Where I live all speed traps are clearly marked. Yes, their purpose should be to make people slow down, not catch them by surprise. But we're talking about warning about caps,nort just speed traps. If it's just unmarked speed traps I agree.

1. I'm assuming you're warning other drivers because you think they can change their behavior and avoid a fine. If it's unavoidable why warn them? It's not going to help anyone. So yeah, you can get a ticket for other things but reckless driving is the main one you can easily avoid when warned ahead of time.
2. Yes, driving outside the law is always risky. It's not up to individual drivers to decide when it's 'safe' to break the law. It's stupid to think that you can just decide that the laws do not apply to you because 'you know better'. Every single driver breaking the law thinks that they know better or that they can do it because they are a better driver than everyone else. Drivers like that can just go fuck themselves.
3. Speed traps that are clearly marked and enforce speed limit instead of just fining unsuspecting drivers do make roads safer.