The highlight of the latest version of Aimsun is, without a doubt, the full support that Aimsun 8 now offers for the four-step transportation planning process. This addition makes Aimsun the first and only software on the market that supports four-step travel demand modeling and dynamic traffic assignment as well as microscopic, mesoscopic and hybrid simulation in a single software application!

Full four-step travel demand modelling is only available in Aimsun 8 Expert edition.

Click here for further information about the new line-up of Aimsun editions.

To try a free trial version of Aimsun 8 Expert, write to info@aimsun.com.

Four-step demand modelling is an important functionality for Aimsun software and another step towards true integration. You can now start a transportation modelling project from scratch, entering raw geographical and socio-economic data and follow the process all the way through to simulation without recourse to a separate package.

With Aimsun you no longer have to interface with an external traffic demand model for importing data as a starting point or as a means of redistributing demand from one mode to another or to estimate the impact that capacity changes have on overall demand levels.

Being able to use a single software application for an entire project means vastly increased cost-efficiency, consistency and quality for Aimsun users.

A Four-step Model Experiment, contained in a Four-step Model Scenario, can be used to manage the whole Four-step Model process. The diagram relates the processes, displays the workflow and can be used to execute everything at once.

You can define the input data and which experiments you want to execute, each of which relates to a box in the diagram. You can choose to execute the processes up to a certain point in the diagram, or a single box (provided all the previous necessary steps have been already executed).

The main steps in this model are:

- Trip Generation
- Trip Distribution
- Modal Split (which in Aimsun 8 is simultaneous with Trip Distribution)
- Traffic Assignment

In order to manage all the data these new features require we have added new objects to Aimsun, including Transportation Modes, Time Periods, Vectors, Areas, Generation/Attraction and Distribution Data Sets, Public Transport Sections and Stations, Distribution + Modal Split and Public Transport functions.

Aimsun Expert also introduces a number of new visualisation modes and outputs that are standard requirements of the traffic demand modelling community. Those include select link analysis, generation-attraction plots, public transport assignment loads, and more.

- Improved assignments results for user-defined components
- More efficient storage of the paths information

Apart from the already available Frank & Wolfe method for the Equilibrium Traffic Assignment, we have introduced:

- All-or-Nothing Assignment: this method looks for the shortest paths in free flow and returns free flow costs and paths
- Incremental Assignment: in this kind of assignment, it is the user who fixes the % of trips that will be loaded to the new calculated shortest paths and discharged from the rest of the previous paths at each iteration
- MSA Assignment: this method is similar to Frank & Wolfe except that the step lambda is not optimised but fixed to 1/n where n is the iteration number
- Stochastic Assignment: this method is recommended for bicycle assignment. It takes into account the flows obtained in another macro experiment, calculates the k-shortest paths and assigns demand based on a utility function

Users can now create their own list of key words when defining a subfunction and they will generate new outputs when the corresponding subfunctions are defined in the used functions. Users can define derived components e.g., if you create a component TravelTime and a component Distance, you can define another component Speed as a derived component Distance/TravelTime and there is no need to declare it in the function when the first two are already there.

- A new object is available in Project->New->Demand Data->Transportation Mode. A vehicle type belongs to one Transportation Mode.
- A new object is available in Project->New->Demand Data->Time Period.
- A new object is available in Project->New->Demand Data->Traffic Profile. It contains the percentages for each interval during a time period. This information will be used by the Apply Profiles operation to generate a set of O/D Matrices from a global matrix.

In addition to O/D Matrices, now vectors are also supported in a Centroid Configuration. We have also defined a new container for Generation/Attraction outputs: the Generation/Attraction vector, which consists of two columns, one for Generation Values and one for Attraction Values.

An O/D matrix has a specific cost distribution based on any of its corresponding skim matrices, e.g., the O/D distances matrix. After an adjustment, this distribution might have been distorted, and it can be recovered by executing a Redistribute operation using the following as input: the matrix to be redistributed; its current corresponding skim matrix; the original demand matrix; the original skim matrix (these last two giving the cost distribution that you want to recover).

This operation will apply the same (multiplicative) changes to the current matrix that were made from an original matrix to a manipulated matrix, such as an adjusted matrix, up to a maximum correction factor.

This operation will apply the profiles selected per cell, obtaining O/D matrices for each interval in the profile and with values multiplied by the percentage stated in the profile.

A new View Style is available for Diagrams (with several options to choose from: Circles, Pie, Histogram or Stacked Histogram). You can use it, for example, to visualise and compare the Generation/Attraction values with a histogram for each centroid.

You can calculate a Control Plan (fixed or actuated) based on the results of a static traffic assignment for one or several nodes. You can also calculate the control plan on a single intersection based on flows that you enter manually.