What is the primary function of Traction Control / Anti-Slip Regulation (ASR / Antriebsschlupfregelung) on a commercial bus?

1. To prevent driven wheels from spinning during startup and acceleration on slippery road surfaces
2. To automatically steer the bus back into its lane if the driver drifts over road markings
3. To automatically apply the service brakes whenever a passenger presses the stop button
4. To prevent the passenger doors from opening while driving