"...until it affects others by impeding normal social interaction."

You are correct that the niqab is a hindrance to normal social interaction, or could certainly be taken that way.  However, someone taking the bus has no obligation to "interact" with me in any way, nor I with them.  I'm driving the bus, you put your coins in the cash box and sit down.  Interaction over.  I don't *need* to see your face.  People wearing balaclavas in winter have been boarding buses for years and we've never heard a word of complaint about it.

The fact that you're advancing such silly arguments tells me what's really going on here.  Like I said, this is about the symbolic politics of Muslim bashing, not any practical or public safety consideration.