Emergency Brake Assist -safety feature or serious hazard?
Discussion
Emergency brake assist –safety feature or serious hazard? ( with an 'R' in BRAKE this time!)
(NB although the vehicle referred to in this post is a Mercedes, I’m sure other marques have similar systems –hence posting it in General Gassing
So there we were fighting our way round the usual Friday afternoon car park on the M25. Stop-start traffic and we’d just peaked at maybe 50mph when a couple of cars ahead someone dabbed his brakes. Usual scenario, the car in front of me then braked a little harder and even though I don’t tail-gate, I had to step on my own brakes a little bit harder still –nothing heavy but still a reasonable degree of deceleration intending to scrub off maybe 20mph.
Ordinarily nothing untoward except I’m driving a nice shiny new Mercedes ML350 (my weekday workhorse) and it’s fitted with the latest M-B collision avoidance and ‘Emergency Brake Assist’ systems as standard spec. This uses sensors in the front of the car to detect a possible impact and sound a warning bleeper. However, if you then touch the brakes while the warning system is activated it assumes it is a genuine emergency and applies the ‘Emergency Brake Assist’. In other words it suddenly overrides your own braking and slams the brakes on to their absolute maximum.
What started as a modest bit of braking resulted in the car’s electronics taking over and suddenly trying to do a full on emergency stop –ABS pulsing and tyres smoking in the outside lane of a packed motorway. The whole thing took maybe one or two seconds before the car ahead stopped braking and the sensors decided we were no longer on collision course (which we never were anyway). At this point the systems relaxed and gave me back control just before we came to a full halt. I fully expected the guy behind to slam into the back of us (odds-on he sh*t himself though!) and no idea if the resulting ‘coil-spring’ effect of the braking resulted in any rear-end shunts further back in the queue. The experience was terrifying and I seriously question whether these systems are genuine safety feature –or completely the opposite!
(NB although the vehicle referred to in this post is a Mercedes, I’m sure other marques have similar systems –hence posting it in General Gassing
So there we were fighting our way round the usual Friday afternoon car park on the M25. Stop-start traffic and we’d just peaked at maybe 50mph when a couple of cars ahead someone dabbed his brakes. Usual scenario, the car in front of me then braked a little harder and even though I don’t tail-gate, I had to step on my own brakes a little bit harder still –nothing heavy but still a reasonable degree of deceleration intending to scrub off maybe 20mph.
Ordinarily nothing untoward except I’m driving a nice shiny new Mercedes ML350 (my weekday workhorse) and it’s fitted with the latest M-B collision avoidance and ‘Emergency Brake Assist’ systems as standard spec. This uses sensors in the front of the car to detect a possible impact and sound a warning bleeper. However, if you then touch the brakes while the warning system is activated it assumes it is a genuine emergency and applies the ‘Emergency Brake Assist’. In other words it suddenly overrides your own braking and slams the brakes on to their absolute maximum.
What started as a modest bit of braking resulted in the car’s electronics taking over and suddenly trying to do a full on emergency stop –ABS pulsing and tyres smoking in the outside lane of a packed motorway. The whole thing took maybe one or two seconds before the car ahead stopped braking and the sensors decided we were no longer on collision course (which we never were anyway). At this point the systems relaxed and gave me back control just before we came to a full halt. I fully expected the guy behind to slam into the back of us (odds-on he sh*t himself though!) and no idea if the resulting ‘coil-spring’ effect of the braking resulted in any rear-end shunts further back in the queue. The experience was terrifying and I seriously question whether these systems are genuine safety feature –or completely the opposite!
Edited by monoloco on Friday 11th September 14:30
apologies for the unfortunate typo in the title! Someone better phone the local plod to send a jam-sandwich to arrest me!
Anyway, I would point out that I was categorically NOT tail-gating. I was a sensible distance back from the car in front, it then braked unexpectedly and fairly sharply. I'd have had to be half a mile back not to have needed to brake to some extent myself. I braked moderately -not more than about 20-25% of full capacity and would have easily stopped comfortably and with a significant safety margin. Trouble is the M-B system doesn't seem to differentiate between that and full on emergency. It simply seems to think if the vehicle is closing on the one in front it will impact the obstacle, regardless of the fact the driver is already controlling the brakes and is going to stop in plenty of time. I can only assume that the system builds in a massive safety margin on the braking distance required -ie it takes a 'worst case' on how slippery the road is and just slams everything on despite the fact we were on a warm, bone dry road.
Meanwhile the link from LordHaveMurci is interesting and indeed along the same lines -shame I hadn't spotted it before I posted the original.
Anyway, I would point out that I was categorically NOT tail-gating. I was a sensible distance back from the car in front, it then braked unexpectedly and fairly sharply. I'd have had to be half a mile back not to have needed to brake to some extent myself. I braked moderately -not more than about 20-25% of full capacity and would have easily stopped comfortably and with a significant safety margin. Trouble is the M-B system doesn't seem to differentiate between that and full on emergency. It simply seems to think if the vehicle is closing on the one in front it will impact the obstacle, regardless of the fact the driver is already controlling the brakes and is going to stop in plenty of time. I can only assume that the system builds in a massive safety margin on the braking distance required -ie it takes a 'worst case' on how slippery the road is and just slams everything on despite the fact we were on a warm, bone dry road.
