# Strange thing!



## Tom Martin (Apr 10, 2021)

I had a strange thing happen to me twice now! I've got a '21 M3P and I was rolling along in cruise when all of a sudden the car slowed down drastically, like I put the brakes on. Fortunately there was nobody close behind me because I'm pretty sure they would have rear-ended me if there had been. I was in cruise, max regen.
Does anybody know why this happened or has it happened to anyone else??


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## FRC (Aug 4, 2018)

Welcome to the forum, and welcome to Tesla. What you experienced is commonly known as "phantom braking". It seems to be caused by low overpasses, shadows, and other things that confuse the car. It used to be much worse, but software updates continually improve this behavior. It is likely the most complained about fault of the Model 3. As disconcerting as it is, everyone who experiences it claims, as you do, that if someone had been behind them they would have been rear-ended. Yet there are no reports of such a rear-end collision caused by phantom braking that I have heard. I suspect that the unexpected sensation of braking, while alarming, is not as severe as it feels.

At any rate, search "phantom braking" in the search bar above and I think you'll learn more than you ever wanted to.


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## Long Ranger (Jun 1, 2018)

Also, keep in mind that you can easily override phantom braking by depressing the accelerator. The braking is really annoying when it happens, but I’ve never been too concerned with being rear-ended because I respond quickly.


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## SalisburySam (Jun 6, 2018)

The other work-around for phantom braking is to avoid using TACC and grouse about not having dumb cruise control.


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## radlaw (Oct 20, 2018)

Thanks for the explanations. I've often wondered what caused that. My wife thought it was me doing something stupid. Now I'm of the hook.


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## Gwgans (Aug 10, 2017)

Tesla‘s answer to phantom braking is to drop radar entirely from FSD so at some point your car should stop doing this.


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## SalisburySam (Jun 6, 2018)

Gwgans said:


> Tesla's answer to phantom braking is to drop radar entirely from FSD so at some point your car should stop doing this.


Hadn't thought of that but I think you're correct! Thanks for pointing that out. Or worse, we then learn it was the camera imaging software all along, but really, really hope not. Now, when and if...

BTW, I might have a radar unit for sale. Just sayin'.


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## Klaus-rf (Mar 6, 2019)

Gwgans said:


> Tesla's answer to phantom braking is to drop radar entirely from FSD so at some point your car should stop doing this.


I can 100% guarantee you that radar is NOT detecting shadows in the roadway that often cause phantom braking. ONLY the camera(s) are "seeing" shadows, puddles, etc..


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## garsh (Apr 4, 2016)

Klaus-rf said:


> I can 100% guarantee you that radar is NOT detecting shadows in the roadway that often cause phantom braking. ONLY the camera(s) are "seeing" shadows, puddles, etc..


I can 100% guarantee you that shadows and puddles are not the cause of phantom braking.
It's things like roadway signs that radar senses but can't determine if it's an obstacle on the road or above/beside the road. The radar unit that Tesla uses has no vertical resolution - only horizontal.

@Gwgans is correct - eliminating radar (and replacing it with a better camera-based method that uses several frames instead of the current system which uses one or two) is very likely to solve these phantom braking events.


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## Klaus-rf (Mar 6, 2019)

garsh said:


> I can 100% guarantee you that shadows and puddles are not the cause of phantom braking.
> It's things like roadway signs that radar senses but can't determine if it's an obstacle on the road or above/beside the road. The radar unit that Tesla uses has no vertical resolution - only horizontal.
> 
> @Gwgans is correct - eliminating radar (and replacing it with a better camera-based method that uses several frames instead of the current system which uses one or two) is very likely to solve these phantom braking events.


I beg to differ.

I can easily show repeatable "phantom braking" situations on the same piece of roadway where only the position of the sun, aka "shadows", change and there is a dramatic difference in how AP "reacts" to the shadows. No signs, bridges, overpasses, tunnels, etc. anywhere nearby for radar to get confused about. When the shadow is 2 feet wide, the car reacts and when it's only 8" wide the car ignores it. I have NEVER had this issue when driving at night (in the dark, with headlights on) on this same piece of roadway.

The only obvious conclusion is the car is afraid of shadows. Shadow PTSD?


