[Mantipath]

Sure. 3 MPH would have made a difference. So would 3 MPH less after that, and 3MPH more... heck, if it had been going 2 MPH total she'd probably be fine! In the aftermath of a front collision it would always have been safer if the car had been going slightly slower, regardless of the speed limit.

The strangest part of watching this story unfold has been discovering that some people have faith that speed limits are anything except a rough guess as to what's mostly safe on a given road. Oh, and subtract 10% from that guess to compensate for the fact that human drivers under 60 years old almost all speed by 4-9 MPH.

[notahacker]

According to the UK DOT the estimated pedestrian accident survival rate is 55% at 30mph and just 5% at 40mph (due to kinetic energy rather than reaction times) so 3mph in this range is not exactly an insignificant difference. The strangest thing about watching this story is that people who are apparently ignorant of this still feel sufficiently certain of the relative unimportance of speed differences in this range to sneer at other people for commenting on it.

Of course, as others have pointed out, human tendency to ignore speed limits and react badly to vehicles driving slower than ambient traffic speed creates another potential hazard to trade increased accident fatality rates off against when deciding if and when self driving vehicles can speed. It's trolley problems all the way down.

[lolc]

I agree fully with your first paragraph and I chuckled at the "trolley problems all the way down" depiction.

However I can't imagine a car company accepting the legal liability of allowing their autonomous cars to go over posted speed limits to "match traffic".

In a trolley problem you have the option to refuse a decision. By participating in "speed matching" you become one of the trolleys and you have accepted the unsafe conditions.

[notahacker]

Oh, I'm pretty sure that self driving software isn't being programmed to systematically disregard speed limits to match traffic, not even by a company with Uber's attitude towards regulations, but it is an example of conditions where rigid adherence to speed limits might increase rather than reduce risk.

[gjm11]

With enough self-driving cars on the road (which of course there aren't now), rigid adherence to speed limits might change the safest-and-easiest driving speed for everyone and lead to everyone driving at the limit rather than everyone everyone driving 5mph (or 10mph or whatever) above the limit.

So the safest reasonable thing for self-driving cars to do could depend on how many of them there are around.

(Of course there are other considerations of that sort. E.g., if all cars on the roads were self-driving then they could coordinate with one another in interesting ways and maybe go substantially faster than human-driven cars for a given level of safety. Maybe not, though, because of risks to pedestrians.)

[lolc]

Hmm, turns out Google cars are programmed to go over speed limits in certain situations. Quote from a Reuters article:

Google’s driverless car is programmed to stay within the speed limit, mostly. Research shows that sticking to the speed limit when other cars are going much faster actually can be dangerous, Dolgov says, so its autonomous car can go up to 10 mph (16 kph) above the speed limit when traffic conditions warrant.

https://uk.reuters.com/article/us-google-driverless/look-no-...

I wonder how they decide over what warrants speeding.

[Dylan16807]

> According to the UK DOT the estimated pedestrian accident survival rate is 55% at 30mph and just 5% at 40mph (due to kinetic energy rather than reaction times) so 3mph in this range is not exactly an insignificant difference.

You'd have that range whether people went at the speed limit, above the speed limit, or below the speed limit. As long as speed limits are made with actual speeds taken into account (they are) that argument is irrelevant to whether you should speed. It just means that if you really want to be safe to hit someone you shouldn't go above 25mph, no matter what the speed limit is.

> The strangest thing about watching this story is that people who are apparently ignorant of this still feel sufficiently certain of the relative unimportance of speed differences in this range to sneer at other people for commenting on it.

The speed limit here was at least 40, wasn't it? Your own numbers say that the differences are unimportant above 40.

[notahacker]

The police spokesperson is quoted saying the vehicle was doing 38 in a 35mph zone, which is pretty much where the curve suggests an extra little bit of speed is most lethal. (Other reports have suggested other speed limits)

[retsibsi]

I see this attitude a lot, but it seems strange to me. Of course any hard line will be somewhat arbitrary, but we need to draw hard lines if we want unambiguous, consistently enforceable laws. What's special about speed limits that makes it okay to ignore them?

To me they seem analogous to age-of-consent laws, which of course are not perfectly chosen for every individual -- and of course there's no magical change that occurs on a person's birthday -- but the law reflects society's best judgment of the balance between (protecting the vulnerable and allowing people to make their own choices :: safety and efficiency).

[rhacker]

I somewhat agree, I keep fighting this on HN - but speed limits are for two categories of people - the people driving, and the people expecting those drivers to be going a specific speed (or range). And 40 - 50 is not a range that is ever safe to put near people.

I grew up near (in the bay area) Almaden Expy where the signs post 45 (which means that in the bay area the limit is really 55). Just search for "Almaden Expy killed" in google and let me know if that's a safe street.

[Dylan16807]

It's analogous to that, except that speed limits very reliably take the reasonable-balance number and then drop it several mph. It's a feedback loop, where people drive faster and limits are set with driving faster in mind.

[herogreen]

You forgot that Ek = 0.5 * m * v² So it is "more dangerous" to be 3 MPH above the speed limit when the speed limit is higher.