TACC lesson: how it tracks cars and makes decisions

[ecarfan]

Recently had a useful example of the capabilities and limitations of TACC so thought I would share it. I’m sure many Tesla owners know all of this but some newer owners may not. I will try to describe this experience as clearly as I can.

Was on a 4 lane divided highway (2 lanes each way), speed limit 55mph, intersections with traffic signals about every mile or so, moderate traffic. TACC on (Auto Steer not engaged), interval setting at 7, I’m in the left hand lane, TACC tracking a car ahead of me. Traffic signal ahead changed to red, car ahead slowed to a stop and my TACC slowed to a stop behind it just like it should.

Signal changed to green, we all accelerated with my car doing so automatically following the car ahead of me (my foot off the pedal, resting on the floor, still on TACC), as we approached the next traffic signal it changed to red, car ahead of me slowed, my car slowed, the car ahead of me changed lanes to the right and now ahead of me was a car that was fully stopped at the intersection. I was prepared to apply the brakes in case my TACC was not able to detect the stopped car ahead of me but to my surprise my car slowed to a stop on its own in safe and gradual manner. (I have been in similar situations many times and sometimes TACC will not slow and stop, it can’t always detect a non-moving object in front of it)

Signal changed to green, we all accelerated again with my car (still on TACC) automatically following the car ahead of me, as we approached the next traffic signal the car ahead of me changed lanes to the right and now there was no car ahead of me, and then the light went to yellow and then red. Of course my car did not slow down because TACC knows nothing about traffic signals or intersections (nor does Auto Pilot at this point) so I applied the brakes and stopped for the light.

My point is that when using TACC you need to know what it can and cannot do and you should never assume it will behave in a certain way but always have your hands on the wheel and foot ready to brake if needed.

I now have over 7,000 miles on TACC and nearly that much on EAP (not a lot compared to many owners!) and am very impressed by their capabilities but know that they have many limitations. We are just at the beginning of the self driving revolution. It’s fun to be a part of it! Motor on.

 

[BerTX]

Some of the TACC capabilities seem to vary with firmware version -- you might mention what version this is talking about.

I love TACC. AP1 has it's place, but once the novelty wears off, I tend to just use it on the open highway with limited traffic. TACC I use all the time, maybe more than driving with no assistance at all.

The one function/option that is missing that I really would like to have is Speed Assist under TACC only. We know it can be done, but Tesla opts not to. Granted, with the somewhat ethereal speed limit database changes and AP2 problems with reading speed limit signs, maybe that's a good feature to hold off on. The constant adjustment of speed settings is just about the only thing that keeps TACC from being really wonderful.

 

[boaterva]

@ecarfan, you were following at 7?! I never use anything but 2.... How far back were you at 7 at 55 MPH? I wonder if some cars have different (individual) settings for how they judge distances, since, I know it’s speed dependent.... I can’t think of when I’d use more than perhaps 4, based on how far back I am at 30 to 40 MPH usually at 2 (about one-two car lengths or a bit less, enough to get wary of a car cutting in).

 

[ecarfan]

you might mention what version this is talking about.

I’m on .42

@ecarfan, you were following at 7?! I never use anything but 2.... How far back were you at 7 at 55 MPH?

I almost always set TACC at 7. Occasionally on this recent 2,300 mile roundtrip to New Mexico I set it at 5 when following a semi truck in very light traffic, since I know that TACC will stop me far more quickly than the semi can brake. Anything less I’m not comfortable with.

I realize that TACC has much faster reflexes than I do, and I have a lot of confidence in it. But I am under no illusions that it is perfect, and see no compelling reason to use a lower number as that leaves less time for TACC to react to emergency braking initiated by the vehicle in front of me which by the way is being driven by a human who is prone to error and poor decision making.

Do you get to your destination significantly quicker with TACC at 2? I don’t think so. Are you safer with TACC set at 2? I don’t think so.

