Depending on how you frame it, that happens all the time. For example, a speedbump causes speed reduction, but at high speeds it causes (momentary) loss of control. Or narrowing the road, which means less margin for error.

People will drive their cars as fast as feels safe. By introducing hazards, you make people slow down which increases safety, but each feature in isolation could be said to be dangerous in some way, y'know?

Look to Europe then. Roads here have less width compared to USA to force you to focus. Or roundabouts which also make the crossing less safe...if you try to keep your speed.

Both end up making roads safer by slowing you down and forcing you to focus.

You either make driving fast less safe, or you put a speed camera on every corner with the appropriate punishment. Cameras are expensive, and drivers will complain about them. If you make driving fast less safe, only the ones that remember how the road was before can complain about it.