To figure out average speed of a vehicle between two known points, you need to find when that specific vehicle was at point a and point b. Likely this would be done by automated license plate readers and storing when that car was where in some database. Great, now we're tracking the movement and location of vehicles (and people) with no reasonable suspicion of a crime. Surely this information won't be misused this time.

Don’t all modern cars have an always on cellular connection anyway?

I’ve got as big a tinfoil hat as anyone, but I think this battle was lost a long time ago. Automobiles transportation will become closer to flights, where everything is logged.