> However, if it's his word against mine ...

I don't know the US point of view, but I have read that in Germany the courts evaluate the "evidence" (which includes statements of witnesses) by importance.

If your word is against the word of the police officer, the court will judge the two statements the following way:

 - The police officer is trained to observe situations in the traffic exactly. He is paid to tell the court exactly what he has seen. He has no motivation to tell anything but the truth.
 - The driver was focused on driving the car so it is possible that he didn't see the stop sign. The driver doesn't have a motivation to tell the truth but he has a motivation to say that he observed the stop sign - even if he didn't do it.

> ... presumed guilty in traffic court

This has neither to do with being presumed guilty, nor with the traffic court:

If somebody stole goods in the supermarket and there is exactly one uninvolved witness who can identify the thief, the court will also believe the uninvolved witness and not the defendant.

In the case of the stop sign, the police officer is the uninvolved witness.