“+ 1/4 mil. commuters”
Minus + 1/4 mil. commuters out of Brooklyn.
Why would you deduct the outbound commuters? Both the inbound and outbound commuters could be on the road at the same time (going in their respective directions) in the 12×12 mil area. That would necessitate inclusion of both. Rav Moshe included the inbound commuters and did not deduct the outbound commuters.