Theme Day : Numbers in your City

Since the first day I saw buses in Montreal, I was wondering what the "POLICE 911" red sign was used for on the top of the left window. My guess was that maybe, in case something goes wrong on board of the bus, the driver simply turn this signal on to warn people outside to call the police. Just like in banks, with a silent alarm. Hum... who knows, maybe I watch too much television...