And no, the mime artists aren't recoiling in terror at the invasion of the touchy blue Oystercard posters (although you cannot move on the tube without seeing them).
The poster says it's dangerous. It is. If you're unsure about the danger, just imagine some manic commuters like me legging it for the train they know they're getting on, come hell or High Street Kensington. And they'll bash into you like an unstoppable force meeting an all too movable object. As they know you're likely to be a tourist or a bunch of tourists standing round there aimlessly wondering whether you're really going southbound or northbound, or why there isn't an Oxford Street station, or with all the Circuses, why there isn't a station called London Zoo.