While there are many variables involved, the average human walking speed is about 3.1 mph. Some people are capable of exceeding 5 mph while still maintaining a walking gait, but the natural tendency is to begin running at much beyond 4 mph.
Many factors influence a person's preferred walking speed, including age, physical condition, body type and emotional temperament. Crowds and other natural barriers are also factors, as is the terrain. People tend to walk more slowly when climbing hills and more rapidly when descending. By definition, walking involves an "inverted pendulum" gait, where the body swings over a stiff leg with each step. Unlike with running, at least one leg is always in contact with the ground.