Tutorial 2 - Demand

Demand, Supply and Congestion

The demand is the number of people that want to travel. The network of roads and pathways is the supply for this demand. If demand is larger than supply, then there will be congestion.

Demand between Areas, and between Zones

There are two "levels" of demand in a hierarchy

Upper Level: Person-Trips are defined between Areas. Each trip defines a door to door journey for a person, which uses one or more modes of transport Lower Level: Vehicle-Trips are defined between Zones. Vehicle trips may exist to carry people on trips defined by the upper level of demand, or they may be generated as background traffic, or because the model is being used to study vehicle traffic only. 6.3 Directed, Undirected and Fixed-Route Demand Within the two-level area-zone hierarchy, demand can be defined in three ways:

Directed demand defines a number of trips, with each trip having an explicit origin and destination. Directed Demand can be used for Person-Trips or Vehicle-Trips.

Undirected demand defines a number of trips from each origin, but no explicit destination. Undirected demand is often accompanied by turn counts (splits) at each junction. The turn counts define the probability of any trip taking each of the possible exits at a junction. Undirected Demand can be used for Person-Trips or Vehicle-Trips. Public Transport (Fixed route) demand defines public transport vehicle trips on the network. This type of demand is for Vehicle- Trips only. These trips use capacity on the lower-level road network, and supply capacity for person-trips to the upper level. Before defining demand you must create Areas or Zones or both. This is described in an earlier chapter.

For an Area to be effective, one of the following must be true:

The Area must contain at least one walkway OR
The Area must be connected directly to a Zone OR
The Area must be connected directly to a Stand

For a Zone to be effective, one of the following must be true:

The Zone must contain at least one running Lane OR
The Zone must contain at least one parking Lane OR
The Zone must be directly connected to an Area

Exercise: Create an Origin-Demand Matrix of Person-Trips

Create a Walkway
Create two Area, one at each end of the Walkway
Select the Walkway, Action Walkway > Bisect (A Walkway can only be associated with one area, this is why you need to bisect the walkway, so you have two walkways, one for each Area)
File / Save
Menu Demand / Demands.
Press Add Matrix.
Select Mode=People, Profile=Flat and Division=D1
Change the name of the matrix if desired, and press OK
The next window, Select Origins and Destination, allows you to use only some of the Areas as origins or destinations. Here we have only two areas, so we need both. Press OK.
In the Demand Editor window, you will now have a grid, with column totals and row totals. Type 200 as the number of trips between Area 1 and Area 2, and 100 from Area 2 to Area 1. When you select a cell in the table, the origin and destination will be highlighted with an arrow on the Graphics panel. You can type values directly into the row total (all trips from this origin), the column total (all trips to this destination), or into the matrix total (all trips). The value you type will be distributed to each of the possible cells, in proportion to the existing values It is not possible to type values into the diagonal cells, as this represents intra-Area trips (from Area X to Area X). If you have Excel or another spreadsheet application on your computer, open that, and try copy-pasting parts of the table to the spreadsheet and vice-versa.

Generating Trips from Demand

Trip “generation” here refers to the process of creating individual disaggregated trips from aggregated demand specification. Trip generators employ pseudo-random number generators to select values from within parameter distributions. For example, a trip may be generated with a smaller than average physical size but a larger than average propensity to change route.

Traffic Analyst separates trip generation from simulation. That is, trips are generated once, before simulation begins, and the trip table is saved as input data. This means that the simulation can be repeated with exactly the same sequence of trips any number of times, even if the model changes. If you have a base model, and a proposed design model, it is often useful to know that changes in measurements resulting from a run of the design model are due to the changes imposed in the design, not due to variations in the sequence of random numbers generated.

The trips generated can be saved as part of the model, or can be saved in an external file. Saving externally is useful in larger models, where the number of trips is large (as a guide, 100,000 +).

Exercise: Generate Trips

Menu Trips > Browse / Generate.
Select Trip Generation Tab Change the “seed” for the random number generator, if required, or use the default
Press Generate Trips Check that all trips you expected have been generated (Success Rate = 100%). If some trips are missing, it is likely that no route exists between one or more origin-destination pairs. Use Menu Assignment > [Person|Vehicle] Routes. then Route Analysis / Unavailable Routes to locate the problems.
If all trips generated, press Yes to save
You can now run the model by pressing Play on the Simulation toolbar

Demand Profiles

In Traffic Analyst, a Profile is 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 lower at the start and end, and higher in the middle 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.

Exercise: Demand Profiles

Open the exercise model of a roundabout. It has four arms, each with two way traffic and two lanes each way. There is a zone located at each end of the arm to generate traffic flow.

The first step of this tutorial is to add a demand matrix to this model.
In the Profile section, there is only one option available, it is called “Flat”. We will use this profile for now.
Leave the default settings for the Origins and Destinations.
Set a total demand, for example, 1200. Then save and generate trips.
Start simulation and you should see traffic flowing.
Notice the volume of traffic is rather consistent and evenly distributed.
Take a look at the demand we created and we know this is because the “Flat” profile was chosen.
Now let’s create a new profile to simulate traffic flow increasing and decreasing over a period of time.
In the “New Profile Parameters” window, we will choose “Simulation” as the Term, because we only want this profile effective during the simulation.
Set the number of intervals to 5. This will divide our simulation term into 5 parts, we will be able to control the intensity of demand in each part individually.
The new window shows the control bars. Since our term is one hour and we have 5 intervals, each represents 12 minutes. You can manually input exact value as well. The sum of these values should be 100, indicating 100%.
Change the values to 10,20,40,20,10 so that we have a gradual increase to a peak flow followed by a decrease.
After typing the value, tick the box on the right hand side, which will automatically adjust the totals so that they sum to 100%, and then click OK.
In the demand window, change the profile from Flat to the one we just created, and re-generate trips.
Start simulation and notice that traffic flow is light to begin with, but as time passes, the flow increases.
High flow is particularly noticeable when simulation is around half way, which is the peak in the profile.
In the demand window, you may have noticed that you can also choose a profile for demand from a certain origin.
So that only traffic from certain origins will generate changing flow, while other origins generate consistent flow or vice versa.
Profiles can be applied to all kinds of demands. Let’s experiment applying the same profile to pedestrian movements.
Firstly, create walkways next to the roads, you can do this by selecting the road and then use Lane> walkway beside.
Turn on Walk Centerlines and Walk Connections plus Handle-End.
Move the end of the walkway towards the roundabout, then create a perpendicular walkway and connect them.
Create an area at the end of the perpendicular walkway; we will generate some pedestrian trips from here.
Do the same at another adjacent arm.
Now we will create a demand of 200 pedestrian trips between the two areas.
In the profile section, choose the new one we created. When finished, save and generate trips.
Run simulation, you should see people appearing and walking.
Turn off Walk Centerlines and Walk Connections.
If they are too hard to be seen, change them to a different color For example, fluorescent green.
We can also add tags to people so they can be observed easier.
Create a tag, choose a bright color, and set origin to Area 1, this will tag people coming from Area 1.
Create another tag, use the same color, and set origin to Area 2 to tag people from the other way.
In the Layer tab, turn on “Tags”. Run simulation and you should be able to observe pedestrian movements a lot easier.
At this moment pedestrian movement is not very heavy.
Turn off the Vehicles layer so we can focus on observing only pedestrians.
With simulation running towards half way, notice the flow of pedestrians becomes heavier.
And toward the end of the simulation, the flow declines as expected.