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## garsh (Apr 4, 2016)

Klaus-rf said:


> I beg to differ.
> 
> I can easily show repeatable "phantom braking" situations on the same piece of roadway where only the position of the sun, aka "shadows", change and there is a dramatic difference in how AP "reacts" to the shadows. No signs, bridges, overpasses, tunnels, etc. anywhere nearby for radar to get confused about. When the shadow is 2 feet wide, the car reacts and when it's only 8" wide the car ignores it. I have NEVER had this issue when driving at night (in the dark, with headlights on) on this same piece of roadway.
> 
> The only obvious conclusion is the car is afraid of shadows. Shadow PTSD?


Does google maps have a street view of this location that you can share?

I find it hard to believe that there is nothing overhead, yet shadows can be cast down onto the road.

I did have one section of highway where the road surface itself changed color (transitioning from asphalt highway to concrete bridge), and that caused some phantom braking. But it also coincided with lane lines disappearing on the bridge, and I haven't run into that particular issue for quite some time now.


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## SalisburySam (Jun 6, 2018)

While I could point to shadows, overpasses, frontage and crossing roads with different speed limits, and the like as causes for early (2018-2019) phantom braking, software updates did address much of that. But phantom braking has not disappeared. I’ve had and still have it happen for no apparent reason on open roads with no traffic in broad daylight just because. That for me is the most disconcerting and why I don’t use TACC and the other AutoGoodies with passengers or our dogs. The behavior is seemingly unpredictable, harsh (>30mph drop in speed from highway speeds), and very frustrating. Yes, I can drive with my right foot hovering over the accelerator pedal attentively awaiting the next event, but not worth the stress. Phantom braking is a nasty behavior for me and the one car’s failing that negates a lot of joy.


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## Needsdecaf (Dec 27, 2018)

Gwgans said:


> Tesla's answer to phantom braking is to drop radar entirely from FSD so at some point your car should stop doing this.


I'll probably stop braking at all at that point.


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## Klaus-rf (Mar 6, 2019)

garsh said:


> Does google maps have a street view of this location that you can share?
> 
> I find it hard to believe that there is nothing overhead, yet shadows can be cast down onto the road.
> 
> I did have one section of highway where the road surface itself changed color (transitioning from asphalt highway to concrete bridge), and that caused some phantom braking. But it also coincided with lane lines disappearing on the bridge, and I haven't run into that particular issue for quite some time now.


This location has a controlled intersection with a traffic light. The shadows come from the overhead lighting poles to illuminate the intersection at night. Those poles and lights are there daylight or not so the "radar view" stays the same. Still AP only panics when the morning shadow of the street light pole are "visible" by the camera(s) since radar can't "see" shadows.

There is also a small section of road just past this intersection where the pavement was recently repaired (fixed a large pothole and associated waviness in surface) so it is blacker. AP tries to avoid this and/or panic brakes for it. Definitely NOT a radar signal;.

With past firmware versions, when the pothole and waviness were more dominant, AP would occasionally attempt to avoid the spot by sharply moving right to the center (for the LF tire) of the hole. Other times there was no AP reaction to that hole at all - very intermittent. If it stayed in the center of the lane lines, it would avoid the hole completely. But it often steered for it. I would usually disengage AP for that section of road. Now that the roadway has been repaired, (and MANY newer firmware versions have past) it no longer tries to steer away/into the new black pavement but it does, intermittently, panic brake just before it. 80 feet or so past the phantom braking of the light pole shadow mentioned previously.

AP usually adds some excitement to every driving event. Like a distracted teenager driving primary in a dual-control vehicle on a very busy road. My family members absolutely refuse to use TACC or AP/FSD when they have to drive the Tesla and frequently complain when I have it engaged. While they have asked for a "normal" CC - which, as we all know, AP does not offer.


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## garsh (Apr 4, 2016)

Andrej Karpathy, Sr. Director of AI at Tesla, just gave a talk about the new vision-only approach.
One of the examples he gave of where radar messes things up is "false slowdowns" due to radar seeing a bridge and mistakenly "merging" that with a car that the vision system sees.


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## TrevP (Oct 20, 2015)

Welcome to the forum! What you've experienced is what we call "famtom braking" which occurs the when the radar sees objects such as overpasses or bridges and thinks it needs to brake. It's a recent complaint even though it doesn't happen all the time.

Tesla has recently moved to all-vision based system for automatic emergency braking and cruise control and while it's not finished yet and no 100% operational they claim it will solve this problem. If you watch the video that @garsh linked Andrej explains it in more detail


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## FRC (Aug 4, 2018)

TrevP said:


> "famtom braking"


He means "phantom braking".


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## Needsdecaf (Dec 27, 2018)

FRC said:


> He means "phantom braking".


Or maybe this dude?


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