Sudden Unexplained Slowing while on Cruise Control with Auto Steering

[Obi Wan]

Our car: 2019 Model 3. Our last 3 trips on the I-5 North from Orange County, CA to LA County, CA, a sudden unexplained slowing occurred at almost the exact same place and around the same time. We were using Cruise Control with Auto Steering. Here are the details from our notes:
5/12/24, 9:50am: driving N on I5; abt 1 m before Westminster Blvd Exit #19; abt 200yds before seeing Westminster Blvd, 1 mile sign; abt 50yds before going under overpass; going 70mph on Cruise Control (and auto steering); clear lane ahead; suddenly slowed from 70 to abt 45mph; after 5-10", corrected to 50mph on Cruise Control.
5/19/24, 9:50am: driving N on I5; abt 1 m before Westminster Blvd Exit #19; abt 200yds before seeing Westminster Blvd, 1 mile sign; abt 50yds before going under overpass; going 70mph on Cruise Control (and auto steering); clear lane ahead; suddenly slowed from 70 to 45mph; after abt 5" I accelerated back to 70mph, then restarted CC; it had self corrected to 45mph.
6/9/23, around 9:10am: driving to Lisa's in Culver City; N on I5; abt 1/4 mile before Magnolia Exit; going 75mph on CC; sudden drop in speed to 65mph for no reason, I quickly returned CC speed to 75mph; then, very shortly, abt 1 m before Westminster Blvd Exit #19; abt 200yds before seeing Westminster Blvd, 1 mile sign; abt 50yds before going under overpass; going 75mph on Cruise Control (and auto steering); clear lane ahead; suddenly slowed from 75 to 45mph; after abt 5" I accelerated back to 75mph, then restarted CC; it had self corrected to 45mph (this was almost identical to events on 5/12 and 5/19.
Weird!!!???
Tom and Lynn driving Obi Wan, Tesla Model 3

 

[Kimmi]

No idea what is causing the car to slow at the same spot all the time I do have mirrors automatically fold at the same spot all the time which I’ve never been able to figure out either. At least my problem is not dangerous.

 

[Tronguy]

All right. Welcome to "phantom braking".

For those of us who've been funning around with various beta versions of FSD and/or highway autopilot, There Was A Time when this would happen, I dunno, every 10 to 50 miles or so. The car would see some shadow or bridge abutment, think that there was Something There, and brake some.

There was a time when people claimed that it was a screeching tires brake, and that may have been true for some tiny minority. But, for the most part, it was a minor braking/lack of gas event. If you were going 75, say, and this happened, you'd probably notice the deceleration and a slow-down to, I dunno, 70 or maybe, worst case, 65. Then a pause, then the car recovers.

I did a trip back and forth to Boston from New Jersey earlier this week. Nothing on the way up; but, when heading back, had one event on I-90 around noon-time on a cloudless, dark-shadow day; another one further down I-90 west of Boston, again near a bridge; and a third on I-84 in central CT. Again with the sun.

And those have been the first times I've noticed this in, I dunno, a year or more.

People have worried that they might get rear-ended, but there's been no reports of same.

My guess: It's a regression. Previous version wasn't doing it; it's back; and Tesla's going to have to fix it (again). Sigh.

If it happens again, hit the right scroll button, say the words, "Bug Report", and say "Phantom braking event!" and the location.

 

[ewoodrick]

Does the speed limits change on the display?

 

[EVRider-FL]

No idea what is causing the car to slow at the same spot all the time I do have mirrors automatically fold at the same spot all the time which I’ve never been able to figure out either. At least my problem is not dangerous.

You might have inadvertently saved that location when folding mirrors previously. If that’s the case, next time it happens, use Controls > Unfold Mirrors and select the Remove Location option.

 

[EVRider-FL]

Does the speed limits change on the display?

I’m on a road trip now, and have experienced some of these slow downs due to incorrect speed limits being detected. One new feature that makes this less of a problem than before is the car will sometimes not slow down in order to maintain the traffic flow (a message appears to that effect), using FSD in 2024.3.25.

 

[enemji]

No idea what is causing the car to slow at the same spot all the time I do have mirrors automatically fold at the same spot all the time which I’ve never been able to figure out either. At least my problem is not dangerous.

All right. Welcome to "phantom braking".

To be clear, this is NOT phantom braking.

I have had these issues myself and with new map updates they have gone away. The slowing down is possibly because earlier the DOT info would possibly have a STOP sign there or some construction noted which causes the slow down. But I have never had the mirrors fold, so it is possible that the geolocation is somehow being interpreted as your garage???

 

[ewoodrick]

To be clear, this is NOT phantom braking.

I have had these issues myself and with new map updates they have gone away. The slowing down is possibly because earlier the DOT info would possibly have a STOP sign there or some construction noted which causes the slow down. But I have never had the mirrors fold, so it is possible that the geolocation is somehow being interpreted as your garage???

Agree,

It's not phantom braking, because it is being done on purpose, to match the speed limit.

Phantom is where there is no apparent cause.