There is a principle called shomer petayim (Meaning "G-d protects the simple". Based on Psalms 116:6.) which rabbis occasionally use to allow people to do something that would probably be considered dangerous. The rabbis make the activity permissible if it becomes evident that a significant populace is willing to take the risks.
For example, Rav Moshe Feinstein ztz"l used this principle to allow smoking despite the large evidence available explaining the dangers. See this article for details.
What criteria allows rabbis to use this principle to permit something? Can they freely use this for anything whatsoever that shows "societal normalcy" (i.e. society accepting the dangers?) For example, numerous people speed or weave on the highway. It's become the "norm" for people to exceed the speed limit by 10 - 20 MPH. There's sufficient evidence that increased speed causes increased as well as more severe crashes. But, since it has become societal norm, can rabbanim use this principle to allow people to do this? (I'm asking only about the halachic aspect of using this principle, here; not about prohibiting it b/c of Dina Demalchuta - i.e. one violates the law by exceeding the speed limit.)
The above is an example. I'm sure there are numerous others where this rule is applied.