Risk / likelihood

In accomplishing any task we seek to minimise risk.  (People who don't will fail to dominate, possibly fatally).  Risk comprises two factors: severity and probability.  Something may be unlikely but VERY nasty if it actually happens.  Something else may be likely to occur but not significantly life-threatening, injurious, or even damaging.

So, moving along the road (or even walking along or across it) we constantly hunt for risks, or hazards, to plan avoiding them.  Often, if we're moving linearly, we can avoid by slowing down ... then the hazard has reduced or is moving away by the time we get closer.  Other times, we may change position to keep more space from it.  Occasionally, we change route entirely to stay a long way from it (like a flooded road).

Ultimately, experience will teach us what frequently happens and what is very rare but extremely dangerous.  It's inefficient to treat every risk as a big threat to safety, so we should compromise and sensibly budget our resources of time, space, fuel, and effort or disruption.