It's specifically 14,115 average between Breithaupt and Victoria. Some of the traffic you mention between Union and Victoria takes Guelph or Louisa or Wellington.
14,115 on average pass over the tracks on Lancaster. For Westmount, only a few segments of Westmount Road East right before it ends at Fischer-Hallman carry that few vehicles.
Edit to add that, were Lancaster closed, some of that traffic really would just disappear. You're right that most of it would move to other streets, but it might or might not be true that the result would be "insanity" there.
14,115 on average pass over the tracks on Lancaster. For Westmount, only a few segments of Westmount Road East right before it ends at Fischer-Hallman carry that few vehicles.
Edit to add that, were Lancaster closed, some of that traffic really would just disappear. You're right that most of it would move to other streets, but it might or might not be true that the result would be "insanity" there.