For calculation purposes, two types of trip form are distinguished:
- outward and return trip (PA)
- path (OD)
Accordingly, the matrix entry 1 for PA equals two trips in the results matrix - one from the origin zone to the destination zone and one for the trip back to the origin zone.
In a calculation with the trip form OD, the matrix entry corresponds to the number of paths.
A special case arises when both the outward and return path are considered and you can choose a time of day in the model structure. The outward and return path can be taken during different times of day, i.e. the number of possible alternatives actually consists of different time pairs. It is assumed that the return path is taken during the same or a later time of day than the outward path. You thus have to take the sequence into account when defining the time of day. Table 41 shows possible alternatives when specifying three times of day.
|
|
|
Return path |
||
|
Outward path |
|
AM |
IP |
PM |
|
AM |
X |
X |
X |
|
|
IP |
- |
X |
X |
|
|
PM |
- |
- |
X |
|
Table 41: Examples of possible alternatives (x) with three given times of day, in the sequence AM, IP, PM
Utilities are always specified per time of day. The utilities of an alternative with a time-of-day pair are calculated based on the input data while the procedure is executed. In this case, the utilities result from the utilities of the time of day of the outward path, plus the transposed utility of the time of day of the return path.
Example OD vs PA:
If we accept the model structure of Image 52, with three alternatives for each mode choice and time of day choice, nine utility matrices (3 modes*3 times of day) are required for recalculation of the demand (“absolute”). As a result, nine demand matrices are calculated for single path trips. When both the outward and return path (trip form PA) are into account, the number of demand matrices increases to 18 (3 modes*6 time of day combinations), as there are six possible combinations of time-of-day pairs.
