observation selection we will get probably 1 in 10 difference, but I still surprised by 1 to 1000 difference in fatalities between most dangerous and most safe cars.
I think that also weight is very important factor: heavier cars are much safer.
Heavier cars are safer for the people in the heavier cars, and more dangerous for others with whom they may collide.
This produces a sort of evolutionary arms race in the direction of larger heavier cars, making everyone less safe overall.
(Other changes in car design have made them much safer over the years. Today's cars are much safer than those of, say, 30 years ago. But not, I think, because they are heavier.)
While at first I thought to agree with you, I have a counterexample. Bigger car is safer if it collides with a wall, small ones and bikes are more dangerous in this case, because they result in quicker deceleration of the driver.
So longer cars are also safer, and longer car tends to be heavy ones. So if all cars will be large we will get safer driving environment.
So it is easy to imagine "absolutely" safe car: it would be large, heavy and very slow (like 30 miles per hour).
Maybe bigger cars are safer if they hit walls, but it's not entirely obvious to me. The more massive the car, the faster it will still be going when its front crumple zone finishes crumpling and wall and windscreen make contact; isn't that bad? Slower driver deceleration isn't a good thing if the still-fast-moving driver starts colliding with bricks.
Anyway, most car accidents don't involve hitting walls, and many car accidents endanger people other than the occupants of the cars, which means more mass => more danger.
A more massive car has more energy to dissipate but also more structure (crumple zones) to apply this energy to. The net balance is not obvious me, either.
While most car accidents do not involve hitting walls, a lot involve hitting objects-other-than-cars (guard railings, trees, animals, etc.) where being heavy can be an advantage (because that excess energy that you have you dissipate into the object). As to pedestrians, the mass discrepancy is big enough to not matter -- the consequences to the pedestrian of being hit by a 1-tonne car are the same as being hit by a 2-tonne car.
Being lighter is better in that case.
The 2-tonne car may be harder to stop or steer so that it doesn't kill the pedestrian.
Sure, if you include when and where people drive as part of what you blame on them. (Safety conscious people might move to particular places, or spend evenings in, or so on, and so even if they're just as good at avoiding accidents conditioned on condition the total distribution is weighted by conditions, which they have some control over.)
Because people believe that the driver's skill is significantly more important factor in determining safety. I don't know if this belief is correct but it sounds plausible. Do insurance companies offer different prices for owners of different cars?
Not only driver skill, I'd have thought; also how they're used. The Dodge Caravan is a big thing with space for lots of people. I bet it's used mostly for transporting large-ish families around. Whether you're an expert driver or a very poor one, I expect you drive more slowly and more carefully when your spouse and children are on board.
I think part of the promise of a BMW is that it's a safe car. However rightly or wrongly 5-star safety ratings is the common measurement of safety and not fatal accidents per mile.
But isn't the fatal accident per mile is better predictive measure?
It might be or it might not be because observational studies don't tend to be good at analysing causation.
In general marketing executives don't focus on using metrics that are good predictive measures. They also don't want to advertise less fatalities but more safety.
I don't think so; different types of car are bought by different people and driven differently. E.g. a person who buys a Lamborghini or Ferrari probably likes to drive fast and show off; a person who buys a Volvo probably drives a lot more carefully.