The standard EMME/2 auto assignment is based on the equilibration of travel times, which are obtained as results from the volume-delay and turn penalty functions. Since these functions are freely definable by the user, it was always possible to formulate them to include other cost factors, such as distance or road tolls. Using this type of work-around to implement a generalized cost function has the disadvantage that the time related assignment results no longer correspond to the true travel times and speeds, but are also expressed in generalized cost equivalents. Furthermore, since in a multiclass assignment all classes share the same volume-delay function, it is not possible to use this method in situations where the generalized cost function is class specific.
With the introduction of the generalized cost auto assignment
in Release 7, there is no longer any need to recode volume-delay
functions for using generalized costs. An arbitrary fixed cost term
can now be added to the travel times during the assignment. This way, the
route choice is based on the sum of travel times and fixed costs, while
all time related assignment results (such as link times, total
vehicle-hours, speeds, etc) are kept in their native units. The fixed
cost term is defined by the user during the assignment preparation
as a link attribute (chosen from length, ul1, ul2,
ul3 or any extra link attribute @xxxxx) and the
corresponding weight coefficient. In a multiclass assignment these fixed
costs are class specific, so for each class a different link attribute
and/or coefficient can be specified (in addition to the class specific
auxiliary auto mode). This allows, e.g., the implementation
of a multiclass assignment in which each class is charged a different
toll, or perceives the fuel cost (distance!) or tolls according to a
different value of time.
From an implementational point of view, the generalized cost assignment
is activated by the modified single class/multiclass select question,
which now reads
Select: 1= single class assignment on auto mode
2= single class assignment with generalized cost
3= multiclass assignment
4= multiclass assignment with generalized cost
Note that the most often used selections 1 and 3 are unchanged from the
previous release, so that in most cases this change will not affect macros
at all.