Thus a refinement is added using the velocity of the average speed distribution.  
This concept is often illustrated using a circle to represent the effective area, treating all targets as point masses.  This method, known as Serway's approach, suffers from the flaw that the target molecules are at rest when their average velocity is actually high.  Thus a refinement is added using the velocity of the average speed distribution.  Usually signified by a lower case lambda, FTP, identify this concept from physical chemistry that measures the average distance between collisions for a gas molecule.
This concept is often illustrated using a circle to represent the effective area, treating all targets as point masses.  
mean free path