Thomas Shey
Legend
Yes, I consider all those multitudes of factors.
But if anything - if we're being realistic - the enemies have the advantage in most cases. If you're in a dungeon or otherwise the home turf of the opponents, they likely know the layout and can navigate it more quickly than the characters. If the characters decide to run away, they are likely taking a beating by a superior foe - so consider that a wounded party would realistically move slower than enemies who are likely less wounded & had no previous tiring combats that day.
Knowing the turf isn't enough, though. Consider other issues: are the opponents going to assume they aren't being lead into a trap? Lead away so someone else can raid their home? At risk of being overextended because not all of them respond instantly?
I don't think it is particularly realistic to assume that flight is more likely to fail than not, barring some special cases (i.e. when the situation was an ambush in the first place). Maybe it works out, maybe it doesn't, and speed and familiarity with the area are only part of the issue (I don't assume the PCs weren't paying attention on the way in and don't have some kind of a retreat route in mind, even if they don't specifically do the pixel-picking old thing of briefing me on it).
So having the party able to escape by the "skin of their teeth" is like a "close the blast doors" moment from A New Hope. Chewie and Han likely wouldn't have been able to escape the Stormtroopers, if not for good luck. If it happened all the time it would get old and really stretch believability in the film - just like it would in a game.
This assumes the PCs have to retreat often. They shouldn't. I've been gaming for 40 years and I can think of it having been necessary a number of times I can count on the fingers one one hand. Yeah, if they had to retreat often they'd be bound to fail sooner or later; the law of large numbers ensures that.
But the gig is, they don't even try often very much because they perceive GMs have the attitude that its a massive failure state, and if that's the case, they might as well fight to the bitter end. Hard to see how that's in any way a virtue.
Fortunately, we can create our own luck in this game. Throwing down oil or caltrops, casting Web or Fog Cloud, summoning monsters to serve as speed bumps. I think it's fair to ask the group what tricks they have up their sleeves before they intend to flee. If they don't have something sufficient, then don't let them.
The problem is that if you follow it too much, none of that helps; carltrops can be run around (again, this isn't just a dungeon/building issue), fog cloud can be run through in one move (and frankly, if that's sufficient a copse of trees or a hill should do; even if the locals know it, do they know where the PCs went when they went around it?) and it all slows you down to boot since games don't usually let you do that at speed.