The average human walking speed is approximately 3 miles per hour, but several answers are possible based on the inclusion of other factors. Individual variance makes the average human running speed impossible to specify, but elite marathon runners sustain a pace of over 12 miles per hour.
Humans face a significant speed disadvantage relative to quadrupeds. Why exactly humans evolved to stand upright has not been satisfactorily explained. The use of tool seems to be the obvious answer, but bipedal hominins emerged long before the age of the oldest recovered stone artifacts. Other possible evolutionary advantages of bipedalism include the opportunity to forage while wading and the ability to see over the tall grasslands of the savannah.