When programming railroad preemption, what is the 'advance preemption time'?

1. The time from when the controller receives the preemption call to when it begins the preemption sequence
2. The time provided by the railroad warning system before the gates descend, allowing the signal to clear traffic from the tracks
3. The time added to the walk interval for pedestrians near the railroad crossing
4. The time the signal remains in flash after the train passes