What do those red UPS safety flags on the back of the trucks mean?
I personally have never seen any ‘flags’ on the backs of the trucks. If you mean the little squares set on their corner that have different colors, those are used to show that the truck is carrying hazardous materials. The HazMats can be classified as flammable, explosive, corrosive and a few other categories. If one of those vehicles is involved in an accident, the local fire dept. responds with a HazMat crew to neutralize any potential threat to people or the environment.
