Assume that the flow at the level of the trachea is 500 milliliter per second, and that tracheal diameter is 2 cm.
What is the gas speed in the trachea?
The surface area, according to the graph, is about 12 cm², the volume flow is 500 cm³ per second. Hence the velocity is 500/12, or about 40 cm/s. This is about 1/4 times the velocity at the level of the trachea.