Calculating demand with the Nested demand model

The Nested demand calculation is a variant of the standard 4-step model to calculate demand. The calculation is based on a Nested Logit model which presupposes the specification of choice model structures for each demand stratum. A choice model structure may be a nested mode choice but also a nested model with destination choice, mode choice, and choice of the time-of-day alternative in general (Fundamentals: Nested demand model).

1.  On the toolbar, click the  Open 'Procedure sequence' window button.

The Procedure sequence window opens.

2.  To the procedure sequence, at the position of your choice, add the procedure Nested demand of the Demand model category (Setting up and starting the procedure sequence).

3.  In the row of the added procedure, click on the button in the Reference object(s) column.

The Demand strata selection window opens.

4.  In the Select demand model list, click the demand model the demand strata belong to.

5.  In the Demand stratum selection list, select the demand strata you would like to calculate the nested demand for.

Tip: To select multiple demand strata, hold down the Ctrl while clicking the desired entries one by one.

6.  Confirm with OK.

The demand strata selected are listed in the Reference object(s) column.

7.  Make sure the added procedure is highlighted. Then, in the Operations section, click the Edit button.

The Parameters: Nested demand window opens.

8.  Make the desired general settings for all demand strata.

Element

Description

For active OD pairs only

If this option has been selected, only those OD pairs which are in the active state are considered (Exception: Filter for OD pairs).

Exclude OD pairs connecting passive zones

If this option has been selected, only OD pairs between active zones are considered (Using filters to set network objects active or passive and Selecting network object types for the spatial selection mode).

Set any result demand matrix to 0 prior to calculation

If this option has been selected, the result demand matrices are set to zero.

If the option has not been selected, the result demand matrices remain unchanged.

Save old result demand matrices for gap calculation

If this option has been selected, already existing result demand matrices will be saved prior to the calculation. They are required for the procedure Nested demand gap calculation.

Model form

There are two variants to calculate the demand (Fundamentals: Absolute and incremental calculation):

  • absolute

In the absolute calculation, the demand is calculated from scratch. The prerequisite is that attraction and production have been calculated with the Trip generation procedure (Calculating trip generation).

  • incremental

In case of an incremental calculation, cost changes and a basic demand are prerequisite for a calculation of the resulting modified demand.

9.  Make the desired changes on the Choice model structures tab.

If required, you can create a choice model structure for each demand stratum and a maximum of one choice level per choice type for each choice model structure ( Create choice level).

Choice model structures tab

Element

Description

Choice type

Choice type of the choice level

Alternatives

Alternatives valid for the choice level

Save demand

If the option is selected, the original demand for the choice type of the level is saved for all demand strata, which use this model structure.

Save logsums

If the option is selected, the logsums calculated on this level are saved for all demand strata, which use this model structure. In case of a nested mode choice, the logsums of the root node are output.

 Create choice model structure

Opens the Create choice model structure window in which you can enter a code for the choice model structure.

 Copy current choice model structure

Copies the marked choice model structure.

 Edit choice model structure

Opens the Edit choice model structure window in which you can edit the code of the choice model structure.

 Delete current choice model structure

Deletes the marked choice model structure.

You can create a maximum of one choice level per choice type for each choice model structure. Use the icons to create or delete a new choice level.

Section Choice levels

Element

Description

Create choice level

The Create choice level window opens. Each type of the choice level can be inserted only once into a choice level.

 Delete marked choice levels

Deletes the marked choice levels from the choice model structure.

 Move marked choice levels upwards

Moves the marked choice levels one row up in the list.

 Move marked choice levels downwards

Moves the marked choice levels one row down in the list.

10.  Click the  Create choice level icon.

11.  In the drop-down list, select the desired choice type.

12.  Confirm with OK.

13.  In the row of the desired choice level Time-of-day choice or Mode choice, click in the icon in column Alternatives.

14.  Make the desired changes:

  • Edit time-of-day alternatives

Time-of-day alternatives must be sorted in ascending order of time.

Icon

Name

Description

Create time-of-day alternative

Adds a new time-of-day alternative TimeOfDay with an ascending number to the list. You can edit the Code. As a rule, the time-of-day alternatives AM (morning peak, IP (inter-peak) and PM (afternoon peak) are used. The table is sorted from the earliest time-of-day alternative (top) to the latest time-of-day alternative (bottom).

Delete marked time-of-day alternatives

Deletes the marked time-of-day alternatives from the list.

Move marked time-of-day alternatives upwards

Moves the marked time-of-day alternatives one row up in the list.

Move marked time-of-day alternatives downwards

Moves the marked time-of-day alternatives one row down in the list.

  • Edit decision tree

How to edit the decision tree for the mode choice is described with procedure Tour-based model - Combined trip distribution / mode choice (Nested Logit model: Edit decision tree). Follow the instructions from step 5.

Note: Cross Nested Logit is not possible during the Nested demand procedure.

15.  Make the desired changes in the Distribution tab.

Distribution tab

Element

Description

Demand stratum

Demand strata set in column Reference object(s) for procedure Nested Demand

Choice model structure code

Selection of the choice model structure code for the demand stratum

Trip form

Selection of the kind of demand:

  • OD (one trip): Single paths are regarded. In the result demand matrices, the entries correspond to the number of paths.
  • PA (two trips): Both directions are regarded. A value of 1 in the result demand matrix corresponds to two paths: one leading from the origin zone to the target zone and one in the opposite direction.

