Traffic Simulation Demand Reference
Demand
In a transport model, the demand is a specification of the number of people that want to travel. Capacity for travel is supplied by a transport system comprised of roads, railways and walkways and the vehicles that travel on that network. Transport modeling examines the relationship between demand, capacity and travel time, and in particular how changes to the transport network can reduce travel time.
InfraWorks Traffic Simulation models vehicle traffic only; demand is expressed as vehicle trips and capacity is supplied by a road network. Demand zones must also be defined. Each vehicle trips begins at an origin zone and ends at a destination zone.
Demand Zones
A demand zone will be created automatically where there is a road at the boundary of the traffic study area. You can add additional demand zones within the boundary of the traffic study area. For any such zone to be effective, it must contain at least one lane of one road. Note that the demand zone has a floor and a ceiling, so you should check that the elevation of the road is not above or below the new demand zone by viewing the zone from the side.
Demand Editor
The Demand Editor has three tabbed panes:
- Directed demand defines a number of trips, with each trip having an explicit origin and destination.
- Undirected demand defines a number of trips from each origin, but no explicit destination. Undirected demand also requires turn counts or splits at each junction. The turn counts define the probability of any trip taking each of the possible exits at a junction.
- Public transport services and departure times simulates public transport vehicles on predefined service routes.
Directed and Undirected Demand Tabs
On the left side there is a list of the demand sources within the model. You can add a new source of demand by clicking the Add button above the list, or delete a source by selecting it and clicking the Delete button below the list.
The table in the center of the window contains details of any source currently selected in the List Panel. This table contains a breakdown of the demand by origin, and also the destination, for directed demand).
The Profile selector allows you to vary the release of the demand over time.
The Division selector allows you to restrict the demand to a subset of all available types for the Mode.
Transport Demand Tab
Each public transport service contains a timetable, but the vehicle trips for a service must be generated at the same time as all other trips.
The Transport Demand Tab repeats much of the information that is available in the Services window. However, it can be useful to see all the demand information in a single window.
The Transport Demand Tab lists all public transport services on the left side. The departure times and associated vehicle types are displayed for each service on the right hand side. You can add or delete entries to the timetable using the buttons below the time table.
To edit the route of a transport Service, you should use the Services window, accessible from the Network menu on the Main Window.
Tips for Defining Demand
- All the sources listed in the demand editor are added together when trips are generated. This includes directed demand, undirected demand and transport demand sources. If you have 3 volumes and 2 matrices defined, then trips will be generated based on the sum of all these sources. Typical reasons for having multiple sources are that you might have one matrix for private cars, another for goods vehicles. Also, it is common to break down a single peak period into 3 hours, and have a separate demand source for each hour.
- Matrices do not require full coverage - when you create a matrix, you are asked to enter a list of origins and a list of destinations. By default, both these lists contain all possible origins and destinations, but you can trim them to suit. For example, if you have one matrix covering highway zones, and another covering city blocks, you can add these separately.
- Matrix size can be reduced by trimming out one-way zones. Models commonly contain zones that contain one-way roads. Such a zones operates as an origin only, or as a destination only, depending on the road direction. Trimming out areas or zones from either the origins or the destinations can reduce the size of the matrix and make the data easier to manage. For example, if you have an off-ramp that is a destination, but does not act as an origin for any trips, then remove it from the list of origins.
Demand Profiles
A demand Profile is a Term and a set of weights that sums to 100%, where each weight applies to a time interval. The weights are applied in sequence to generate a time varying quantity. The Term defines the start and end time. A profile is use to vary the number of trips generated over the course of a term. It is often higher in the middle than at the ends to represent a peak hour.
A profile can have any number of intervals; all of the time intervals are of the same length. If for example, you have an hourly demand, but want to weight the demand by some 5-minute counts, you would define a profile with 12 x 5-minute intervals, and set the corresponding 12 weights according to the count values.
The Term applied to a Profile can be independent of all other Terms. For example, it does not need to coincide with the Terms defined for restrictions or signal timings, or even for other profiles.
The default profile “Flat” applies a constant weight, of value 100%, all the time.
Using the Demand Profiles Window
To define a new profile
Click the “New Profile” button in the top left corner.
Enter a name, for example “AM Peak”.
Choose a Term for the Profile, or define a new one using the drop-down selector.
Define the number of intervals within the Profile. The list on the left enables you to select a profile for editing.
The number on the right hand side below the buttons shows you how many intervals are in the current profile. You may need to scroll the central pane left or right to see all the weights.
To accept a new or edited profile, you must tick the ∑ = 1 (Sum to 100%) box. This will normalize the weights so that their sum is 100%.
After creating a profile, you can
- Add an interval using the “New Interval” button top right.
- Delete an interval.
- Reset all the weights to be equal using the “Reset” button. For example if you have 4 intervals in the profile, reset will set all 4 weights to 25%.
- Move the slider when the ∑ = 1 box is ticked, the other weights will adjust so the sum is 100%.
- Type a value into the box below a slider; this will clear the ∑ = 1 box.
- Paste a row of values from a spreadsheet into the weight value boxes. Select the first (leftmost) box and paste using Control-V.
Demand Divisions
A Demand Division is a weighted group of Types. You can define any number of divisions, each of which can contain any number of types. Divisions are used by the trip generation process to apply a demand matrix or volume in given proportions to a group of vehicles. Divisions are also used to select vehicles for parking. This is also a type of demand allocation, because it decides the proportions of vehicles to generate in a parking zone if none are available.
Divisions may overlap, that is, any one type may be contained in any number of divisions. A division may contain a single type.
A division may be assigned a mode to restrict it to vehicle types of that mode. For example, if you create a new division with a pedestrian mode, only pedestrian types may be included in that division. However, a division can be mixed-mode, so that a single division can contain a combination of vehicle and pedestrian types.
The Division Window shows a table with all types as columns, and all divisions as rows.
To create a new division, click the button in the top left corner of the Division Window. Choose a name for the division and select a mode, or one of the mixed-mode combinations. Then select the types you want in the division by ticking the boxes in the column for each type.