> which part of the fatalities happened > 130kmph?
Usually (not always) you realize that something bad is going to happen so you are going to brake. Let's assume there is an obstacle in front of you that is 150m away. After a reaction time of 1 second, and with continuous braking over 100m you are able to get from 180kmh to 100kmh at time of impact, but if you had started from the generally suggested 130kmh, you would have stopped 40m before the target (assuming -8.83m/s^2 braking acceleration).
> In other words, these drivers did not leave enough distance to react and brake.
Again, recommended following distance is ~250-300 feet when stopping & reaction time can be 350-400 feet. This is OK when the car in front of you brakes, because you'll both be decelerating with a similar profile and you'll have more than 250-300 feet to stop. It is not OK when the car in front of you stops substantially faster than braking with no warning because it has hit an unknown obstacle.
> When the vehicle ahead emergency brakes or stops you have the growth of size of the target to gauge by. That's not much movement, until the last moment when it's far too late to evade.
You see the brake lights up, and as long as you're alert and keep some distance you're going to be alright. If you're not alert and/or driving too close you hit it.
> every other car could slam on their brakes at any time
Or veer out of the way of an obstacle you can't see, leaving you to either similarly veer or collide with it. IOW, do not assume you only need to account for your own reaction time; rather assume the vehicle in front of you can instantly stop moving and you must account for both your reaction time and your vehicle's stopping distance.
> At least 2-3 seconds, minimum. Did you not take any driver's ed classes?
GP is right, though, it is highly context dependent. If you are following another car and paying attention, then two seconds is enough if they slam on the brakes. There is an implicit assumption that two cars have roughly equivalent braking ability.
But what if they do some sort of assisted stop? Bridge abutment. Head-on collision. Or what if there is a stopped car in front of the car in front of you, and the car in front of you swerves out of the way leaving you bearing down on an unexpectedly stopped car in the lane? 3 seconds won't be enough.
I still remember vividly many years ago coming around a high speed sweeping corner behind another car, at a perfectly reasonable stopping distance, and then they jerked the wheel into the oncoming lane and suddenly I was facing the back of a stopped car. That was exciting. I did not hit the car, but it was close, very close. The only way to avoid that is a stopping distance much greater than 2-3 seconds.
> If the car in front of you hits something and comes to an extremely abrupt stop and you don't have the stopping distance to stop, you're too close.
It is physically impossible for a car to come to an instantaneous stop for no reason. It would have to e.g. drive into a concrete wall, but if there's a wall across the freeway you should've seen that from afar.
You only really need to have enough distance to be able to notice the car ahead braking and start braking yourself.
>whose speed probably mostly depends on human physiology (e.g. reaction time, personal comfort level with distance to next vehicle, etc.)
I think it has more to do with the suddenness of the stop; each car behind it has to stop suddenly in response, as the change only spreads locally. I’m pretty sure all you need to modify is deceleration speed for braking and acceleration speed to catch back up, to see like 90% of the effect
I’d test it with the simulation but it was too difficult to setup on mobile :-)
> You might see it slowing down first, and you start slowing down, or continue because there is enough distance anyway.
That's the thing, though - if you were already at the distance limit, and the car in front of you starts slowing down, you have to also start slowing down right away to maintain said limit. If you do not, then it's already "not enough distance" by definition at that point, and you're the one responsible for that.
And at a moderate sprint, like most adults do when they try to cross a roadway with vehicular traffic, that is 4-5 m/s, giving the vehicle 0.4 - 0.5 seconds to stop. 40 MPH ~ 18 m/s that gives the vehicle 7-9 meters to stop.
No human could brake that well, and simply jamming the brakes would engage the ABS leading to a longer stopping distance. Not to mention the human reaction time of 0.5 - 0.75 seconds would have prevented most people from even lifting the foot off the accelerator pedal before the collision, even if they were perfectly focused on driving.
