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Phantom Braking Measured

Wingsy

Member
Apr 30, 2019
178
276
Mocksville, NC
M3 G-Force Plot.png


When I try to describe phantom braking I get a little frustrated with myself in not being able to find the words to describe just what it is that I feel when the car suddenly decelerates for no apparent reason. The graph above shows just what happens (TACC on, Autopilot off). The data was generated via the Accelerometer app on my phone. At the 6-second mark it panicked. A car or two ahead of me, yes, but not in my lane. Everyone was just rolling right along.

0.15 Gs will topple a bag of groceries in the back seat; it will tilt your head forward with a couple pounds of force; it will create a loud noise in your ear from the passenger; and after hundreds of these it is starting to get mighty annoying. OK, hundreds is an exaggeration since I've only had the M3 for about a month now, but I get 1 or more of these every time I go somewhere. It's not just this phantom braking that annoys me. It's all the abrupt little corrections the TACC makes before it settles down when a speed change occurs. In fairness it is not always abrupt; sometimes it's as smooth as silk. It reminds me of the sensation you get when running out of gas (ever done that?). This has been around for a long time, and I wonder why it hasn't been given the attention I think it deserves. My $50,000 car isn't supposed to scare the hell out of my passenger every other day.

Mr. Musk, please fix this.

(I would post this on the Tesla forums but they don't post pictures.)
 

Msjulie

Active Member
Jun 26, 2016
2,449
1,809
Monterey Peninsula
@Wingsy thanks for this post. I've had similar things happen, grunts of disapproval from both passenger and driver when the car thinks cars, in front but in other lanes, are in it's lane -

There needs to be a way to disable TACC w/cruise for when that is the better choice..
 

Knightshade

Well-Known Member
Jul 31, 2017
13,625
23,117
NC
Would be helpful if you could post the matching dashcam footage

I've yet to have my car brake hard with NO reason visible, even if it's not immediately obvious at a glance- and I do like 90% of my driving with TACC/AP on.
 
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Wingsy

Member
Apr 30, 2019
178
276
Mocksville, NC
@Wingsy thanks for this post. I've had similar things happen, grunts of disapproval from both passenger and driver when the car thinks cars, in front but in other lanes, are in it's lane -

There needs to be a way to disable TACC w/cruise for when that is the better choice..
Even when cars ARE in my lane and holding a steady speed it can happen, like it did about an hour ago. I was behind a flatbed semi, country road, straight, normal width, when out of the blue it braked hard. There was absolutely nothing going on ahead of me. Just me and the truck and a person behind me (lucky they were paying attention). Speed was around 50. I got on the GO pedal withing 2-3 seconds, but I wish no one was behind me AND I could think fast enough I would've let it go to see what it wanted to do.

This is becoming insane.


@Knightshade: As soon as I round up another camera (have to use my iPhone for the accelerometer) I'll try to make a video showing the screen and the dash cam at the same time.

It's good to know you aren't having problems as I am. Tells me there is hope that Tesla service can make mine behave as yours does.
 
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anticowboyism

Member
May 31, 2019
32
17
Los Angeles
Had mine only 2 weeks, but have had this happen once while exiting the freeway on NOAP. The car gave a few sudden brakes with nothing in front of me but plenty of road. Not sure why. I suspect the computer errs on the side of paranoid braking rather than ignoring a signal that could be noise. Until the AP computer is able to improve signal to noise, this is probably the preferred behavior.
 

JasonCinOtt

Member
Apr 5, 2019
32
17
Ottawa
I've been having lots of phantom decel issues, to the point of wondering if I need a camera recalibration. I've managed to hit record on the dashcam for the last few, will try and figure out how to transfer it on here. As much as I love this car, I've almost given up on any automated driving. The likelihood of getting rear ended at highway speeds is a very real threat now and I don't trust the car, like at all. My wife almost wore a whole cup of hot coffee today when the car decided to slam on the brakes on the highway with TACC engaged.

Is this something that I should submit a service request for?
 
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JulienW

Active Member
Jul 7, 2018
2,895
3,695
Atlanta
Oddly yesterday I noticed it happening at 2x at two different locations where a concrete bridge ended and back to asphalt road. Not sure if it was just a coincidence but was odd.
 

Msjulie

Active Member
Jun 26, 2016
2,449
1,809
Monterey Peninsula
I've been having lots of phantom decel issues, to the point of wondering if I need a camera recalibration. I've managed to hit record on the dashcam for the last few, will try and figure out how to transfer it on here. As much as I love this car, I've almost given up on any automated driving. The likelihood of getting rear ended at highway speeds is a very real threat now and I don't trust the car, like at all. My wife almost wore a whole cup of hot coffee today when the car decided to slam on the brakes on the highway with TACC engaged.

Is this something that I should submit a service request for?

Phantom braking is really common, to the point of being called out in the manual. Unfortunately I believe you would get the same response I did, beta software. I don't use the traffic-aware cruise or AP anymore ;(
 
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Msjulie

Active Member
Jun 26, 2016
2,449
1,809
Monterey Peninsula
If they are it's because others are illegally following too closely.

But from what limited data we have, AP gets into accidents far less often than teslas not running AP.

Following too close is the unfortunate defacto-standard on every road I've ever driven. Also, brake-checking by humans is illegal in lots of places, so it should be for AP as well.

