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TACC in stop-and-go traffic

MUnangst

Member
Oct 15, 2017
27
17
Pittsburgh, PA
When stuck in traffic, TACC (and autosteer) do a great job of reducing mental load...you can just sit there and let the car speed up and slow down for you. However, passengers have commented that the car is much “jerkier” when accelerating and braking than a human driver (me) would be. Putting the car in CHILL mode helps a bit, but it still tends to do some things that don’t make sense — for example, if traffic speeds up quickly and then slows down, my car will keep accelerating to close the gap with the car ahead, even after the car one or two cars ahead has started to brake, until it gets “close enough” to the next car, and then brakes hard.

TACC seems to rely solely on radar distance sensing and not on other clues (i.e., brake lights from the car ahead) to predict slowing traffic. This works fine at 70 MPH (where you don’t want to slow down until you have to), but in stop-and-go traffic it would be better for it to drive more gently and maintain a speed closer to the average, rather than constantly speeding up and slowing down. This would also help improve traffic flow behind the car by “absorbing” traffic waves, and could actually dissipate traffic jams.

I have an Aug. 2017 AP2 X100D in case that matters...
 

Tdreamer

Supporting Member
Sep 10, 2017
967
1,396
Bedford, Massachusetts
What distance setting do you have on TACC? I agree completely with your observations if the setting is in the 1-3 range (follow closely to cars ahead). But if you set it to 5-7, I find the braking and acceleration to be very gradual - although you end up being farther away from cars ahead which can lead to opportunity seekers jumping in front of you. A balancing act...
 

dmode

Member
Oct 29, 2015
241
372
United States
What distance setting do you have on TACC? I agree completely with your observations if the setting is in the 1-3 range (follow closely to cars ahead). But if you set it to 5-7, I find the braking and acceleration to be very gradual - although you end up being farther away from cars ahead which can lead to opportunity seekers jumping in front of you. A balancing act...

Yes, agreed. I have found setting distance to 4 is optimal for stop and go traffic
 
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MUnangst

Member
Oct 15, 2017
27
17
Pittsburgh, PA
What distance setting do you have on TACC? I agree completely with your observations if the setting is in the 1-3 range (follow closely to cars ahead). But if you set it to 5-7, I find the braking and acceleration to be very gradual - although you end up being farther away from cars ahead which can lead to opportunity seekers jumping in front of you. A balancing act...

Interesting...I’ve usually been keeping it at 3, which is where I usually have it set for highway speeds. I’ll have to try the higher settings the next time I’m stuck in traffic and see how it is! The behavior I’ve experienced so far is bad enough to make my wife and son carsick, and they’ve asked that I just drive “the old fashioned way” when we’re in a jam with them in the car, so I’d like to find a way to make it palatable to them.

I don’t really care if people jump in front of me, they’re not going to get there much sooner...although I did have a scary moment the other day, where traffic in front of me started to move, a gap opened between my car and the car in front, and the Tesla started to accelerate hard just as the car in the next lane started to move over. Fortunately I was paying attention and hit the brake, but if I hadn’t intervened I assume that the Tesla would have either had to brake much harder (once the car was far enough over for the radar to see it) or might have rear-ended it.
 

Need

Active Member
Nov 22, 2017
2,876
2,196
SoCal
I noticed the exact same thing. The radar cruise control does not take into consideration of the deceleration of the car in front (or see the brake light). It does take a while to get used to it speeding up when all the cars in front got brake lights on. I set the distance to 4 and it is not really 4 cars length in traffic.. more like 1.5 car length.
 

kiwi

Member
Oct 1, 2017
75
50
New Zealand
My experience is that TACC has some smoothness issues in heavy traffic. There are a number of specific cases where it performs poorly - most of which have been mentioned above:

- harsh acceleration and deceleration. I run it at 6-7 for spacing to try and reduce harsh acceleration and still experience this. Passengers don't like it.
- cars moving into my lane are ignored until they are well into the lane. The following case is common: space opens ahead of me, my car starts to accelerate, car from other lane moves into space, my car continues to accelerate to fill the space even though another car is well into my lane. Either I take control to slow, or the car suddenly realises there's another car and brakes hard.
- cars leaving my lane are tracked until well after they leave (especially turning into side street). My car will continue to slow down/stop well after the other car has left the lane, causing confusion behind me.
- I'm still seeing phantom breaking - but that happens more at high speed, not much in slow traffic jams.
- Leaving the lane on cornering. .52 is way better than .42 and .36 that I had previously. When there are cars parked on the side of the road, you really can't depart the lane at all - you need to stay centered.

