When the plane reaches V1, pilots take the hand off the throttle: they're committed to takeoff, even if an engine fails. It is better to take off and fix the problem or land again, than to smash into whatever is beyond the end of the runway.
Surely uncertainty about the situation contributes to defaulting to committing, but what if it's a passenger plane and at V1 pilots know they've lost power? Wouldn't veering into highway at 30 mph be weighted against certain, big loss of life?
Edit: I now see that this has been partially answered by uncle comment
The margin is for example that the plane must not just be able to fly, but also reach a minimum climb gradient to clear obstacles with a bit of safety margin. There is also an allowance for the time it takes from calling abort to actually hitting the brakes. And for example headwind is part of the calculation (it makes the takeoff distance shorter) but only 50% of the headwind is used in the calculations.
But all of those margins are not for the crew to use, the crew must just execute the procedure exactly as trained which means at V1 you're committed to continue the takeoff. And before V1 in case of an engine failure you have to hit the brakes to make sure you can stop before the end of the runway.
Between V1, Rotate and V2, there’s like a 2-3kts difference (between each of them).
I am not familiar what the procedure is if you have dual-engine failure at or above V1.
Captains can make the decision to abort the takeoff in the case of absolute power loss or for 'failure to fly' (where the aircraft is clearly not going to fly, e.g. the elevator/pitch controls aren't responding). But the training is adamant: if you're uncertain what has happened after V1 you try to fly the plane away from the runway.
That's what I'm getting at. I want to abort unsafely. Imagine 400 meters of grass field after the end of the runway, and a water body. I'm asking wether such factors are accounted for, or if plane on ground beyond runway does-not-compute.
That "extra" 400m of grass? That's for all the other things that can still go wrong even when you follow procedure. e.g., you're below V1 so you abort takeoff, close throttles and hit the brakes. You should be able to safely stop on the runway.
But now your brakes fail because maybe the reason you had to abort was a fire that also managed to burn through your brake lines, or it started to rain just as you were taking off, or...
Now that's where the 400m of safety margin comes in to save your ass (hopefully). It's "extra", you don't plan on using it.
V1 for this plane in those conditions is nearly 200 mph. Even if they shut down all engines and applied full brakes (and assuming the brakes/tires didn't catch fire), they'd still run off the end of the runway with enough kinetic energy to kill themselves and anyone else in the way.
I expect pilots are trained explicitly not to do that.
If you can't abort safely, than it follows that the safer course of action is to try to fly. I'm sure there are exceptions to that, but a pilot has barely seconds in which to decide if any of those exceptions apply, so they're not going to abandon procedure unless the situation is clear.