A quote from the latest edition I have of the Model S Owner's manual which I find on My Tesla (page 69) under the heading of Traffic-Aware Cruise Control and has been there, at least, since 7.1 - maybe before.....
"In right hand traffic, engaging the right turn signal when driving in the right-most lane within 164 feet (50 meters) of an exit (on a restricted access road only, such as a highway or freeway), causes Traffic-Aware Cruise Control to assume you are exiting. As a result, Traffic c-Aware Cruise Control begins to slow down the vehicle. Likewise in left hand traffic, when engaging the left turn signal when driving in the left-most lane within 164 feet (50 meters) of an exit. The onboard GPS (Global Positioning System) determines if you are driving in a region with right or left hand traffic. In situations where GPS data is unavailable (for example, inadequate signal), engaging the turn signal near an exit does not cause Traffic-Aware Cruise Control to slow down the vehicle. "
So it's under TACC, not Auto Steering. I recall that it worked, but had a problem when you tried to use it to change from one highway to another as it assumed you wanted to slow down, when, under that condition you wanted to maintain your speed since you were still on the highway and not an exit.
