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Emergency Braking Question

During rush hour traffic this morning, a BMW 2 cars ahead of me suddenly slammed on his brakes to stop allowing a pedestrian to cross the crosswalk. This caused the car in front of me and myself to also slam on our brakes. My M3 collision avoidance alarm went off and I felt the brake pedal depress even further than had been able to press (my foot was hanging mid-air). Luckily we were all able to stop without any accident.

Has anyone else experienced this where the brake pedal depresses even further than you can press down on during a sudden stop?
 

AlanSubie4Life

Efficiency Obsessed Member
Oct 22, 2018
14,274
18,406
San Diego
That feature would be very good for salespeople to demonstrate during test drives, so potential owners wouldn't be surprised when it happens to them after delivery.

Unfortunately it may not always work, so this sales strategy would likely end in tears. Or at least an airbag to the face.
 

JulienW

Active Member
Jul 7, 2018
4,223
6,363
Atlanta
That feature would be very good for salespeople to demonstrate during test drives, so potential owners wouldn't be surprised when it happens to them after delivery.
WHAT? You might want to try reading the manual. Yea that would make a GREAT demo.

Screen Shot 2019-09-11 at 7.06.39 PM.png
 
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AlanSubie4Life

Efficiency Obsessed Member
Oct 22, 2018
14,274
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San Diego
Comforting.

LOL

They are safety-enhancing features. Combined with an alert driver who is fully engaged in the driving task, they can really increase safety overall, even if they don’t always work. They don’t have to work all the time, and they aren’t really “supposed” to. I am certain the design requirement is not 100% detection rate. There’s also always a balance to be struck between false negatives and false positives.

Really glad the OP had no problems here and avoided a possible accident. There are a lot of studies out there that people don’t press the brakes hard enough in an emergency, so you can see that at work here too.

In general, I do feel like driver ed courses should practice a lot of 100% brake-force emergency stops on a closed course. I think if people learn that feeling when they are young, it might stick with them and help them apply full brake force when a real emergency arises. A bit expensive on the tires I suppose. Fortunately the computers are finally stepping in, though, so less necessary now.
 

C141medic

Active Member
Apr 9, 2016
1,714
1,582
New Jersey
There are a lot of studies out there that people don’t press the brakes hard enough in an emergency, so you can see that at work here too.
I remember years ago when I used to drive emergency vehicles we were taught to not slam on the brakes but to pump them repeatedly in order to not skid and stop effectively. With the advent of ABS, the brakes are most effective if you slam on them as hard as possible and not let off. The ABS system will correctly modulate the brakes to prevent skidding.
I also, recently, had the opportunity to attend a tactical vehicle operator’s course and this time around all the vehicles had ABS. Way better than the old system, but I guess I’m dating myself;)
 
I'm curious how, if you were standing on the brakes, the pedal retracting further would leave your foot 'hanging in the air' rather than continuing to press it down.

I guess it was the shock and how fast it all happened. I do remember pushing the brakes down as hard as I could, feeling and hearing the ABS kick in, and then feeling the pedal go off my foot and even lower. Keep in mind at the same time my mind was saying "Oh Sh*t Oh Sh*t" while my wife was screaming and the collision alarm is going off in the background.

When the pedal went even lower the car came to a complete stop and that's when I noticed the pedal come back up and tap my foot to tell me "Np bro I got you"
 
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AlanSubie4Life

Efficiency Obsessed Member
Oct 22, 2018
14,274
18,406
San Diego
then feeling the pedal go off my foot and even lower.

Almost certainly your foot never actually left the pedal, but this is what AEB is supposed to do - apply maximum braking force just in case you can't. It was probably possible for you to press the brakes harder than you did, but it depends on seating position, etc. Thanks for reporting a success! Just don't rely on it...

Any dashcam footage?
 

AlanSubie4Life

Efficiency Obsessed Member
Oct 22, 2018
14,274
18,406
San Diego
I'm still a little confused. In stop and go traffic, creeping forward at under 7 mph, if you take no action after the alert goes off, will the car actually come to a full stop before hitting the car in front?

No idea. It's not the sort of thing people test. Personally I think it looks at large changes in velocity of vehicles in front, and evaluates whether it is an actual emergency, then bases actions on that. Just a guess. It has to find a way to reduce false positives otherwise the system becomes useless and annoying - so it'll try to discriminate based on certain defining factors which are common to collision scenarios.

So my guess is that in the situation you describe, maybe sometimes it will react, but eventually it will hit the car in front. I would guess it would stop if the vehicle in front slammed on its brakes, but wouldn't stop if the car in front gradually slowed to a stop. But that's also not a particularly serious situation - doesn't really matter much if you hit someone at 7mph - other than a little whiplash and maybe a few airbags, all will be well! ;) In any case, I'm not doing any testing.
 
I'm still a little confused. In stop and go traffic, creeping forward at under 7 mph, if you take no action after the alert goes off, will the car actually come to a full stop before hitting the car in front?
That's the scenario I had in mind when I suggested they show potential buyers in a test drive (presuming it works at low speed and 100% of the time). Do it in a parking lot...aimed at a wall or post.

However, if it hurts then you do that...then don't do that. ;)
 

AlanSubie4Life

Efficiency Obsessed Member
Oct 22, 2018
14,274
18,406
San Diego
You did read that I quoted the comment asking about automatic braking at 7 mph, right? :rolleyes:

Yes. It will not work 100% of the time at low speed, and will very likely hit the post or wall some or all of the time. I would think this is not what the AEB is tuned for - it’s much more common to hit other vehicles, and even if it were tuned for stationary objects, it wouldn’t work all the time.

Main point: do not rely on AEB under any circumstances, yet be thrilled when it works and helps avoid or mitigate a collision!
 
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