I often see trucks (school buses, ambulances, sometimes dump trucks) which have metal chains hanging off the rear axle, on either side of the differential. What are these for?
Theoretically, they discharge static electricity, so you don't get a shock when opening the doors, etc.
Whether they really work, I'm not so sure.
Vehicles where static electricity is a real issue (for example fuel tankers) use much better-engineered solutions to avoid sparks causing fires and/or explosions.
Ambulances are an interesting case - they have a lot of static-sensitive electrical equipment inside (e.g. EEG monitors) but I don't recall ever seeing an ambulance with anti-static chains in the UK.
I think you may be seeing these automatic snow chains - https://m.youtube.com/watch?v=LxOh8Qav5tY