Ah, okay. I suppose it'll be part of the investigation but I wonder if the RELs were indiciating an unsafe runway which prompted the firetruck to ask or if they always ask for permission. Either way, I think my assesment is still correct: there are a lot of edge cases that neither lights nor humans are going to stop. O'Hare apparently has tunnels/underpasses for ground vehicles to use which seems basically foolproof for avoiding collisions like this.