Vehicles carrying inflammable materials usually have metallic ropes touching the ground during motion. Why?
Due to air friction, the body of vehicle also gets charged in addition to the charge developed on tyres.
In case, the charge accumulation becomes large, it may prove hazardous to the vehicle carrying inflammable materials. So that, it does not happen, metallic ropes from the vehicle are made to touch the ground. Due to this, the charge on the vehicle leaks to the earth.