Fear description 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, where all happen in a narrow corridor and the caster is at front of you:
Behind you is a deep pit of 10 ft wide. You could jump, but if you fail your Athletic roll you fall to the pit and die.
Same as 1, but you won’t die but take a lot of damage.
Same as 1, but you have fear of height.
Behind you is a flaming corridor, which deals damage for every 5 ft 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 not necessarily kills you, if you try to escape using that route.
What is the treshold of ‘safe’ in the 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? Do those routes considered available, but safest?