| | Curb line
State laws are generally in accord that a vehicle is considered to be in the intersection once its front wheels have passed the curb line. The curb line is a theoretical line that connects the four closest corners of a four-way intersection. Itís always farther into the intersection than the stop line, usually by 4-6 feet or more.
Most RLCs have two parallel piezo strips (induction loops), one just past the stop line, the other at about the curb line. Crossing the first strip starts a timer, adjustable from zero to as much as one second, which is the leeway given drivers before the first camera is triggered. Most RLC contractors, with the city's hearty approval, set the tolerance to as little as 0.1 second, not wishing to miss any revenue.
If you look at the data box at the upper corner of the violation photo, it'll have a value for the duration of the red light before the camera was triggered. On Gatso systems it's a yellow "R" with a X.X (seconds/tenths) after it. Competing systems are similar.
Hope this helps.