Honestly the only thing I see weird here is the cars simply passing through players in New LA. Would have been better if they simply came to a stop to avoid hitting players to try and make it a bit less apparent that they have no collision detection.
I want to say that it's intended to prevent them from crashing into your mechs. You can drive on the roads in your mech if you want but if cars had collision that might be a problem and a lot of fumbling around town.
EDIT: oops, meant that for VariantX