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Sudden Large Increase in TACC Set Speed ?

jsmay311

Active Member
Apr 22, 2016
1,178
1,693
Chicago suburbs
Yesterday I was driving my 2018 Model 3 with TACC engaged while driving through a highway interchange (one highway was ending and merging onto another), when out of nowhere my set speed suddenly jumped from ~62mph up to 90mph and the car started aggressively accelerating up towards 90mph.

Right before this occurred, I was using the right scroll wheel to very slowly increase my set speed up from 60 to 65mph — bumping it up by 1mph every couple seconds or so — but I only made it to 62mph (or so) before it jumped to 90mph. I definitely did not do a “quick” scroll wheel roll that would cause the set speed to increase by 5mph, nor did I touch the stalk or the accelerator pedal. And I have the set speed programmed in my car to engage at the current speed, not an offset from the speed limit. In the minutes prior, I was either using AP or TACC to maintain a speed right around 60mph.

This wasn’t the first time that I've had the TACC/AP set speed suddenly jump way up on me, but this was the first time I made a point of remembering all of the relevant details to post about later. Overall, I’d guess that this has happened to me 3-5 times over the past 6 months or so. And I think that in each prior instance the set speed has similarly jumped by ~30mph, but I’m from from certain about this detail.

I found this behavior pretty alarming and potentially dangerous, so I wanted to post and see if anyone else has experienced behavior like this in their car?

And/Or does anyone have an explanation or good theory about what might have happened here?


Other possibly relevant details…
  • "Cruise Set Speed" setting: Current speed. (Not "speed limit".)
  • FSD enabled?: Yes
  • H/w version: HW3
  • S/w version: 2021.4.18
  • Location: Eastbound IL-56 merging into I-88 near Sugar Grove, IL.
  • Speed limits: IL-56 is 55mph and I-88 is 65mph, but there was a 60mph construction-zone speed limit sign on the interchange ramp.
  • Lead vehicle?: There was no one in front of me.
  • Sharp curves?: There are no sharp curves along this route. (See map screenshot below.)

1625639207247.png
 

jsmay311

Active Member
Apr 22, 2016
1,178
1,693
Chicago suburbs
I found this snippet in the Model 3 manual about TACC set speeds while driving on highway interchanges, which I suspect might be relevant if there’s a bug with the “feature” described below:

“When enabled while on a highway interchange or offramp, Traffic-Aware Cruise Control may reduce your set speed in 5 mph (5 km/h) increments – to as slow as 25 mph (40 km/h) – to better match the reported speeds of other Tesla vehicles that have driven at that specific location. To override this and continue cruising at your set speed, tap the accelerator pedal or touch the plus (+) or minus (-) button on the touchscreen. The new set speed is maintained for the duration of the interchange or off-ramp (unless you override it or cancel Traffic-Aware Cruise Control). After the interchange or off-ramp, the set speed may revert or change as necessary based on the new location. For example, if you merged onto a different highway, the set speed reverts back to the set speed that was in use before driving on the interchange.
 

jsmay311

Active Member
Apr 22, 2016
1,178
1,693
Chicago suburbs
Did you notice if the current speed limit displayed on the touchscreen was also 90 when the car sped up?

I did glance quickly at the screen after it start accelerating and I'm pretty sure it did not show a 90mph speed limit on the screen at that moment. (But it all happened very quickly, so I can't say with 100% certainty.)
 

Stach

Member
Mar 23, 2019
464
246
Madison, WI
I have just the opposite happen quite frequently (twice yesterday) where I'm cruising along at 65mph on TACC (speed limit is 55mph) and suddenly the TACC speed is reduced to 50mph or even 40mph, for no reason (4-lane highway with nobody around me). Also, I always double-check to see what the car thinks the speed limit is, and it has not changed from 55mph.

I'd say this easily happens at least once a week for me and makes driving on TACC very frustrating, especially when you add in the phantom braking incidents, which have become more rare, thankfully. I can see why Tesla continues to call TACC in Beta, but it's beyond ridiculous that a basic feature like TACC is still in Beta years after release.
 

jsmay311

Active Member
Apr 22, 2016
1,178
1,693
Chicago suburbs
I have just the opposite happen quite frequently (twice yesterday) where I'm cruising along at 65mph on TACC (speed limit is 55mph) and suddenly the TACC speed is reduced to 50mph or even 40mph, for no reason (4-lane highway with nobody around me). Also, I always double-check to see what the car thinks the speed limit is, and it has not changed from 55mph.

I'd say this easily happens at least once a week for me and makes driving on TACC very frustrating, especially when you add in the phantom braking incidents, which have become more rare, thankfully. I can see why Tesla continues to call TACC in Beta, but it's beyond ridiculous that a basic feature like TACC is still in Beta years after release.

That does sound awfully annoying, especially if happening as frequently as once per week.

For instances where the TACC set speed unexpectedly drops rather than increases, I wonder if an explanation might be found in this tidbit in the TACC section of the manual:

"WARNING: Due to limitations inherent in the onboard GPS (Global Positioning System), you may experience situations in which Traffic-Aware Cruise Control slows down the vehicle, especially near highway exits where a curve is detected and/or you are actively navigating to a destination and not following the route."
 

