On the freeway, the time it takes to react to seeing the next car's brake lights isn't the main problem. The bigger problem is how long it takes to judge that car's deceleration.

Are they just tapping the brakes because the road is bending---some people always do---or are they slamming on the brakes because there's a traffic incident that you can't yet see? You're 200-millisecond reaction needs to be completely different in these two cases, but you just can't immediately tell the difference.

Both for people and for automatic cars, judging deceleration is a harder problem than just reacting to a red light going on, but the sensors on automatic cars should give them a big advantage.

> Ideal human reaction time is around 200 milliseconds.

Sure, if they're not looking down at the radio or at the accident on the other side of the freeway.

And then there's the time to actually make your movements. Getting your foot onto the brake pedal and pushing on it takes appreciable time, both for the foot to make the movement and for the nerve impulses to get there to make it move.

I recall from looking into this stuff a bit that a good reaction time from "oh shit" to "brakes are applied" is around 750ms. Accounting for people who aren't fully on top of their game (distracted, tired, injured, old, etc.), road designers assume more like 1.5-2s delay for applying brakes. And even that will be optimistic in some cases, as many car crashes demonstrate.

There's that, but also the realization that you need to brake "as hard as possible"-- we're not trained to do that often so there's additional delay before that happens.

Hence why he said "ideal".

And my point is that talking about the ideal human response time is absurd in the context of driving.

Why? It provides an lower boundary on the human response time. It's significant because the ideal response already represents _at least a whole car length_ of travel distance. We don't even need to delve into what a realistic response time is: it's already so bad that we should be looking at alternatives like our machine-overlord-self-driving-cars.

The protection is useful because for a highway rear-end accident to occur, a chain of at least two events has to occur. The car ahead of you has to significantly decelerate, and in the same window of time, the rear-ending driver has to be distracted for longer than the time it takes to close the gap and stop. Most of the time both of those events don't occur at the same time, but just look at traffic accident reports to see that in areas with heavy traffic - the probability that both events occur over the population of cars in traffic is 100%+ (because it happens multiple times every day).

Adding some sort of auto brake feature adds another layer of safety (presuming that the ratio of 'saves' to added risk is very high with this tech.)

With autobrake and safe following distance, even approaching 25% of cars using the systems would drastically reduce the amount of multi-car pileups.

I think a recent one in the news was 20+ cars. If just one of those cars had autobrake and safe following distance this could have been halved or quartered and wouldn't have blocked a highway for a few hours.

You will never be as fast as software, hardware and sensors.

This was the point I was trying to say, but I think I didn't communicate it well.

I would like to know just how fast these cars can react. I'm betting they could be actually slowing the car before a human is even aware something is wrong, but I would like to have some numbers to cite.

Also - keep in mind that this type of Driver Assist is (in the scheme of things) a very straightforward problem compared to others. It's pretty easy to calculate what type of braking is required to avoid running into something in front of you based on the relative velocities.

Not sure about the Model S, but Google's Prius and Lexus autonomous vehicles use a radar sensor on the front of the vehicle that can see past a semi tractor trailer in front of it, providing sensor input a human would never have.

Ideal humans do not cause accidents, normal humans do and normal human reaction time while driving is not around 200 milliseconds.