Lastly... one extra point. The display doesn't give me confidence that the visual system is accurately describing what's ahead and around. On corners (mostly) or straight ahead, the car ahead of me often shows as outside the lane, and bounces around quite a bit. (I'm AP2 BTW). And those cars moving from other lanes into mine don't show up until they are well into the lane.
 
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Qbenjamin

Frugal But Classy!
Jan 7, 2017
1,139
673
Bravos
Utilizing TACC in stop-and-go traffic was a life saver for me. I agree with the others mentioning that people cutting in front of you is very annoying, but it's to be expected due to your desired gap range.
 

Seattle Tom

Member
Mar 31, 2016
502
525
Seattle, WA
I use 4 for TACC and have a good experience generally, but do agree with most posts above.

As not all cameras are being used now, cars moving into and out of lanes really can cause some “jumpiness” and I hope that’s being addressed soon with some EAP upgrade. This is most problematic in slow traffic, not stop and go traffic. If we’re all crawling it’s much less likely to have cars jump in and out. But if it’s just crowded the normal lance changes cause problems with current autopilot use.

I also agree that it doesn’t use brake lights for any clue to stop, but a decelerating car should be taken into account. The Tesla should also decelerate to maintain distance so it’s odd yours seems to ignore that slowing and brake suddenly. I don’t have that experience myself!
 

vandacca

ReActive Member
Oct 13, 2014
3,371
2,202
Hamilton
IMO, brake lights should not be used in AP because it is a low-quality input which doesn't provide any feedback on how hard the car in front is braking. Since Tesla is equipped with RADAR, its a much better sensor to determine braking speed and Tesla should simply keep it's speed within a few mph than the vehicle up front. If it wants to bridge the gap, then drive a few mph higher than the car in front and if it needs to keep the same gap or increase it, then drive a few mph less than the car in front.

Of course, this is an easy algorithm to write, but that's not how AP works, since it uses AI. Tesla must have used some aggressive driving videos to train the current AI iteration. ;)
 

Vapor6

Member
Jan 15, 2018
15
4
Olympia, WA
In stop and go freeway traffic I have found that decreasing the target speed to a lower number (from 65 to 35 mph) decreases the “jumpiness”. When traffic is running smoothly again I increase it back to the full speed I want. This seems to work for me.
 
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obiwin

Member
Dec 28, 2017
6
0
Vienna, VA
I've generally found AP2 (2017 MX) to be much jerkier than AP1(2016 MS) was. On my AP1 MS I would often have desired following number set at 1 or 2 in heavy traffic with minimal cut-in issues and minimal jerkiness.

I had to tinker around a lot with AP2 settings to try and find a sweet spot, and this is what I do now:
Chill mode on
At higher speeds: following number 6 (with a max speed btw 62 to 70 MPH)
in stop and go traffic: following number 4 (with max speed setting at 25 or 30 MPH )

I do get more cut-ins now, but I'd rather deal with that than the sudden acceleration and braking.

I'm not sure it's a placebo effect or not, but lowering the max speed in stop and go does seem to help reduce hard accelerations and hard stops. At the very least, it provides some peace of mind that I'm not going to accelerate into someone at 60 MPH.
 

ecarfan

Well-Known Member
Sep 21, 2013
19,197
13,846
San Mateo, CA
I can't imagine setting TAC to 4 much less 5-7. People cut in front of me when set at 1. I'd like a .7 setting!
After 13K miles in my X, the majority of it on TACC and/or AP, I use 7 and occasionally 5-6 and have no problems. Yes, sometimes another driver will cut in front of me. I don’t care. I get to my destination a small fraction of a second later for each car that does that. So what?

TACC is fantastic in heavy traffic, in fact in any situation on a divided highway (I never use it on roads with cross traffic). Much more relaxing driving. Of course you still have to pay attention and keep your hands on the wheel. But it requires less active monitoring than AP.
 

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