Show that inertial frame moves with constant velocity related yo each other

Assume 2 bodies moving with velocities ‘V’ and ‘U’ opposite to each other (for example a car and bus on a 2 lane road). According to the law of inertia, both the drivers see themselves at rest. Now let there be some nonzero distance between the two. 

 

The distance covered by the car in ‘S’ seconds is (V*S) and the distance covered by the bus is (U*S). We know that the bus driver feels himself to be at rest, so he sees the car come closer by V*S + U*S distance = (V+U)*S. Hence he sees the car driving at a velocity of V+U towards himself. By similar reasoning, you will also be able to deduce that the bus driver is moving with velocity V+U according to the car driver, towards the car. Thus we can say that 2 inertial frames move with constant velocity with respect to each other.