> I'm talking about optimal conditions here, wet roads would change things obviously but each of these cars was able to stop within it's own car length (around 15 feet) from 30mph, simply by stamping on the brake pedal with maximum force, triggering the ABS until the car stops
How did you measure that? Because plugging these figures into a uniform acceleration calculator, 50km/h to 0 in 4.47m requires a deceleration of 2.2g but "Analysis of emergency braking of a vehicle"[0] experimentally measured very best case deceleration as barely scraping 1g (with ABS at 80km/h, significantly lower at lower speeds or without ABS).
> You should be able to safely not rear end the car in front of you even if it immediately hits a brick wall and stops completely, […]
I don't think that's the usual expectation. Maintaining absolute braking distance is what railways do, whereas cars commonly operate on relative braking distance plus a safety margin accounting for your reaction time.
Over here, both driving schools and other general road safety education talk about keeping 2 seconds following distance (or half your kph-speed in metres, which corresponds to 1.8 seconds following distance), and traffic engineers commonly give the capacity of a single lane of traffic as 1800 vehicles/hour, which again corresponds to 2 seconds distance (this time as measured vehicle front to vehicle front, instead of tail to front of following vehicle).
> Computers have better reaction times than humans. Forcibly braking is a simple maneuver.
The question isn't about the MHZ speed of a computer or the theoretical reaction time but the lack of reaction by the computer in the actual real world. It doesn't matter if this is a sensor failure, a software bug, or a slow reaction by a computer. The result is the same. A person died.
From the article:
*"... the lack of braking or swerving whatsoever is alarming and suggests that the system never anticipated the collision.”
>It's likely that the accident still would have been fatal, or at least the pedestrian would have been seriously injured if the safety driver was driving. Based on the video there was only a second or two to react from the time the pedestrian stepped out of the shadows, and human reaction time varies from 0.7 to 3 seconds [1]. One study found the average driver's reaction time is 2.3 seconds.
Yes, but it is difficult to state that it would have been "likely" fatal, and even sevral fractures are IMHO a big improvement over death, the whole point is not about the "average" braking time (BTW that same study gives 1.64 seconds average for "steering away" which is probably what most human drivers would have done in a similar situation, instead of braking or instead of only braking), it is the peaks that count.
I mean, it is about the (rightful) expectation that an automated system is safer (faster, more reactive) than the best you can find (not the average, the best) "in nature" among a random set of drivers, just like the state codes (correctly) take the worse case in the sample or however incereases the recommendation to three seconds.
If you play the game suggested on the same site you gave a link to and deliberately count up to three before clicking, you get:
Reaction time: 2916.00ms
Reaction speed age: 89 years old
Playing fairly, you are likely to get values in the range 300-500 ms.
The car should have slowed at that six second mark, and at least reduced speed dramatically by the time of impact (if not completely).
The risk of fatality would have been severely reduced if the car was (at most) travelling at 30mph (likely around ~10% - instead of between 25% and 60% for the speed at the time of impact depending on what study you choose).
That calculation would probably be wrong anyway, unless you have a lot of practical experience with emergency braking. I constantly underestimate my reaction times.
> The difference of speed between cars is what causes accidents.
A cause, but most definitely not the cause. There's the not-so-trivial factors of reaction time (separation / speed) and breaking distance (proportional to the square of the speed) too. And many other factors too, of course.
> Still, you would think that there would be some threshold where the car decides, "Hey, this stationary obstacle is right in front of me. I should slow down"
I think there is, it's just that by the time it reaches that threshold, the distance to obstacle is less than the stopping distance for the car at 60+ mph. A human making that judgment might steer around the obstacle, but AEB systems don't have that option.
Usually (not always) you realize that something bad is going to happen so you are going to brake. Let's assume there is an obstacle in front of you that is 150m away. After a reaction time of 1 second, and with continuous braking over 100m you are able to get from 180kmh to 100kmh at time of impact, but if you had started from the generally suggested 130kmh, you would have stopped 40m before the target (assuming -8.83m/s^2 braking acceleration).
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