and a volume dependent congestion function 
in the form
The congestion function can be any non-decreasing function with d(0)=0. It models the discomfort of traveling on a segment at a volume 
. Since the function 
is specified as a network calculator expression, it can access any other attribute of the transit line as well, such as: headway, seated and total vehicle capacity, user attributes. By default, BPR-type and conical congestion functions are provided (Spiess, 1990), but the macro allows easy integration of other functional forms that might be required for particular applications.
The fixed travel costs are, as usual, coded directly into the transit time functions. In order to enable the transit time functions to reflect congestion costs, all transit time functions have to be multiplied with the term *(1+US1). During the assignment steps, the user defined segment attribute US1 will contain the value of the congestion function .
In terms of the so defined congestion function , the objective function of
