Under the rules, video evidence used to determine whether a ticket is issued must start before the car enters the intersection. So sensors in the roadway measured the speed of approaching cars to predict whether or not they have time to stop at the light. If the car is moving faster than a preset "trigger speed" the camera is activated. According to Ferguson's report, the trigger speed during the spike from April to June of 2011 mysteriously dropped from 15 miles per hour to as low as 5 miles per hour.