> So slower cars, closer together, probably result in more cars passing per second than at higher speeds with greater stopping distances between cars. Exactly the same as with roads.

I hear this from time to time but it doesn’t pass the sniff test. If we assume the vehicles are points, their flow is precisely determined by stopping distance (as measured in seconds). If on the other hand we assume the vehicles are very long, their flow is determined by speed. Thus flow at minimum does not change with respect to speed, but in practice increases with respect to speed, assuming time-constant stopping distances (i.e. you pass X point Y seconds after the vehicle in front of you does)