For any electrical engineers in the crowd, if we derive a formula for averaging speeds, it would look alot like the formula for calculating parallel resistance.


Say the total distance each way is d, and the speeds are s1 and s2 for times t1 and t2. Then:


avg speed = 2d/(t1+t2)


= 2d/(d/v1 + d/v2) = 2/(1/v1 + 1/v2)


= 2v1v2/(v1+v2)


Same as parallel resistance except for the 2.