Two cars approaching each other with speed 100km/h each. Relative to one of the cars the road moves with the speed of 100km/h (car pulls it toward itself), the other car moves on the moving road, vectors add - relative velocity 200 km/h just the way it should be. Relative to the road two cars move with speed 100km/h, vectors point in opposite directions, add vectors, relative velocity equals 0. Why?? Changing system of coordinates should not matter, they both inertial. Galilean transformation does not work either the second car vector turns into 0. I don’t understand.