Meanwhile the link from LordHaveMurci is interesting and indeed along the same lines -shame I hadn't spotted it before I posted the original.
Common sense (which admittedly seems to be absent from many drivers on the M25) says you leave a gap big enough to see, react and stop in. That's exactly what I'd done -supported by the fact the car is also fitted with an anti-tailgate system that flashes a different warning light if you get too close to the vehicle in front at steady speed (ie not closing on it). That light was not illuminated before this incident took place. However, as the car in front braked, obviously the distance between me and it closed slightly before I reacted and started to brake myself so presumably at this point the sensors picked up the other car. To avoid this happening you'd need to leave double the safety gap -ie enough room to see, react, brake AND the same distance again for the sensors to think you'd got enough room to do that all over again. At no time did the gap narrow to a point where it was looking like an accident was going to happen -I started braking moderately and had plenty of room to stop even at the moderate level of brake pressure I was applying. The system just totally over-reacted and instead of preventing an accident it very nearly caused one. Its easy for anyone who hasn't experienced this to say its all my fault for being too close but I categorically wasn't -the system may be a good idea in theory but its just too crude -all or nothing. In my view the algorithm is flawed and turns what could be a clever safety feature into a significant hazard. Doing an unnecessary emergency stop in the outside lane of the M25 would get you a ticket for dangerous driving -so how come the car can do it for you?
IanCress said:
Think I may have seen this in action this morning. Outside lane of M62 around 9.30am, rubber necking at a broken down HGV on the opposite carriage way, white Merc slams on far harder than it needs to, lighting up the hazard lights. A Galaxy (not a whole solar system, the big car thing) goes in to the back of the Merc, and another car in to the back of the Galaxy.
No need to brake that hard, but I wonder if it was the Merc drivers choice or the cars decision.
Sounds very much like it -particularly the hazard lights coming on which I seem to recall is part of it. No need to brake that hard, but I wonder if it was the Merc drivers choice or the cars decision.
Edited by IanCress on Thursday 17th September 12:05
In the week since I made the original posting I'm starting to think I might have a fault on the system as the collision warning buzzer/light has activated twice more when it wasn't required -once as I was passing an HGV in the lane along side me and the second time in heavy rain but no other vehicle or obstacle in sight. A trip to the dealer required I think.
However, possible faults in the triggering system aside, I think I've concluded the system basically a good idea but it is currently under-developed and needs to be more selective about when and how it kicks in. When it activated on me it really wasn't necessary -there categorically wasn't an emergency and everything was under control -whether this was down to a over-zealous sensor or not is uncertain of course but it does highlight Chris Watton's comment above about future reliability! However, triggering aside and given the way everything else on the car is 'adaptive' -eg dynamic dip-beam range on headlights depending on distance to the next vehicle, dynamic cruise control etc, it must be feasible to vary the braking force it applies dependent on the distance/speed/etc in order to bring you to a controlled halt in the space available rather than slamming everything on in a panic and ending up with the guy behind slamming into the back of you.
I would also add to all the 'doubters' -wait until it happens to you when it's unnecessary and you're not expecting it -its terrifying!
ORD said:
I agree that the OP was following too closely. You should aim to almost never use your brakes on a motorway, let alone having to use them with force. If you cannot brake very gently or, even better, just ease off, you are too close.
While I agree in principle and as you say try to leave sufficient room so I can attempt to avoid using my brakes on a motorway (or in any other heavy traffic), this is far too simplistic especially in a heavy car which like most modern Mercs has an autobox (and also proves you haven't read the rest of the thread). Just to make the point, while I was out at lunch a few minutes ago I tried to test how long it would take to slow naturally from 50mph. So, doing exactly 50mph on a flat, straight (and empty!) road, I took my foot off the throttle. By the time I got to the end of the 300metre+ straight and had to apply the brakes I was still doing 40mph. How long it would take to scrub off the other 40mph is anyone's guess -another half mile? More? Would it ever actually stop without applying the brakes? Apply your your holier-than-thou theory would mean leaving a gap between you and the car in front of at least half a mile.Sheepshanks said:
You're not thinking about this like the authorities do. The solution is cars which drive themselves, with all control (other than keying in a destination) taken away from humans.
...and they'll probably have seats fitted with commodes that automatically wipe your arse afterwards -although come to think of it I wouldn't have minded one of those after the original 'event' that sparked the topic in the first place 
!!ORD said:
There's your answer, then. Half a mile is the appropriate distance.
Happy to help.
Excellent. Lets do some simple maths: M25 is 117 miles long, lets say an average of 3.5 lanes, and one car every 1/2 mile in each lane. That means the entire M25 could hold a total of 819 cars at any one time. I wish......Happy to help.
yep -I've had mine kick in due to long grass/cowparsley etc in the verge -particularly if its wet and bending over into the road slightly. It trips the proximity sensors and the buzzer goes off but if you just happen to touch the brake at the same time the emergency braking kicks in for a second or so until you've gone past the offending tuft of grass..
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