This is an interesting point. The Subaru's adaptive cruise control system treats a "car length" as a fairly large unit of measurement. One car length seems to b

This is an interesting point. The Subaru's adaptive cruise control system treats a "car length" as a fairly large unit of measurement. One car length seems to be the literal length of the vehicle, plus some reasonable amount of buffer on both ends. Probably twice as large as the vehicle itself. And the faster you're going, the larger a single car length becomes. Infinite nuance :P But in general, very good commentary on systems that provide useful feedback for the human operator versus ones that don't. ACC makes it clear what a car length should be, and if you're paying attention you can gauge how much more space it leaves when you're moving fast. So that's what I go by.