Activity & Trip Based Travel Models
The following is summarized from “Activity Based Travel Demand Models” to give students an overview.
- How to choose a model type
- Modeling Techniques
- “Trip” vs “Tour” vs “Activity”
- Trip-Based Models
- Activity-Based Models
- Gravity Models and O/D Pairs
- Advanced Integrated Models
Why model travel?
Understand questions such as:
How will the national, regional, or even local transportation system perform 30 years into the future?
How will economic, demographic, or land use changes affect transportation performance?
Will intelligent transportation alleviate congestion or attract riders?
What model type should I choose?
The type of model you choose to use will also depend on the type of question you aim to answer.
Where do you need detail? Different model types allow for varying levels of fidelity in various areas. However there are tradeoffs for higher fidelity models because they take more computing time and can be more costly to develop models that reflect all of the information of interest. At lower fidelities, sketch planning (using GIS and spreadsheets) may be sufficient in early stages of the design before creating higher fidelity models.
Modeling Techniques
“Trip” vs “Tour” vs “Activity”
- Trip (aka four step model) — individual person trips, doesn’t understand that trips are interrelated
- Tour — strings together trips that will be typically done in sequence, but doesn’t account for vehicle capacities or total course of the day
- Activity — accounts for vehicle trips (accounts for vehicle capacity) and how trips would be strung together over the course of a day
Trip-Based Model — aka Four Step Model (FSM)
A trip-based model is structured in four steps: trip generation, trip distribution, mode choice, and route choice.
1 Trip Generation
ITE: Trip Generation Report — ex: will show number of trips to a proposed shopping mall based on amount of sq ft
2 — Distribution
desire lines between two zones, then attractiveness applied between two zones (if more jobs in one zone = more attractive)
3 — Mode
mode — likelihood of someone to take one transit type versus another based on the amount of time it will take
4 — Route
route — which route is taken
Link node network represents many network paths (highways, small roads, train lines and stops) where each node shows capacity
Activity-Based Model
- Distinguished from trip based — represent each person’s activity and travel choices across the entire day, considering the types of activities the individual needs to participate in and setting the priorities for scheduling these activities. As a persons schedule fills up, they no longer have time left to travel. This is particularly important for account for the differences in travel flexibility experienced by various travelers.
- are based on behavioral theories about how people make decisions to participate in activities based on when and where those activities are and how to get to them. Activity based models tend to represent behaviors and decisions at higher fidelity and thus can be more realistic.
Bias & Assumptions
- Aggregation Bias — Aggregation bias refers to the assumption that group characteristics are shared by all the individuals who are members of that group. This is a large issue in travel models because individuals will not all behave according to assumptions of the model.
“There is tremendous diversity in how different types of persons and households make travel decisions depending on factors such as income, transit accessibility, competition with other household members for vehicles, travel times by detailed time of day, and many other influences.
The use of average values applied to aggregate populations across aggregate spatial zones and time periods distorts a model’s sensitivity to investment and policy alternatives. Although it may be theoretically possible to incorporate additional detail in trip based models through the use of additional market segmentation (such as including more household income categories), zones or time periods, it is practically challenging because the aggregate trip-based model’s reliance on two-dimensional origin–destination (O-D) or production–attraction matrices causes model run times, storage, and memory requirements to increase exponentially as segmentation increases.
As a result, most trip-based models incorporate significant levels of aggregation, which compromises their sensitivities to different alternatives and limits their ability to provide detailed information on the impacts of these alternatives, reducing their usefulness as decision-support tools.”
Stakeholder Buy-in
Getting buy-in on models is a particular area of concern as models are based on assumptions and thus can be difficult to quantify how closely they reflect reality. Do stakeholders think the model is valid? Does the person creating the model have a conflict of interest? Is there a way to have an external entity audit a model?
Mode Decisions — Greater Fidelity
- Various models account for greater levels of information in how modes are chosen, for example: it is highly unlikely that a traveler who has used transit to get to work is going to drive home alone.
- Multi-Modes — A person may chain together different types of modes (walk→ train→taxi) throughout their trip. Higher fidelity models account for how modes switch throughout a trip.
- Sharing Rides — Modes are also impacted by the capacity of vehicles and whether rides are shared between household members etc.
Other Model Types
- Integrated Travel Demand Model System
- Population synthesis models create detailed, synthetic representations of populations of individuals within households (agents) whose choices are simulated in activity-based models. This population is based on information produced by regional economic models, land use models, and demographic models.
- Activity-based travel demand models predict the long-term choices (such as work location and automobile ownership) and the daily activity patterns of a given synthetic population, including activity purposes, locations, timing, and modes of access. These estimates of travel demand can be used to help evaluate alternative transportation, land use, and other scenarios.
- Auxiliary models provide information about truck and commercial travel, as well as special purpose travel such as trips to and from airports or travel made by visitors. The travel demand represented in auxiliary models complements the personal travel generated by the activity-based model.
- Network supply models are tightly linked with activity-based demand models. The flows of travel by time of day and mode predicted by activity-based travel demand models and auxiliary models are assigned to roadway, transit, and other networks to generate estimates of volumes and travel times. Measures of impedance output from network supply models are usually used as input to activity-based models and other integrated model components.
Advanced Integrated Models
Parts of a model:
- Trip — origin and destination pairs (O/D pairs)
- Trip distribution — matches O/D pairs using a “gravity model”
- Gravity model — used to estimate the amount of interaction between two cities or two places (say homes and workplaces). Gravity models have increasing weight depending on their importance, closer distances, shorter times, greater activity.
