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.

Answer 2

"Safest" doesn't mean "safe"

The target has to take the safest route, which doesn't mean only routes that are totally safe. If the safest route is still life-threatening, they'll do it, because that's what fear does. Attempting risky maneuvers to escape from the source of their fear is entirely within the purview of the spell, even if the path is itself terrifying, dangerous, or irrational. The spell overrides normal concerns.

It really almost never comes up, though. It's very rare that the situation prevents all movement except in a damaging or actually suicidal direction.

But "safest" and "nowhere" are up to DM interpretation

At my table, I have ruled in the past that obvious instant death (such as going over the side of a flying airship) is functionally impassible and counts as having "nowhere to move". But most cliffs are at least theoretically climbable, so the target could try to descend that way instead of just leaping to its doom.

However, it has to be instant death. Jumping off a ship in the middle of the ocean is definitely deadly in the long term for most land-dwellers, but it isn't immediately fatal, so a creature under the effects of fear would be willing to jump in the ocean and swim away with no clear destination, at least until the spell ended.

There's a certain amount of judgement call in terms of whether a given target thinks a particular action is instantly deadly or not, though, and thus whether or not it's a valid path. A high level barbarian might look at a five-story drop and think, "I can totally tank that" where a low level sorcerer might well consider that same drop to be instant death (unless he happens to know featherfall). I don't think it's useful to try to create a list of what does or doesn't count as 'instantly deadly' ahead of time, because it's very difficult to predict all the options that will be available to any given character in any given case.