I'm confused. But then again, I know little/none about US laws. My naive understanding of the 5th is that you can remain quiet if you'd otherwise have to point at yourself (or your family?). If your coworker is the offender, can you plead the 5th?

If that's close enough: How is that a problem? Yes, people over here might just pay the fine (which is completely legal for a lot of cases: In all cases where the state doesn't care about the offender's identity).

But if the 5th only means that you're allowed to protect yourself (and yours), isn't it correct to pay? I currently parse your comment as "Wouldn't the people pay up if they are guilty in the first place?".

Not my native language.

If you are suspected of doing something illegal (speeding) because your car was photographed using photo radar, I'm reasonably sure the 5th amendment applies to the police or prosecutor asking you to answer any questions about that accusation.

The answer could be "X person borrowed my car", which isn't, itself necessarily covered, however the answer could be "I was driving my car at that time", which is definitely covered by the 5th amendment. Since one of the possible truthful responses counts as testimony against yourself, you don't have to answer the question at all, otherwise non-answers could be seen as implicit confessions.