How do the traffic cameras know when to take the pictures.?

they have sensors for when it turns red and it always takes a picture.How do the traffic cameras know when to take the pictures.?
Last week there was an accident at the intersection and cops were directing the traffic. I was coming down the intersection as the cop was waving the traffic thru and I noticed that the camera on opposing lane was constantly taking pictures once every second or so because it was activated by red light. It was noticeable because of flash even on a sunny day. So my guess is that the camera has a sensor linked to red light. What I don't know is how it differentiates the vehicles moving from left to right on green against vehicles moving up to down.How do the traffic cameras know when to take the pictures.?
Beecuz them cameras bee smart !!How do the traffic cameras know when to take the pictures.?
If you have not gone through the intersection by the time the light hits red then the camera snaps you and you get a ticket in the mail usualyy within 1-3 business days.
I'm guessing that it takes pictures every second, or even less.