On an aerodrome equipped with controllable taxiway stop bars at runway holding points, what visual indication signals permission to proceed past the stop bar?

1. The red stop bar lights are switched OFF (extinguished) by ATC, accompanied by a verbal taxi/crossing clearance
2. The red stop bar lights begin flashing rapidly while remaining illuminated
3. The runway edge lights are dimmed to 50% intensity
4. The stop bar lights change color to solid blue