Welcome to Tesla Motors Club
Discuss Tesla's Model S, Model 3, Model X, Model Y, Cybertruck, Roadster and More.
Register

Autopilot dangerously reducing speed on highways

LoveMyM3

New Member
Jan 28, 2020
1
1
Colorado
Hi and sorry if this was posted before. Have anyone experienced this? I will be on a highway on autopilot at the speed of 70 where the speed limit is 65. Then suddenly the car would decrease the speed to 55! I have been paying close attention and it seems to happen at the same spot where for approximately 100 yards the speed limit symbol disappears from the screen. It’s almost like if it doesn’t know the limit at that particular section of the highway it will default to 55? I’ve had many close calls where I had to take over and slam on the accelerator to keep the car following me from rear ending “my precious.” Is there a thread about this? Is it a bug? Seems it should just rely on following the car in front rather than slowing the car down like this.
 
  • Like
Reactions: jsmay311

LowBattery

Member
Dec 19, 2019
27
20
NJ
As Fresnel said, it is fairly common. I generally only turn on autosteer if the drivers behind me have been maintaining a healthy distance.
 

Knightshade

Well-Known Member
Jul 31, 2017
11,402
15,020
NC
It's a common thing. Called Phantom breaking. I've also noticed it happens when there is a lot of traffic in the lane next to you that's going slower then you are. It's more complicated then just tracking the car in front because it needs to know how to work when there are no cars in front.


That's not phantom braking at all. Phantom means it'd doing it without any obvious reason... (a phenomenon I've yet to experience in over 15,000 miles of driving, mostly on autopilot)

The thing you describe isn't phantom braking- lane-adjacent speed reduction was added intentionally in a recent update.

The OPs case isn't either- there IS an obvious reason in the OPs case- the car no longer thinks it's in a 65 mph zone because the maps are wrong.

So long as Tesla relies on map data for speed limits, and that data isn't perfect, this is going to happen.

Relying on the car in front of you to pick a max speed (other than TACC insuring you don't hit the guy) makes no sense because if the guy in front of you is a maniac who wants to drive 30 over the limit you probably don't want YOUR car doing that do you?
 

HyperionMark

Member
Aug 28, 2017
495
221
Brookings, SD
I think this is different than phantom braking. This is where it doesn't know the speed limit, therefore 45 or 55 is the top autopilot speed slowing the car down dangerously. I've had it happen a fair amount on this version. Will likely go away with the next one.
 

Fresnel

Member
Nov 27, 2019
384
167
USA
I think this is different than phantom braking. This is where it doesn't know the speed limit, therefore 45 or 55 is the top autopilot speed slowing the car down dangerously. I've had it happen a fair amount on this version. Will likely go away with the next one.

I thought the top autopilot speed was 90?
 

Knightshade

Well-Known Member
Jul 31, 2017
11,402
15,020
NC
I thought the top autopilot speed was 90?


On a divided limited access highway with a known speed limit- it is.

The OP notes his speed limit on the display vanished- at which point AP usually reduces max speed- the owners manual points this out in the AP section.

This isn't an AP bug- AP is working exactly as intended.

It's a map problem.
 

Gondi63

Member
Apr 26, 2019
164
233
Tampa
That's not phantom braking at all. Phantom means it'd doing it without any obvious reason... (a phenomenon I've yet to experience in over 15,000 miles of driving, mostly on autopilot)
I really disagree with this definition. In the vernacular of the community as I understand it, phantom braking is braking done without a need regardless of the obvious reason. The most common one being braking while adjacent to a large truck regardless of if the truck is actually moving into your lane.
 

Avid

Member
Oct 1, 2018
327
192
Houston, Tx.
I agree with @Knightshade, we drive on Hwy 290 in Houston where it had been under major renovation for about 7 years. It's pretty much done, about 3 years behind schedule, the two biggest issue I have with the renovation is 1) striping is not complete and 2) speeds are not updated for Tesla usage. The speed signs appear to be correct but the database that Tesla uses is not. Same portion of the hwy will go from 65mph to 45mph. 45 was the construction speed years ago, it is annoying and dangerous.
 

dsvick

Closed
Jun 10, 2016
2,198
2,214
NE Ohio
Has there been construction there within the last 6 months to a year? There is a stretch of highway near me that had construction and a temporarily reduced speed limit and for the last 6 months (since the construction ended) my car slows down there from 60mph to 50mph. I don't know that the prior construction is the reason it does it, but it's the only reason I can think of.
 

