Autopilot speed reduction—explain?

[RubinRA]

Okay, can anyone explain what’s going on with my autopilot (2020 Model 3 w FSD, latest updates)?

This has happened multiple times, both day and night: I’m driving home at 75 mph on a limited access freeway. Speed limit is 70, and that’s what shows on the GPS display. I’m on autosteer (not following a GPS route). I reach a gentle curve where an exit ramp is coming up on the right, and am about to pass under the overhead sign showing the state route number (Rt. 55). The car slows down by about 10-15 mph! even though the GPS display shows a 70 mph speed limit, and the TACC is set at 75 and hasn’t dialed down to a lower speed. Then the car speeds back up to 75. Ten miles later, I pass an exit sign: Exit 55. The car slows down again!

Is it reading the 55 signs as speed limits? I thought that wasn’t yet functional. Other explanations?

 

[diplomat33]

Okay, can anyone explain what’s going on with my autopilot (2020 Model 3 w FSD, latest updates)?

This has happened multiple times, both day and night: I’m driving home at 75 mph on a limited access freeway. Speed limit is 70, and that’s what shows on the GPS display. I’m on autosteer (not following a GPS route). I reach a gentle curve where an exit ramp is coming up on the right, and am about to pass under the overhead sign showing the state route number (Rt. 55). The car slows down by about 10-15 mph! even though the GPS display shows a 70 mph speed limit, and the TACC is set at 75 and hasn’t dialed down to a lower speed. Then the car speeds back up to 75. Ten miles later, I pass an exit sign: Exit 55. The car slows down again!

Is it reading the 55 signs as speed limits? I thought that wasn’t yet functional. Other explanations?

No, it's not reading the sign as a speed limit. It sounds like just your typical phantom braking issue. The radar is probably seeing the overhead sign as an object in front of the car so the car freaks out a little bit and brakes until it realizes that there is no risk of collision.

 

[LowlyOilBurner]

What software are you on? I had this issue a while back, it was gone on 2020.20.1. I’m on 2020.24.6.1 now, I’ll have to see if this returned.

 

[swaltner]

There are many reasons a Tesla will drive slower than the set speed.

• TACC (traffic aware cruise control) - there is a vehicle in front of you going slower than your set speed. In this case, the vehicle that your car is tracking is shown on the main display in a slightly darker shade of gray. You don’t mention nearby vehicles, so this likely isn’t your specific issue.
  
• TACC on a “tight” corner - when using TACC, As you approach a tight corner, the vehicle will automatically slow down to a suitable speed. This is a very gradual deceleration and acceleration process. You can tell this is what is going on because there is an animation of the blue circle around your set speed when the car is slowing down or speeding up before/after a “tight” curve, at least there is when it changes your speed more than 5 mph..
• “Phantom braking” - this is what Tesla owners call the sometimes aggressive braking when the car “sees” an overhead sign, a shadow, or some other sensor reading that confuses it, so it brakes at an undesirable time. People that are tailgating you at the time may this you are brake checking them because these can be slightly aggressive braking events. The feel particularly aggressive because they happen when you aren’t expecting them. These are the annoying braking events. A “jab” on the accelerator pedal aborts these when they happen. I usually don’t let these play out long enough to see if the car slows down 5, 10, or even more mph when they happen.
• AEB (Automatic emergency braking) - this happens right before a crash in an attempt to slow the car down some. By this time, you have already received an alert to the vehicle in front of you and the car will issue a full on panic braking to try and limit the impact of an unavoidable crash. This is not what you are describing. This happens right before you have to make a call to your insurance company’s claims department.

You mention a curve in the highway, so it sounds like TACC is slowing down for the “tight” curve ahead. In my case, I rarely see these happen on the highway, but I do see it on ramps between highways. This would be my guess as to what you are seeing.

 

[Saghost]

Which lane are you in when this happens?

I’ve found that the speed behavior under Autopilot is much less predictable in the rightmost lane. I’m not sure if the car is bouncing in and out of the speed limitations it would put on the exit ramp, or reacting more to things on the road side, or if there’s something else going on, but I know mine will do things like you describe in the right lane that just don’t happen one lane over.

 

[J1mbo]

There is a new "phantom" braking scenario: if there is a car up ahead that starts drifting towards the lane divider, your car will slow and not overtake, even if the car ahead stays in lane.

 

[mrkisskiss]

There is a new "phantom" braking scenario: if there is a car up ahead that starts drifting towards the lane divider, your car will slow and not overtake, even if the car ahead stays in lane.

I'm not sure this is new -- I've seen the rapid slowing and refusal to overtake once a car starts drifting close to the lane lines before - it was added as part of "Cut in detection" I think. It can also occur if the sunlight "glints" off the back of a cars rear headlight enclosure; presumably it looks like the indicator is flashing. It's happened a few times with me when driving on GLORIOUS days

 

[mrkisskiss]

Another thing I've found that does not help... I'd. be interested to know if our friends in the US see the same thing...

My Model S has a penchant for hugging the left lane line. When I'm in the right-most lane (the overtaking lane), it'll still drive closer to (and sometimes nearly on) the left lane line - i.e much closer to other cars I'm overtaking than the barriers on my right. The car must be well aware it's doing it, since the position of the car vs the lane lanes is accurately visualised.

Do our friends in the US see a penchant for AP to drive closer to the right lane line? I wondered if this is a 'behaviour' that AP has learned from Americans, and in the "right hand vs left hand drive" logic flip, they just... didn't do that bit? I'm not sure there's any benefit for me to drive closer to cars I'm passing, and indeed puts everyone at greater risk.

Surely, it should either aim for a "bang in the middle" approach, or favour the farthest edge, where possible. I realise this sometimes interferes with signals from the ultrasonics, especially in the case of barriers which give a consistent boundary "ping", but still... it should be smart enough to know about path delimiters like barriers, walls, kerbs etc vs other cars and _still_ prefer proximity to stationary delimiters.

 

[MXLRplus]

Okay, can anyone explain what’s going on with my autopilot (2020 Model 3 w FSD, latest updates)?

This has happened multiple times, both day and night: I’m driving home at 75 mph on a limited access freeway. Speed limit is 70, and that’s what shows on the GPS display. I’m on autosteer (not following a GPS route). I reach a gentle curve where an exit ramp is coming up on the right, and am about to pass under the overhead sign showing the state route number (Rt. 55). The car slows down by about 10-15 mph! even though the GPS display shows a 70 mph speed limit, and the TACC is set at 75 and hasn’t dialed down to a lower speed. Then the car speeds back up to 75. Ten miles later, I pass an exit sign: Exit 55. The car slows down again!

Is it reading the 55 signs as speed limits? I thought that wasn’t yet functional. Other explanations?

My Tesla cannot read speed limit signs at all. It wants to drive 45 mph in a 25 mph residential tract that is well marked with a large clear sign.

My car appears to be changing the TACC speed at certain locations like clockwork. It's appears to be programmed into the maps.
I get hit with this about 10 times a day. The trick is to use the speed lever to bump the speed back up.

I really wish they would make a button to turn off this nonsense under the AutoPilot menu. It's irritating especially since my other ACC cars don't do this. One stupid place it does it is right before a Toll Reader Sensor so I always remember to have my hand ready.