Average and instantaneous velocity coincide when the velocity over the interval is:

1. The two agree only under a special condition. 2. Average velocity is displacement over an interval of time. 3. Over a small enough interval, average velocity becomes instantaneous velocity. 4. If the velocity never changes, every interval gives the same value. 5. So the two coincide when velocity is constant. _Source: OpenStax College Physics for AP Courses 2e (CC BY 4.0), Ch 2 'Kinematics', sections 2.1-2.8_