The signal cameras should not ‘arm’ until the light turns red. When it turns red, there is a sensor near the start of the intersection, just beyond where you stop.. that will trigger the camera. It is basically a wire loop put into the pavement by cutting a slot in it (kind of an over sized metal detector). There should be two picts taken, to verify that you didn’t ‘stop’ on that sensor.. and to validate speed.
Technically, the yellow light should last 1 second for every 10mph of the speed limit on the road that the red light will be stopping. This value is roughly tied to the amount of time a car takes to stop from that speed (safely), as a minimum. The lights don’t always obey it. NOTE: For 45mph, that would be nearly 5 seconds of yellow.
