Multilayer Volume Plots in Module 6.12

Traditionally, three categories of auto volumes have been used in EMME/2: auto volumes, additional volumes and total volumes, defined as the sum of auto and additional volumes. As the continuous developments rendered the auto assignment within EMME/2 more powerful with every new release, today these three categories are no longer really sufficient, and the following needs have become apparent:

• The additional volumes, originally limited to fixed predefined volumes, are now mostly used to store the results of an additional options assignment. As such, they are not necessarily additive to the auto volumes, but often contain a selected part of the latter. In these cases, the total volumes do not represent anything meaningful, whereas the difference of auto volumes minus additional volumes would be useful, since it represents the complementary volumes, i.e. the trips not retained in the additional options assignment.
• With the introduction of the multiclass assignment, and in particular with the new feature that allows storing the class specific volumes directly during the assignment, tools are also needed to display these volumes.
• For auto volumes that are composed of different components (such as multiple user classes, or selected vs. non-selected trips in an additional options assignment), the graphic display should not be limited to showing one set of volumes at a time, but should support the display of multiple volume layers on the same plot.

The above needs are addressed in Release 8 by providing a new and greatly enhanced version of module 6.12. In this new version, the old option ``3= total volumes on auto network'' has been replaced by the new option ``3= composed volumes on auto network''. When activated, the user can choose among the following possibilities:

Select: 1= sum of auto and additional volumes
        2= difference auto minus additional volumes
        3= auto volumes and additional volumes
        4= additional volumes and auto volumes
        5= additional volumes as first part of auto volumes
        6= additional volumes as second part of auto volumes
        7= class specific auto volumes

The first selection corresponds exactly to the old total volumes, whereas the second allows displaying the complement of the additional volumes as a standard single-layer volume plot. The remaining selections produce multi-layer volume plots in which each selected volume component is displayed separately, using parallel volume bands of different colors.

The following example shows the results of a multiclass assignment with three classes of users, each associated with a different value of time:

A word of caution: Even though displaying auto volumes in the above way gives a new and very powerful means of visualizing composed volumes, one should not forget that, from a mathematical point of view, the decomposition of the total volumes into its components is not necessarily unique. Thus, other decompositions than the one obtained by the assignment may exist.