Stach

Member
Mar 23, 2019
464
246
Madison, WI
That does sound awfully annoying, especially if happening as frequently as once per week.

For instances where the TACC set speed unexpectedly drops rather than increases, I wonder if an explanation might be found in this tidbit in the TACC section of the manual:

"WARNING: Due to limitations inherent in the onboard GPS (Global Positioning System), you may experience situations in which Traffic-Aware Cruise Control slows down the vehicle, especially near highway exits where a curve is detected and/or you are actively navigating to a destination and not following the route."
I really appreciate the tip and I submitted a bug report when it happened last night. I may submit a service request soon, but I've been holding out hoping that the massive new FSD update would help TACC as well, but who knows how long it'll be before we actually see that release.
 

jsmay311

Active Member
Apr 22, 2016
1,178
1,693
Chicago suburbs
I got curious enough drive through the same route and try to reproduce what happened, and I had no problem reproducing it two times in a row.

While driving in TACC at between 60-70mph, the set speed simply jumps up to 90mph right after the speed limit sign on the screen changes to 70mph. (Side note: there are no actual 70mph speed limit signs.) Doesn’t appear to matter whether or not I’m manually increasing the set speed with the scroll wheel like the first time it happened. I tried it once each way.

I took a video and here are before/after screenshots taken a fraction of a second before and after the set speed jumps up to 90mph. The full video is at th bottom, and the jump happens at the 14-second mark.

Before: 65mph set speed.
E2AFB6B8-5549-48B5-9328-262FA212BE94.jpeg


0.1 seconds later: 90mph set speed.
72BBC871-D46F-4CA2-A7E3-BE1595CB12C4.jpeg



WTF.
 
Last edited:

mikes_fsd

Banned
May 23, 2014
2,562
2,093
Charlotte, NC
I got curious enough drive through the same route and try to reproduce what happened, and I had no problem reproducing it two times in a row.

While driving in TACC at between 60-70mph, the set speed simply jumps up to 90mph right after the speed limit sign on the screen changes to 70mph. (Side note: there are no actual 70mph speed limit signs.) Doesn’t appear to matter whether or not I’m manually increasing the set speed with the scroll wheel like the first time it happened. I tried it once each way.

I took a video and here are before/after screenshots taken a fraction of a second before and after the set speed jumps up to 90mph. The full video is at th bottom, and the jump happens at the 14-second mark.

Before: 65mph set speed.
View attachment 682437

0.1 seconds later: 90mph set speed.
View attachment 682438


WTF.
This is really good info, create a service request with a link to the video. They will send it to the right ppl.
 

Tam

Well-Known Member
Nov 25, 2012
9,144
7,862
Visalia, CA
....I took a video and here are before/after screenshots taken a fraction of a second before and after the set speed jumps up to 90mph. The full video is at th bottom, and the jump happens at the 14-second mark...
Thanks for sharing the document. It's a bug and of course, it will be fixed by the time the real robotaxis arrive.
 
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Reactions: jsmay311

Huskyf

Member
Jan 6, 2018
520
1,126
Geneva - CH
I got curious enough drive through the same route and try to reproduce what happened, and I had no problem reproducing it two times in a row.

While driving in TACC at between 60-70mph, the set speed simply jumps up to 90mph right after the speed limit sign on the screen changes to 70mph. (Side note: there are no actual 70mph speed limit signs.) Doesn’t appear to matter whether or not I’m manually increasing the set speed with the scroll wheel like the first time it happened. I tried it once each way.

I took a video and here are before/after screenshots taken a fraction of a second before and after the set speed jumps up to 90mph. The full video is at th bottom, and the jump happens at the 14-second mark.

Before: 65mph set speed.
View attachment 682437

0.1 seconds later: 90mph set speed.
View attachment 682438


WTF.
Did you add on the settings of the car in the section "Autopilot"an +- speed limit ???? or a % of speed limit ?
 

jsmay311

Active Member
Apr 22, 2016
1,178
1,693
Chicago suburbs
This is really good info, create a service request with a link to the video. They will send it to the right ppl.

Do you know what is the current recommended method to contact Tesla about bugs like this? Is it tesla.com/contactus ?

(All of my previous attempts to contact Tesla to report bugs has seemed like a complete waste of time. But maybe/hopefully they're more responsive now?)
 
Last edited:

mikes_fsd

Banned
May 23, 2014
2,562
2,093
Charlotte, NC
Do you know what is the current recommended method to contact Tesla about bugs like this? Is it tesla.com/contactus ?

(All of my previous attempts to contact Tesla to report bugs has seemed like a complete waste of time. But maybe/hopefully they're more responsive now?)
The absolute best is to create a service request through the app tied to the particular car you had an issue with.

I've even reported broken supercharger stalls this way... when the number on the stall sticker did not work...
 
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jsmay311

Active Member
Apr 22, 2016
1,178
1,693
Chicago suburbs
It just happened again, but this time on a different interchange: driving from Northbound I-355 onto Westbound I-88 (in the western Chicago burbs).

TACC set speed was ~65mph going through the interchange. Then near the end of the ramp it jumped to 90mph.

I’m surprised there aren’t more posts on this. I can’t believe I’m the only one this is happening to. And it’s such a bizarre behavior.
 
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