What is relative speed between two objects?

The difference in speeds is known as the relative speed.

The other driver's relative speed is 20 mph from my perspective if I am traveling at 60 mph and the car next to me is traveling at 80 mph. In other words, I am essentially not moving and the other car appears to be moving at 20 mph.

If I were the other driver, the first car appears to be moving backward at a speed of 20 mph in relation to me; therefore, my relative speed is -20 mph in relation to the other driver.

Technically, you can be relatively stationary (as I feel in my own car) or relatively moving (as a 60 mph car relative the ground), but all speeds are relative because "relative" simply means that we are moving in relation to someone in a different perspective.

The relative speed between two objects is the speed at which one object appears to be moving with respect to the other object. It is calculated by subtracting the velocity of one object from the velocity of the other object.