You do realize that in order for the assistant to know you're in a car, and that you're "at speed" it would likely need access to your location information, right? Aggregated over time (even if only when the assistant is "on" - ie, the iOS "while using" setting), that's a pattern of your movements.

Either Apple/Google/etc and granulize the permissions such that things like "boolean: user is in vehicle at speed" are addressable separately (possible but unlikely) or such knowledge comes with a cost - the app knows where you are and have been.

Same applies doubly for "noise it has heard".

Consequently my decision is: no assistant, thankyouverymuch.