Assumptions - Average Speed As A Weigthed Average
The applet simulates that the average speed of an object during a time interval is the weighted average of the object's speeds in portions of that time interval.

This is a "theoretical" applet and therefore no physical assumptions are required.

The applet simulates the motion of an object (car) whose speed can be varied with a slider and performs calculations based on the values of the speed maintained by the object and the lenghts of the time intervals during which these speeds are maintained.