Define 'excessive'

Photography is not a crime. Obviously. But authorities on both sides of the Atlantic suggest otherwise. There's been a number of well-documented over-zealous police actions in the UK, which prompted the protests in Trafalgar Square this weekend. But just so you don't think its just UK snappers getting a rum deal - check this out from the Chicago Transit Authority. Can someone clear up what constitutes 'excessive' photography? No, me neither. Read more on this here