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.