What is the 'Starting Inhibit' or Door Interlock (Haltestellenbremse / Anfahrsperre) on transit buses?

1. A device that prevents the bus from starting in the morning if cold
2. A safety system that automatically holds the service brakes applied and cuts accelerator throttle while any passenger door is open or the wheelchair ramp is extended, preventing drive-away accidents
3. A speed limiter that locks the bus to 10 km/h permanently
4. An ignition lock linked to passenger ticket validation