Blvn Lakshman

Rate of change of displacement is called velocity means change of displacement with respect to time v=s/t why dont we spell change of time with respect to displacement I.e., v=t/s Here both displacement and time are

Well I guess that the person who explained this concept for the first time would have taken displacement factor as his interest rather than considering time and also there should be a standard to be followed all over the world. This is according to my point of view , i don't know whether others have solution for your question, let's see if i am wrong.

The fact is that time is relative and depends on the speed.
We do not know whether the road is constant - it is only an assumption.
However, for practical reasons is most conveniently define the velocity as a change in the way in a specified time.
Guided by the principle of three significant digits difference the passage of time at speeds of Earth (in a vehicle or an aircraft) is negligible.
Everything is a matter of convention - for example, we can represent the number of ternary system - but what? 😀