The truck filled with skeletons

Something that's always baffled me about the trailer is the shot of H-K machines flying past a truck in a road filled with skeletons, and one of the skulls rolls off the truck into a puddle, with a hole in it like it was shot. What was the truck full of, illegal immigrants or something? Just people hitchhiking? And why were they all apparently shot? I figured the truck was on the move when the nuclear blast hit and they just all died and decomposed there, but what's with the gunshot wound?