The question though, surely can't be whether there's an expectation of privacy on public roads, but the total effect of knowing where people are at essentially all times through a combination of things like location data from phones, license plate readers, facial recognition etc.

Whether there is an expectation of privacy can't be what matters, what matters has to be whether the total effect allows a level of control that is dangerous or might have chilling effects on speech or on participation in things that are controversial.