What constitutes a 'safe route' for the Fear spell?

Question

The description of the fear spell says:

While frightened by this spell, a creature must take the Dash action and move away from you by the safest available route on each of its turns, unless there is nowhere to move.

Imagine these scenarios, which all happen in a narrow corridor with the caster in front of you:

1. Behind you is a deep pit of 10 feet wide. You could jump, but if you fail your Athletics check, you'll fall into the pit and die.
2. Same as 1, except you won't die but take a lot of damage.
3. Same as 1, except you have a fear of heights.
4. Behind you is a flaming corridor, which deals damage for every 5 feet you move.

Those represent uncertain situations where it might or might not be safe to cross the escape route. The fourth scenario represents a certain situation where you will take damage, but it won't necessarily kill you, if you try to escape using that route.

What is the threshold of 'safe' in the fear spell description? Must the victim run through the route even if there is certain danger? What if the danger is highly probable, but not certain? Can those routes be considered the "safest available route"?

Answer 1

The victim has to take the "safest available", even if it is not "safe"

In all the circumstances you describe, none of the routes are safe but they are each the "safest available". If the victim has a choice between 2 or more unsafe routes I would let the victim use their own judgement as to which was the "safest available".

Only if there is "nowhere to move" - i.e. every possible route brings the victim closer to "you" - can the victim stand still.