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Comment by HarHarVeryFunny | Hacker News Reader

I don't know how valid the "brakes shedding energy at same rate per unit distance" is, but other sources confirm how massively braking distance increases with speed.

For example, from here:

https://www.automotive-fleet.com/driver-care/239402/driver-c...

At 50 mph braking distance is 125 ft.

At 60 mph it's 245 ft

At 80 mph it's 439 ft

Now, when you consider the emergency circumstances where you might be flooring the brakes, this is not good ...

e.g.

A deer or child runs out in front of you, so close that emergency braking is called for .. how far away are they going to be from you ?

Or, say the car in front of you on highway slams on their brakes, and they were travelling at 50mph, with you approaching at 60mph. Unless you are at least 120 ft behind them (vs the typical highway following distance of a few car lengths), you are going to be in a bad head-on collision.

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> At 50 mph braking distance is 125 ft.

> At 60 mph it's 245 ft

> At 80 mph it's 439 ft

It looks like you're jumping between columns and sometimes transposing rows here. 125' @ 50 mph is from the raw braking distance column, 245' is from the same column but for 70 mph rather than 60 mph, and 439' @ 80 mph is from the overall stopping distance column. I'm not pointing this out just to nit, rather because:

> say the car in front of you on highway slams on their brakes, and they were travelling at 50mph, with you approaching at 60mph. Unless you are at least 120 ft behind them (vs the typical highway following distance of a few car lengths), you are going to be in a bad head-on collision.

You've concluded nearly twice the stopping footage as the table actually says and, as is more directly said in the last column, it's really roughly a 4 car braking length difference between the two speeds (including reaction time).

Umm.. yeah. Not sure how I botched up reading that quite so badly!

I think the point still stands though - by definition in an emergency situation there is not going to be much distance between you and that deer/whatever, and an extra 10mph can make all the difference!

> I don't know how valid the "brakes shedding energy at same rate per unit distance" is

From the follow-up post, the author gives a justification for that.

> the force applied by the brake pads on the brake disc is going to be roughly constant, and so the work done, which in this case is the energy shed, is the force times the distance over which the force is applied.

There will of course be confounding factors like temperature, etc. But it's probably pretty close.

Kinetic energy scales with velocity squared (0.5mv^2), so this result makes sense. Brakes dissipate constant energy, but the amount of energy it needs to disapate is going up much faster.

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