I haven't seen anyone mention this so I thought I would post my observations on the Model 3 TAC/Autopilot touchscreen controls. When the touchscreen shows a speed limit sign, if you tap on the sign in the touchscreen display, the TAC speed will adjust to the speed limit on the sign (plus any offset you might set in the customizations).
I find that if I've adjusted the speed limit up (or down) through the '+' or '-' control on the touchscreen and strayed from the posted limit, if want to quickly get back to the posted limit (plus my offset - which I have set at 4MPH over the posted limit), I just tap the sign and I'm good to go.
If you tap the '+' or '-' buttons on the touchscreen, it will adjust the speed in 1MPH increments. If you hold the control down, it will also adjust up or down to the nearest 5MPH increment (at least I think it is the nearest 5MPH increment 0-5-10-15-20 etc., not absolute - 22-27-32-37 etc.).
For example, if you are going 39MPH and want to quickly go down to 35, just hold the '-' button and it will adjust down to 35. It appears to only adjust one 5MPH increment per press, so if you want to go up or down another 5MPH, you have to release and then hold the button again to get the next increment adjustment. I think you only have to hold the control down for two seconds or so for it to adjust. It's a very short period of time, but not as short as tapping the control.
I know there has been discussion about there being no hard controls on the steering wheel or the steering column to adjust the TAC/Autpilot and it's been a concern. I would also like to see the ability to adjust speed and following distance through the scroll/click wheels, but I find that it's not too troublesome for me to adjust the speed of the TAC/Autopilot and maintain safe control of the car by using the touch screen. I keep my left hand on the steering wheel and I use my right hand to make the speed adjustments. It's basically a paradigm adjustment and I think Tesla thought a lot about this UI choice.
It's actually nice to have the controls accessible on the touchscreen if you are on Autopilot. If my hands happen to be off the wheel for some reason while on Autopilot, it's a more natural motion to just touch the screen to adjust the speed. I would like to have the following distance control to also be accessible on the same screen as the speed control in a persistent menu when TAC/Autopilot is active, so I don't have to dig through layers of menus to get to the following distance adjustment.
Once I found that I could quickly get back to the posted speed (Plus my offset) by tapping the white speed limit icon on the touchscreen, it wasn't quite so troublesome. The adjustment will only work if the TAC/Autopilot is active, so there isn't the danger of accidentally tapping the speed limit icon and having the car accelerate to the speed if you are in manual drive.