Not a fox news fan but Brake-Checkers, Beware
 
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Knightshade

Well-Known Member
Jul 31, 2017
13,625
23,117
NC
Following too close is the unfortunate defacto-standard on every road I've ever driven. Also, brake-checking by humans is illegal in lots of places, so it should be for AP as well.

Not a fox news fan but Brake-Checkers, Beware


AFAIK the only relevant law in this state is:

NC law said:
The driver of a motor vehicle shall not follow another vehicle more closely than is reasonable and prudent, having due regard for the speed of such vehicles and the traffic upon and the condition of the highway.


Thankfully I live in a contributory negligence state- if they're following too close they'll get $0.00 from me.



But then again I don't seem to experience this "car brakes hard for no reason all the time" some other owners appear to experience either.
 

67King

Member
Feb 2, 2018
323
218
Knoxville, TN
If they are it's because others are illegally following too closely

I apologize for my ignorance, but can you please show me where "too closely" is defined, legally?

That perspective reminds me of the vigilantes who feel justified driving in the left lane going the speed limit, because no one should be going faster than them.

Might as well blame them for going 56 in a 55. At highway speed, even following a reasonable distance, it may not be the first car. But there will be a delayed reaction with it, then a delayed reaction with the 2nd, then 3rd. You add up all of those delays, and the accident may be several cars removed from the car causing the accident.
 
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Daniel in SD

Well-Known Member
Jan 25, 2018
7,128
10,613
San Diego
This has been around for a long time, and I wonder why it hasn't been given the attention I think it deserves.
If they “fixed it” then people would complain about their cars running into things, unfortunately it’s a trade off. There was a funny thread where someone was complaining that AP1 had better lane changing than AP2. It has better lane changing because it doesn’t check to see if there’s a car there!
I thought I had never experienced a phantom braking event but looking at you plots I definitely have. I always thought people were talking about hard braking, 0.15g is about 15% braking.
 

Knightshade

Well-Known Member
Jul 31, 2017
13,625
23,117
NC
I apologize for my ignorance, but can you please show me where "too closely" is defined, legally?

I did. Literally in the post above yours.

Generally it's why if someone hits you from behind it's going to be their fault- because they did not leave enough room to be able to brake their own car in time if the one in front of them braked for whatever reason.

If you do that you won't run into the back of other people. If you don't, you're likely driving illegally.


Your own states laws on this, with virtually identical wording to mine, are here:
» Tennessee Code 55-8-124 – Following too closelyLawServer



I
That perspective reminds me of the vigilantes who feel justified driving in the left lane going the speed limit, because no one should be going faster than them.

This reminds me of people making up excuses to illegally tailgate.
 

67King

Member
Feb 2, 2018
323
218
Knoxville, TN
I did. Literally in the post above yours.

Generally it's why if someone hits you from behind it's going to be their fault- because they did not leave enough room to be able to brake their own car in time if the one in front of them braked for whatever reason.

If you do that you won't run into the back of other people. If you don't, you're likely driving illegally.


Your own states laws on this, with virtually identical wording to mine, are here:
» Tennessee Code 55-8-124 – Following too closelyLawServer

This reminds me of people making up excuses to illegally tailgate.

I started replying, and walked away for a while before I finished. Ironically because I'm at an event in your state (Raleigh) this weekend, and get pulled away at times.

"Reasonable and prudent" is a horrendously ambiguous term that is not going to stand up in any court of law when a vehicle brake checks for no reason, whatsoever. Defining "too closely" requires quantifiable measures (i.e. distance in units such as feet or car lengths), not ambiguous ones that are only qualitative and judgement calls.

And the perspective that leaving enough space to always avoid an accident if being brake checked is reasonable and prudent does not come from those who live and drive in bigger cities such as Atlanta (where I learned to drive). Keeping the car length for every 10MPH rule of thumb will keep a driver constantly slowing as others merge into the space. And more and more studies are showing that the slower drivers are the ones who cause more accidents on freeways, though they often are not even involved.

And like I said, it isn't usually the car behind the one that brake checks, it is several cars behind. Happens all the time. Happened to me a few months ago, crested a hill to find traffic slowed drastically, guy ahead of me hit the brakes hard, I hit the brakes hard, the guy behind me did, the guy behind him ran off the road, and two cars later there was a rear-end collision. Watched if all unfold in my rear view mirror, as as soon as I saw the brake lights come on and the rear end of the car ahead of me light (from weight transfer), my first thought was that I might get rear ended. Literally, add a few tenths of reaction time, and at 60MPH, that closes the gap by 25 feet (26.4 to be exact) assuming the same deceleration. Which of course won't happen, the second car will brake harder. Meaning a higher decel rate, then add that 0.3 seconds for the car behind it, and it brakes even harder. It is the accordian affect, and if traffic is heavy, there WILL be a collision.

I've been pulled over for following too closely, actually. Not so calmly told the cop that the car in front of me slammed on brakes and cut me off when the driver saw him. In other words, one doesn't always control the distance between them and the car in front of them. FWIW, cop's response was "Okay, have a nice day."

At the end of the day, this all boils down to one thing that drivers depend on from other drivers - predictability. The TACC/AP often behaves in ways that are very unpredictable. And frankly, I'm a whole lot less concerned about whether a guy behind me gets a ticket for following too closely than I am about fixing my car if I am rear ended. Reasonable people are more concerned about avoiding accidents than they are about scapegoating others so their dangerous behaviors are not punished when they cause an accident.
 
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