Other irresponsible drivers will foolishly cut in front of you whether TACC is at 2 or at 7. I see it happen regularly at highway speeds with other cars following very closely at distances comparable to a setting of 2 on TACC.

My perception is that each single digit change on the 1 to 7 scale is equal to less than three feet at highway speeds (meaning over 65, less at lower speeds since the scale is heavily speed dependent). But since I have no way to accurately measure it I am only guessing. So going from 2 to 7 is maybe a total of 15 feet difference at 65. I want that extra 15 feet, for safety. If you do not think that is important, that’s your call. But I do not want to be in the car you are following.

I wonder if some cars have different (individual) settings for how they judge distances,

All EAP cars should in theory provide the same distance interval at a specific speed for a specific TACC setting. If one car differs from another, I would suspect a camera or radar calibration issue.

 

[boaterva]

I’m on .42
I almost always set TACC at 7. Occasionally on this recent 2,300 mile roundtrip to New Mexico I set it at 5 when following a semi truck in very light traffic, since I know that TACC will stop me far more quickly than the semi can brake. Anything less I’m not comfortable with.

I realize that TACC has much faster reflexes than I do, and I have a lot of confidence in it. But I am under no illusions that it is perfect, and see no compelling reason to use a lower number as that leaves less time for TACC to react to emergency braking initiated by the vehicle in front of me which by the way is being driven by a human who is prone to error and poor decision making.

Do you get to your destination significantly quicker with TACC at 2? I don’t think so. Are you safer with TACC set at 2? I don’t think so.

Other irresponsible drivers will foolishly cut in front of you whether TACC is at 2 or at 7. I see it happen regularly at highway speeds with other cars following very closely at distances comparable to a setting of 2 on TACC.

My perception is that each single digit change on the 1 to 7 scale is equal to less than three feet at highway speeds (meaning over 65, less at lower speeds since the scale is heavily speed dependent). But since I have no way to accurately measure it I am only guessing. So going from 2 to 7 is maybe a total of 15 feet difference at 65. I want that extra 15 feet, for safety. If you do not think that is important, that’s your call. But I do not want to be in the car you are following.

All EAP cars should in theory provide the same distance interval at a specific speed for a specific TACC setting. If one car differs from another, I would suspect a camera or radar calibration issue.

Interesting that the difference is that small per digit. I’ll have to try it further out and see how it is! Thanks for your details!

 

[ecarfan]

Interesting that the difference is that small per digit. I’ll have to try it further out and see how it is! Thanks for your details!

Please keep in mind that I said “my perception...” and “I have no way to accurately measure...” those distances. Please post what your perceptions are.

 

[trmmcd]

I use a TACC setting of 4. It most closely models my own reaction time and therefore is most comfortable for me.. The only time I go below 4 is in stop and go traffic where the tracking distance invites cars to continuously jump ahead.

 

[Gwgan]

7 except in traffic then 5, 1-2 for Boston traffic.
What irks me is when the dash shows the slow lead car going through a bend but accelerates anyway as if the lead car were no longer there to modulate the cruise speed setting. TACC knows about the car and about the curve (camera) but stops adjusting interval disrance (radar). Version .42

 

[vandacca]

I find that TACC set to 1 would be fine if I trusted it completely. But I find it nerve-racking, especially when I see the red brake lights on the car ahead. However, when I sit there all tense with the foot over the brake and the hands on the wheel and let TACC do it's thing, it tends to do a much better job than I could at keeping the same distance and avoiding "accordion-style" traffic oscillations.

The problem is that I don't trust it enough (yet) and I don't want to be watching over it so carefully. This is with AP1 on 2017.46. I had a AP2 loaner for a few weeks and I would feel less comfortable with it, since after 30 minutes of sane driving, AutoPilot would slowly creep into the next lane over, even if there was a car there. TACC might have been just as good, but I didn't have it long enough to find out.