Note

If the OD (one trip) option has been selected, time-of-day alternatives correspond to time periods. If the PA (two trips) option has been selected, time-of-day alternatives correspond to a combination of two time periods: one in which the outward journey takes place and one in which the return journey takes place. The return journey must take place in the same time-of-day period or afterwards. Possible alternatives are determined by the chronological order. They correspond to combinations of two time periods (Fundamentals: Trip form in combination with time of day choice).

First column Input demand matrix

Click the icon to open the Select matrix window, in which you can select an input demand matrix (Selecting matrices).

Note

The matrix is only required if no choice type Destination choice is calculated.

Second column Input demand matrix

Display of the input demand matrix

Attraction attribute

Selection of a zone attribute for the attraction

If no selection has been made, attribute Attraction (<Demand stratum>) will be used.

Note

The attribute is only available if the choice type Destination choice is calculated.

Normalization mode

Scaling of the attractions

Note

The attribute is only available if the choice type Destination choice is calculated and absolute is selected as the model form.

Constraint type

Selection of the constraint type:

  • singly-constrained
  • doubly-constrained

Note

The attribute is only available if the choice type Destination choice is calculated (Fundamentals: Singly or doubly-constrained destination choice).

Group for doubly-constrained distribution

Displays the demand strata that are balanced together. If the value is empty, the demand stratum forms its own group.

Maximum number of iterations

Maximum number of iterations before the procedure is terminated.

Note

The option is only available if the doubly-constrained option has been selected in the Constraint type column.

Tolerance

Termination criterion. The calculation is terminated if the tolerance value or the maximum number of iterations is reached.

Note

The option is only available if the doubly-constrained option has been selected in the Constraint type column.

16.  Make the desired changes on the Utility tab.

Utility tab

Element

Description

Key

Unique value generated by Visum for each combination of Demand stratum and Mode and Time-of-day, where applicable

Demand stratum

Code of the demand stratum

Mode

Code of the mode if a mode choice is calculated

Time-of-day

Code of the time-of-day if a time-of-day choice is calculated

Utility function

Click the icon to open window Utility definition for <key>, in which you can specify the utility function (Fundamentals: Using matrix references for demand calculation).

17.  Make the desired changes in the Output / basic demand tab.

Note: The sections of the tab on basic demand are only displayed in case of an incremental calculation (Fundamentals: Calculation for the incremental model form).

Output / basic demand tab

Element

Description

Key

Unique value generated by Visum for each combination of Demand stratum and Mode and Time-of-day, where applicable

Demand stratum

Code of the demand stratum

Mode

Code of the mode if a mode choice is calculated

Time-of-day

Code of the time-of-day if a time-of-day choice is calculated

First column Result demand matrix

Click the button to open the Select matrix window, in which you can select a demand matrix for zones as result demand matrix (Selecting matrices).

Second column Result demand matrix

Display of the result demand matrix

First column Base demand matrix

Click the button to open the Select matrix window, in which you can select a demand matrix for zones as base demand matrix (Selecting matrices).

Second column Base demand matrix

Display of the matrix selected in the first column

First column Last demand matrix

Use the button to open the Select matrix window, in which you can select a result demand matrix for zones as result demand matrix (Selecting matrices).

Note

The column is only displayed if option Save old result demand matrices for gap calculation has been selected.

Second column Last demand matrix

Display of the last demand matrix

18.  Specify the settings you need on the Alternative-specific constants tab.

The Alternative-specific constants tab is only displayed if you have selected the option absolute under Model form and an absolute calculation is to be carried out. For each choice model structure, a new tab is displayed. On those tabs, you can specify the respective alternative-specific constants (Fundamentals: Alternative-specific constants (ASC)). The definition of the constants is optional.

Alternative-specific constants tab

Element

Description

Key

  • Code of the demand stratum
  • Code of the mode if mode choice is calculated
  • Code of the time-of-day if time-of-day choice is calculated If the option PA (two trips) is selected as Trip from under Distribution, the time-of-day pairs are displayed, for example AM-PM.

<Code of the demand stratum> columns

Use the button to open window Alternative-specific constants, demand stratum <Code of the demand stratum>. Above the destination choice level, specify a zone attribute. From the destination choice level, specify a matrix. Use the arrow keys to move through the rows of the table. A separate column is displayed for each demand stratum.

19.  Make the desired changes on the Scaling parameters (lambda) tab.

For each choice model structure, a new tab is displayed. On those tabs, you can specify the respective scaling parameters (lambda) (Fundamentals: Scaling parameters (λ)). The definition of the constants is optional.

Scaling parameters (lambda) tab

Element

Description

Key

Displays the alternative of the choice type

Choice type

Displays the choice type

<Code of the demand stratum>

Scaling parameter lambda for each node of the decision tree

They must become greater from top to bottom within the choice model structure, i.e. at a node, the scaling parameter must be at least as great as the scaling parameter of the overlying node in the model structure.

20.  Execute the procedure (Executing procedures in the procedure sequence).

The distribution of the demand is calculated using Nested demand. The calculation results are saved in the result demand matrices of the zones.

Note: You can display the results in zone lists or in the network graphics (Displaying data graphically and in lists).