Brilliant! Motion is always relative. Whenever you say that an object is moving, you must also mention with respect to (wrt) what.
When we say that a car is moving at \(30 \, \mathrm{km/h}\), we mean that it is moving at \(30 \, \mathrm{km/h}\) wrt the Earth.

Look at this animation. In one case, two cars are moving at \(30 \, \mathrm{km/h}\) and \(40 \, \mathrm{km/h}\) in the same direction. In another case, the cars are moving with the same speed in opposite directions.
In what case is the car accident more damaging?
  • Case A
  • Case B
  • Both cases are equally damaging