If you're speeding, something's measuring your speed based on camera images. The method used to determine speed is not described in detail, just "trust us, they were going the speed we say". It's not generally the city who is doing this, but a third-party private company which sometimes is paid per ticket issued.
You can face the city in court, and the city will just say "Our contractor says you did this." The contractor will say "we totally think you did this. No, we will not show you our proprietary software that says you did it." In my opinion, that last statement violates the rights of the accused.
There have been numerous cases in the last decade of red light and speed camera companies issuing hundreds of illegitimate tickets. The system is ripe for abuse, and gets abused all the times. Happy to pull up some examples of this if you're curious.