I can recall seeing gasoline tractor trailers dragging chains on the ground to avoid risk of building up a static charge that could lead to an explosion by static discharge. But, I’d say I haven’t seen this since the 1980s, and I really did not see it too much even then. So, what has changed over the decades? Is the truck grounded by some less obvious means?
Carbon black in the tires to make them conductive, and grounded filler nozzles.
First thing a tanker driver does when loading or unloading, is to earth (ground) his truck. A filling station will have specific places for this.
Fuel flowing through the pipes will create a static charge and that will eventually cause a spark without earthing. You might also notice that all the couplings are made of brass te avoid any sparks from metal/metal contact.
Dragging chains create sparks, that’s not something you want from something carrying a bunch highly combustible liquids. They use anti static strapsto dissipate static electricity. Years ago I worked for a company that delivered fuel oil to homes and businesses, one of my jobs was checking the straps on the trucks and replacing them if necessary. This was done weekly.
I agree that dragging chains create sparks. But back in the 1960s every gasoline tanker was dragging a chain or chains down the road. I agree with the OP that I haven’t seen such grounding-while-driving installations on tankers for many, many years.
Although I did recently (last 18 months) see a modern big cubical paramedic van which had about 6 grounding chains dragging from both sides of the rear axle.