Why is a metallic chain of a petrol supply tanker made to roll on the road?

A metallic chain on a petrol supply tanker is designed to roll on the road to ensure it remains grounded, preventing the buildup of static electricity. As fuel is transferred, static charges can accumulate due to the movement of fuel within the tanker and friction with the air. By grounding the tanker, the chain helps dissipate any static charge safely, reducing the risk of sparks that could ignite flammable vapors. This safety measure is crucial in preventing fire hazards during fuel delivery.