Transport demand arises when a series of activities (home - work - shopping - home) cannot be carried out at the same location and therefore a change of location is necessary.
The travel demand is stored in a matrix, whose columns and rows contain all zones contained in the traffic model.
- A PrT demand matrix element has the unit car trips, a PuT demand matrix element has the unit passenger trips (do not mistake with the vehicle journey of a PuT line). It contains the number of travel demand from a traffic zone i to a traffic zone j.
- A travel demand matrix refers to a time interval (analysis time interval) and thus only contains trips that depart within the time interval.
- Trips of a demand matrix can refer to the total transport system, to partial transport systems (for example pedestrian, bicycle, PuT, car), to person groups (for example employed, students, retired persons) or to purposes (for example commuting, shopping, leisure).
- A demand matrix is assigned to exactly one demand segment. A demand segment describes a group of road users with homogeneous travel behavior.
Travel demand can be divided into surveyed and calculated demand as well as into today's and future demand.
Surveyed travel demand describes the number of trips and the trip distribution within a fixed time interval for an existing transport supply system. It represents a snapshot of the current traffic situation and cannot be reproduced again practically. An exact survey of today's current travel demand in an area of interest is not possible in practice because all travelers would have to be interviewed at the same time. For this reason, only a representative, random sample of travelers is interviewed to determine travel demand for the purposes of transportation planning. From this survey a matrix of today's travel demand is then deducted. It represents the travel demand for the existing supply system.
Calculated travel demand contains assumptions about the number of trips and trip distribution. To calculate travel demand, demand models are used which, for example, differentiate between the three steps of Trip generation, Trip distribution and Mode choice. The calculated travel demand can be designated differently depending on the used input data.
- Calculated travel demand is called today's travel demand if the input of the demand calculation is today's land use structure, today's population and economic structure, and today's transport supply system.
- Forecasted travel demand is based on data on future land use, future population and economic structure and the future transport supply system.
An overview of the procedures for determining travel demand can be found in Leutzbach et al. (1988).
Within Visum all 4 stages of the classical transport model (4-step model) can be calculated, besides traffic assignment (choice and volume of the route to get from origin zone to destination zone) the other three steps Trip generation, trip distribution and Mode choice (choice of means of transport), too.
In the first step of the classical model, Trip generation, the production and attraction (origin and destination traffic) of each zone is determined on the basis of socio-demographic data (for example, number of inhabitants and jobs). These production and attraction values define the totals of the total demand matrix, which is determined by means of relevant skim data (for example, journey times, fares etc.) in the second step, Trip distribution. In the third step the total demand matrix is distributed onto the different traffic modes (for example, PrT, PuT) on the basis of mode-specific skims. In a fourth step the resulting mode-dependent demand matrices can be assigned to the supply (Visum network) by means of the PrT and PuT assignment procedures in order to obtain link volumes and new skims. This skim data can again be used as inputs for trip distribution or mode choice of a new demand calculation. The Go to the procedure function allows you to iterate the calculations until a convergence criterion concerning link volumes or matrix values is satisfied.
Visum contains three alternative calculation models for the demand modeling.
- The Standard 4-step model is based on North American practice for aggregated demand models (Standard 4-step model in two variants).
- The EVA model is another aggregated demand model for passenger demand. It differs from the Standard-4-Step Model by a simultaneous trip distribution and mode choice as well as by its particular method of balancing the differences between origin and destination traffic (EVA (passenger demand model)).
- When calculating demand matrices, the Tour-based model (traffic in cities generation model) takes into consideration activity chains which homogenous-behavior user groups (for example employees with or without a car, pupils, students) perform during the course of the day (Activity chain based model (tour-based model)).
The matrix editor integrated in Visum supports matrix processing and provides a gravity model.
The calculation models are based on specific Visum demand objects describing the characteristics of trip purposes and road users. Person groups combine road users featuring comparable mobility behavior to groups. The break-down of the population into person groups may be based on their job status (employed, students, retired persons) and (optionally) their car ownership (with/without car). Activities are activities or locations of a person in the course of the day which are not traffic related (work, school, home). Activity pairs describe transitions between two activities and may imply trips from one place to the other (home - work, home - school). They are then called trip purposes.
A demand stratum combines one or several person groups with an activity. Almost all calculations of the first three stages of the model are carried through separately for each demand stratum and their results are stored separately for a better illustration and verification. The resulting demand matrices always have the unit [persons].
By aggregating the demand strata to demand segments parts of the demand jointly to be assigned are combined prior to the fourth stage, which is the assignment. Hereby, the PrT demand matrices are converted into the [Vehicles] unit by dividing the demand stratum matrices by the occupancy rate of the respective transport system.
Temporal distribution of travel demand
The trips from one traffic zone to another traffic zone in reality take place at different times. The temporal distribution of travel demand within the analysis period is described by a start time and a time series when modeling in Visum. The time series is taken into consideration at the PuT assignments and the dynamic PrT assignment. The time series is ignored in the case of static PrT assignments. Temporal distribution of the trips within each time interval of an observed time period can therefore not be set for this procedure.
The start time specifies the time and – if the weekly or annual calendar is used - the day on which the period referred to by the demand in the matrix starts. The end of the period is calculated from the length of the assigned time series.
Time series can be defined in two different ways:
- Time series by percentage of one demand matrix
- as a time series consisting of several demand matrices
A time series by percentage specifies the proportion of trips with the desired departure time within the respective time interval. Demand time series can cover more than 24 hours if a weekly or annual calendar is used. An equal distribution of travel demand during the observed time period is assumed as default. Instead of this default, a user-defined demand time series can be specified for the entire matrix. This user-defined demand time series can be overwritten again for selected pairs of origin-destination zone types with specific demand time series. In this way, it is possible to specify deviating time series for zones, for example, with known structural features (for example purely residential or commercial areas) that reflect the different traffic loads in one direction (Image 2) at certain times of the day for journeys between home and work.
Image 2: Example of a time series of the travel demand by four intervals of 30 minutes
A time series of demand matrices allocates a separate matrix to each time interval which contains the demand with the desired departure time in the respective time interval. It should be used if for example matrices on an hourly basis already exist based on a trip generation model. Contrasting time series, here the time dependent course of the demand can be freely selected for each matrix item. However, the data entry expenditure and the memory requirements are higher accordingly, because several complete matrices are supplied.
