Half the time the cameras dont work or arent turned on.
I can vouch for this as i was knocked off at Canary Wharf who have more cameras whatching the streets than most places. The 5. Yep 5! cameras watching the junction were either broken or turned off.
Ah you may say. They were having you on because they didnt want to get involved. I was working for CW at the time and have been freinds with the guy who manages security for the last 6 years, I drank with the guy who manages the cameras. Twas true. So. This thing can happen.
Rule of thumb. The more cameras there are, the greater chance that some will be turned of or broken.