New physics concept I developed while in the streets of Varanasi, India. Kinks have not yet been worked out.

FLY SPEED: The velocity above which flies cannot follow a walking person. This is strongly affected by immediate environmental airflow. Thus, as the amount of airflow in a given walking area increases fly speed can move down from two steps per one second all the way to zero steps per second (at a certain, not yet measured amount of airflow).

Lets define the variables:

Fly = F
Wind Speed = P
Walking Speed = W

W >= P + F

Simple case no wind

Therefore P = 0

W= F

If wind is in the face of the walker then

W >= -P + F

If the wind is at the back of the walker then it is in the favor of the fly

W >= P + F