The one-dimensional thinking of the "you can't add lanes because they'll fill up" crowd is always astounding.  They act like that single road is the only thing that counts, and completely ignore the positive impact on the alternates when a lane is added and some traffic shifts over to it.  

For most of them, it just seems to come down to their environmentalist anti-car bias to begin with.  When I read "sustainability advocate" from the guy above, that was a pretty good clue regarding his ideology.