  The presented explanations are provided for the one--dimensional diffusion
process with constant drift by using forward Fokker--Planck technique. We are
interested in the outflow probability in a finite interval, i.e. first passage
time probability density distribution taking into account reflecting boundary
on left hand side and absorbing border on right hand side. This quantity is
calculated from balance equation which follows from conservation of
probability. At first, the initial--boundary--value problem is solved
analytically in terms of eigenfunction expansion which relates to
Sturm--Liouville analysis. The results are obtained for all possible values of
drift (positive, zero, negative). As application we get the cumulative
breakdown probability which is used in theory of traffic flow.