EMTM3

Member
Jan 28, 2020
16
2
New Jersey
Hi and sorry if this was posted before. Have anyone experienced this? I will be on a highway on autopilot at the speed of 70 where the speed limit is 65. Then suddenly the car would decrease the speed to 55! I have been paying close attention and it seems to happen at the same spot where for approximately 100 yards the speed limit symbol disappears from the screen. It’s almost like if it doesn’t know the limit at that particular section of the highway it will default to 55? I’ve had many close calls where I had to take over and slam on the accelerator to keep the car following me from rear ending “my precious.” Is there a thread about this? Is it a bug? Seems it should just rely on following the car in front rather than slowing the car down like this.

This happened to me as well few times on high way. No matter what we call it (Phantom braking or what so ever..) but I really feel it is dangerous and the passenger was really felt like the car hit somebody in the front. It is really that slow all of sudden.
Just wondering if this can be fixed with any of the available AP settings in the car without waiting for next software update. Any thoughts?
 

Knightshade

Well-Known Member
Jul 31, 2017
11,402
15,020
NC
This happened to me as well few times on high way. No matter what we call it (Phantom braking or what so ever..) but I really feel it is dangerous and the passenger was really felt like the car hit somebody in the front. It is really that slow all of sudden.
Just wondering if this can be fixed with any of the available AP settings in the car without waiting for next software update. Any thoughts?


No.

There's nothing to "fix" with AP.

AP is working exactly as intended

The problem is the map has the wrong speed limit (or in this case, apparently NO speed limit since he said the sign vanished rather than just became a lower speed)


I think a lot of folks in this thread would benefit by reading page 85 of the current Model 3 Owners manual that explains how AP speed restrictions work.
 

EMTM3

Member
Jan 28, 2020
16
2
New Jersey
That is totally understandable. Two things here that can avoid this dangerous situation:
1. Let the car move with the speed that was set to in AP (65, 70 mph etc..) without interfering with posted speed and this should be controlled by the driver not posted speed in maps
2. There should be a setting in AP that can give us options a) you want to follow the speed as per maps b) manually control the speed in AP.
 

arghx7

Member
Aug 6, 2019
471
492
Michigan
I'm sure they've known about this problem for a very long time (over reliance on maps), we just have no visibility to how they are planning to address it. There really isn't much of a financial incentive for them to devote resources to fixing it in the near future. They are likely focusing on features that can be released as FSD revenue. The whole business model is based on pushing out partially working software (typical of a tech company).

A lot of "buggy" (even though it's not a TACC bug, more like an overall flaw in the driver assistance system) features generates more revenue than chasing down all the problematic parts of existing feature sets.

We're all waiting for the pot of gold at the end of the HW3 rainbow.
 
  • Love
Reactions: MarkBrokeIt

arghx7

Member
Aug 6, 2019
471
492
Michigan
That is totally understandable. Two things here that can avoid this dangerous situation:
1. Let the car move with the speed that was set to in AP (65, 70 mph etc..) without interfering with posted speed and this should be controlled by the driver not posted speed in maps
2. There should be a setting in AP that can give us options a) you want to follow the speed as per maps b) manually control the speed in AP.

yes and there should also be a feature to completely turn off TACC and run dumb cruise control sometimes for especially problematic situations like this.

they could also put in an "anti brake-check" algorithm to the automatic speed change subsystem that would cap the amount of braking force/deceleration based on the distance of the car behind. This is called "anti-jerk" logic in conventional ICE vehicles, often used to cap jarring behavior on acceleration.
 
  • Like
Reactions: MarkBrokeIt

About Us

Formed in 2006, Tesla Motors Club (TMC) was the first independent online Tesla community. Today it remains the largest and most dynamic community of Tesla enthusiasts. Learn more.

Do you value your experience at TMC? Consider becoming a Supporting Member of Tesla Motors Club. As a thank you for your contribution, you'll get nearly no ads in the Community and Groups sections. Additional perks are available depending on the level of contribution. Please visit the Account Upgrades page for more details.


SUPPORT TMC
Top