According to Article 81 of the PRC Road Traffic Safety Law Implementation Regulations, what are the speed and following distance rules when visibility on an expressway is less than 50 meters due to dense fog?

1. Turn on fog lights, low beams, clearance lamps, and hazard lights; limit speed to <= 20 km/h, and exit the expressway at the nearest ramp immediately
2. Maintain 60 km/h in the fast lane and follow the taillights of the leading truck at 10 meters distance
3. Stop immediately in the middle travel lane, turn off all lights, and wait for the fog to dissipate
4. Activate high beam headlights and drive at 50 km/h on the emergency shoulder