Under national standard GB 11567-2017, what is the primary life-saving function of Side and Rear Underrun Protection Devices (侧面及后下部防护装置) installed on heavy trucks?

1. Prevent passenger cars, motorcyclists, and cyclists from sliding or wedging underneath the truck chassis in rear-end or side-impact collisions
2. Serve as exterior cargo racks to carry spare tires and luggage outside the vehicle
3. Act as mechanical plow blades to clear snow and gravel from highway lanes
4. Reinforce the truck chassis so the truck can pull three additional trailers