“The cop has no way to know ahead of time if there is or if there is not one in the car.”
Indeed, and under our Constitution, when the police have no cause to believe you have committed a crime, they are not allowed to harass you. They are not allowed to go on fishing expeditions in an attempt to drum up business.
Hey, the guy blew a yellow light that he had plenty of time to stop for. This is trucking training 101, see the yellow, prepare to stop, do not accelerate.