No but they're also flying in much closer tighter quarters.

I think that's kinda the parent comment's point. Just because sonar is good for indoor localization doesn't mean it's got the range for highway speeds. In fact, ultrasound on cars is for exactly this: low speed, close quarters detection. This is why backup sensors or self parking systems rely on ultrasound.

Is there the equivalent of Reynolds number for this? I'm guessing something based on a characteristic length and velocity? It looks like I want the Walking Froude Number [0] which is Fr = u^2 / g.l it seems...

0. https://en.wikipedia.org/wiki/Froude_number#Walking_Froude_n...

Closer relative to their size? Cars often travel in packs closer together than unit size.