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Comment by BobaFloutist | Hacker News Reader

At 25 mph, which I would hope would be the speed limit on roads next to schools, slamming on the brakes even seconds before colliding with children can make an enormous difference in how fast the car is going when it hits the kid.

Speed is the factor in collisions (other than weight), and modern brakes are incredibly good.

Not to mention that the car, with it's 360 degree sensors, could safely and efficiently swerve around the children even faster than it can brake, as long as there's not a car right next to you in another lane -- and even if there is, hitting another car is far less dangerous to their life than hitting the children is to yours.

These things should be so much better than we are, since they're not limited by unidirectional binocular vision, but somehow they're largely just worse. Waymo is, at best, a bit better. On average.

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> At 25 mph, which I would hope would be the speed limit on roads next to schools

I regularly drive on a two lane 55mph highway that school buses stop on and let kids out.

It runs through a reservation and has no sidewalks at all.

> modern brakes are incredibly good.

They're probably not worth that much of a superlative, and they're fundamentally limited by the tires.

This is just a pet peeve of mine, since it is used by people to argue that modern vehicles are so much vastly better than cars in the 1980s that we should be able to drive at 90mph like it is nothing.

But reaction times and kinetic energy are a bitch, and once traction control / stability assist hits its limits and can't bail you out, you might find out the hard way that you're not as good of a driver as you think you are, and your brakes won't stop you in time.

> Speed is the factor in collisions

This I will definitely agree with. Say it louder for everyone in the back.

25mph is 36 feet per second, about the length of a school bus. The stopping distance at 25mph is 30 feet, assuming perfect reaction time and dry pavement. Human reaction time is about 750ms, so stopping distance is about 2 school bus lengths. You don't have seconds.

25mph is too fast for any street where kids may jump out behind parked cars. Not just school zones, but all residential streets. There's a knee at about 20mph in the traffic fatality stats where below that speed pedestrian collisions are unlikely to be fatal. Above 20mph fatalities become more common quite quickly.

> The stopping distance at 25mph is 30 feet,

Stopping is nice, but not the entire point of braking. The lower the collision speed, the better.

> Human reaction time is about 750ms

No. Human reaction time is around 250ms on average. What you point to is the time it takes to react to an unexpected stimuli. The number I've seen quoted is about 1s. But that assumes a completely unexpected event that you're not prepared for at all.

So if you're mindlessly passing a school bus at 25mph, then a 1s delay is expected. But if you're doing so with your foot covering the brake while hyper focused on reacting to any sudden movement in front of you, you can do much, much better than 1s. Of course, at that point you might as well drive correctly by slowing down.

This is why school buses flash yellow warning lights before deploying the stop sign and opening the doors.

It should never be the case that someone is surprised by an instantaneous bus stop. The are plenty of context clues in advance. Including the fact that the bus is around at all, which should already heighten attention.

Even if you won't prevent the collision, reducing the velocity at which it happens is still very desirable, And, given that kinetic energy of the car is proportional to velocity squared, even a little bit of reduction means a lot less energy dumped into the pedestrian.

Sure, if you've hit the brakes by then. The reaction time delay means you travel significant distance before hitting the brakes.

For the knee to be useful you have to go below it -- thus 15 mph is a much better neighborhood and school speed limit.

To add a data point. In Belgium, the max speed around schools is enforced to be 30 km/h. That should be a bit less than 20mph.

I wasn't talking about human reaction time.

Kids don't get dropped off only at school. They get dropped of at their homes and can be on 55 mph roads. You are quibbling with the main point anyway. The whole reason for the stop signs on busses is because kids will be kids and really its too late when they run out.

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