When walking back to place the warning triangle sign behind a disabled vehicle on an expressway, what is the safest pedestrian procedure?

1. Walk backward in the middle of the left overtaking lane
2. Walk along the outside of the right-side steel guardrail (护栏外侧), facing oncoming traffic, and step onto the shoulder only to position the triangle (面朝来车方向，沿护栏外侧行走)
3. Run down the center lane waving arms
4. Walk with back turned to incoming traffic while looking at a smartphone