What is the primary purpose of the mandatory minimum 50-metre (or 100-metre) safety following distance required for heavy commercial vehicles inside Austrian motorway tunnels?

1. To improve mobile telephone reception for passengers
2. To provide sufficient stopping room in smoke/low visibility and prevent multi-vehicle pileups that could trap emergency responders
3. To allow passenger cars to continuously weave back and forth between lanes
4. To save fuel by reducing aerodynamic drag