## Distribution and Modal Split Functions

### Technical Note #29

January 2018

Since version 8.2 of Aimsun, the Distribution and Modal Split steps have been separated into two different processes, each with their own Scenario and Experiment. Each of these two steps needs its own set of two types of function:

Figure 1 – Distribution functions and Modal Split functions

#### Distribution

The Distribution process implements a gravity model, where you need to specify (a function to model) the resistance for making a trip between each particular OD pair, given the (combined skim costs or) impedance for that OD pair.

– Distribution Deterrence functions model the reluctance to make a trip given its impedance. You select the Deterrence function in the Distribution Experiment, so if you want to set up different discrete choice functions, such as per trip purpose, you will need to define a distribution scenario and experiment per purpose, changing the distribution deterrence function that you want to use each time.

Figure 2 – Distribution experiment editor

An example of Distribution Deterrence function could be a Gamma function:

Figure 3 – Example of Distribution Deterrence function

– Distribution Impedance functions give the formula that combines the different available costs (distance, travel time, price, etc.) to take into account.

Figure 4 – Distribution Area editor

An example of Distribution Impedance function is a linear function that combines distance and travel time:

Figure 5 – Example of Distribution Impedance function

#### Modal Split

The Modal Split step calculates, for each OD pair, the probability of use for each transportation mode available, based on the utilities per transportation mode.

– Modal Split Discrete Choice functions calculate the probabilities within the available transportation modes. You select the Discrete Choice function in the Modal Split Experiment, so if you want to set up a different discrete choice function, e.g., per trip purpose, you will need to define a modal split scenario and experiment per purpose, changing the discrete choice function that you want to use each time.

Figure 6 – Modal Split experiment editor

An example of a Discrete Choice function is the Multinomial Logit, where probabilities are calculated according to the utilities, following the formula:

Figure 7 – Example of Modal Split Discrete Choice function

– Modal Split Utility functions represent the utility of each transportation mode according to the values of different variables, usually in linear form.

Figure 8 – Modal Split Area editor

Utility functions are usually linear functions depending on different cost variables (xi):

Figure 9 – Example of Modal Split Utility function

### Ajuste estático OD: el impacto de la función de ponderación e iteraciones de descenso de gradiente

Febrero 2019: Mohammad Saifuzzaman explora el uso de funciones de ponderación de ajuste para mejorar el GEH y no solo el R2.

### Cómo definir diferentes costes por propósito de viaje en los modelos estáticos

Enero de 2021: En la Nota Técnica nº 53, dimos algunos consejos sobre cómo modificar el coste de la asignación estática en función del tipo de vehículo. En el consejo profesional de este mes, Marga Delgado explica cómo hacerlo por propósito de viaje.

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### Citar Aimsun Next

Aimsun Next 20

Aimsun (2021). Aimsun Next 20 User's Manual, Aimsun Next Version 20.0.3, Barcelona, Spain. Accessed on: May. 1, 2021. [In software].
Available: qthelp://aimsun.com.aimsun.20.0/doc/UsersManual/Intro.html

Aimsun Next 8.4

Aimsun (2021). Aimsun Next 8.4 User's Manual, Aimsun Next Version 8.4.4, Barcelona, Spain. Accessed on: May. 1, 2021. [In software]. Available: qthelp://aimsun.com.aimsun.8.4/doc/UsersManual/Intro.html

Aimsun Next 20

@manual {​​​​​​​​AimsunManual,

title = {​​​​​​​​Aimsun Next 20 User's Manual}​​​​​​​​,

author = {​​​​​​​​Aimsun}​​​​​​​​,

edition = {​​​​​​​​​​​​​​​Aimsun Next 20.0.3}​​​​​​​​​​​​​​​,

year = {​​​​​​​​​​​​​​​2021. [In software]}​​​​​​​​​​​​​​​,

month = {​​​​​​​​​​​​​​​Accessed on: Month, Day, Year}​​​​​​​​​​​​​​​,

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Aimsun Next 8.4

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title = {​​​​​​​​Aimsun Next 8.4 User's Manual}​​​​​​​​,

author = {​​​​​​​​Aimsun}​​​​​​​​,

edition = {​​​​​​​​​​​​​​​Aimsun Next 8.4.4}​​​​​​​​​​​​​​​,

year = {​​​​​​​​​​​​​​​2021. [In software]}​​​​​​​​​​​​​​​,

month = {​​​​​​​​​​​​​​​Accessed on: Month, Day, Year}​​​​​​​​​​​​​​​,

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Aimsun Next 20

TY - COMP

T1 - Aimsun Next 20 User's Manual

A1 - Aimsun

ET - Aimsun Next Version 20.0.3

Y1 - 2021

Y2 - Accessed on: Month, Day, Year

CY - Barcelona, Spain

PB - Aimsun

UR - [In software]. Available: qthelp://aimsun.com.aimsun.20.0/doc/UsersManual/Intro.html

Aimsun Next 8.4

TY - COMP

T1 - Aimsun Next 8.4 User's Manual

A1 - Aimsun

ET - Aimsun Next Version 8.4.4

Y1 - 2021

Y2 - Accessed on: Month, Day, Year

CY - Barcelona, Spain

PB - Aimsun

UR - [In software]. Available: qthelp://aimsun.com.aimsun.8.4/doc/UsersManual/Intro.html