Why do petrol tankers which run on the highways have metal chains attached that drag along the road?

The chain is supposed to get rid of any static electricity that builds up within the vehicle by directing it from the frame and tanks and into the ground or road. Preventing the buildup of static electricity will prevent the possibility of a spark occurring if somebody happens to touch the unit. We all know that gasoline is highly flammable, and its vapors are explosive, and the spark could provide an ignition source.