This reminds me of the trouble they've had upgrading Air Traffic Control systems.

I grew up in Manhattan and rode the subway constantly. I remember sometimes taking the front car, looking out the window and wondering what the odds were of a collision. Then a friend explained how the red light control system made it physically impossible for one train to crash into another.

Ha! Glad I didn't know the truth back then, I might have developed a phobia about using the subway.