The number that decides whether a track is fair
Redline builds its tunnel just ahead of your front wheel. Most of the work in that code isn't making interesting terrain. It's stopping it from killing you in ways you couldn't avoid.
In Redline you ride a motorbike through a mine. You never steer; you only control the throttle, and the tunnel is generated as you go. Every obstacle is a promise that it can be beaten if you ride well. Break that promise once and players stop trusting the game, which is much worse than the game being hard. Two numbers keep the promise, and we found both by getting killed by our own game.
Sixteen tiles
A full-speed jump covers about 100 pixels, but it isn't even: you take off about 42 pixels before an obstacle and land about 58 after it. If two obstacles are too close, you land right on the second one, and there's nothing you could have done. So the generator never puts two obstacles closer than 16 tiles (128 pixels), however hard the game gets. Everything else about the difficulty can be tuned by feel. This one is maths.
Four hundred and eighty pixels
When you crash, you start again from a standstill. At first we put you back just before whatever killed you. But the widest pits need top gear to clear, and a few metres of road isn't enough to get there. So you'd hit the pit too slowly, crash, restart in the same spot, and crash again until your lives ran out. Now you restart at least 480 pixels back, which is exactly how much road you need to reach top gear. It feels further back than it should, and that's the point.
The bat
The bat is the one obstacle you beat by slowing down. It swoops up and down across the tunnel, so sometimes the way is blocked. You see it 184 pixels away and need 116 to stop at full speed, which leaves a little time to decide. To keep it fair, the bat only blocks the tunnel for about a third of its swing, and it moves slowly enough that you can tell whether it's going up or down.
Make it, then check it
The general lesson: let the generator do what it likes, then check the result is actually playable. Trying to write a generator that never makes a mistake is hard and makes every level feel the same. Checking afterwards is easy and keeps the variety. We do the same in Castaway, where every island is checked to make sure it can be finished before the storm.