Average velocity is equal to the instantaneous velocity when acceleration is zero. In order for acceleration of an object to equal zero, there can be no change in speed or direction. For example, when a car is traveling down a straight road on flat land using cruise control.

Average velocity = the distance traveled divided by the amount of time it took to travel that distance.

Instantaneous velocity = the velocity at a particular instant of time.

average velocity = instantaneous velocity

when the acceleration = 0

if the car is speeding up or down, its instantaneous velocity changes. The average velocity is always one number. The conclusion that the instantaneous velocity must be the same throughout. If the instantaneous velocity is same throughout, than there is no acceleration , which means no change in speed AND direction. (Remember velocity is a vector, not scalar)