I think the rules of due process for minor cases like traffic infractions are, to some extent, just legal fictions.
I observed something else that day I was sitting in court watching cases: In many cases, cops had staked out a major highway, and were looking at cars approaching them from the opposite direction. Thus, they were looking at the fronts of cars rather than the backs. They had radar guns.
When speeding drivers see a cop, or their fuzzbusters sound off, they tend to step on the brakes and slow down. This causes the front of the car to dip. This is something that the police know to watch for.
So it became a sort of “magic phrase” for cops to utter. In several of the cases, the police stated their observations, almost word-for-word alike as if they were reading from a standardized script, and it always included “I observed the front of the car dip.” It was almost like, saying this was a magic incantation, the very pronouncement of which established the credibility of the officers’ cases, capturing the judge’s mind, and forcing a guilty verdict.