Introduction to the Four Step Travel Demand Model
The center of most travel demand models is the “Four Step Model”. This model was created in the 1950s to determine the demand on roadways. The four steps include:
- Trip Generation
- Trip Distribution
- Mode Choice
- Trip Assignment
The first step in the process, Trip Generation, uses socioeconomic data to determine the number of trips produced by a traffic analysis zone (or census tract, census blockgroup, or other geographic division). The socioeconomic data normally includes population, auto ownership, and employment information at the very least.
The second step is trip distribution. Once the number of trips are known, trip distribution determines where the trips will go. This normally uses a gravity model, which will be discussed in a future post. This step takes in several factors, including the number of trip productions, the number of trip attractions, and an impedance value. The impedance value is the resistance to travel, which could include distance, time, tolls, or a combination of those. Each model is different in this regard.
The third step is mode choice. This step determines what vehicle trips will utilize when going from one zone to another. This step can be particularly complex or extremely simple, depending on the area included in the model. Some models have very simple mode choice steps because transit isn’t available, or it doesn’t have a significant effect. Other models can be extremely complex, such as a model for Chicago or New York, which would include autos, bus transit, subway transit, urban rail (such as Chicago’s Metra Rail), and intercity rail transit (such as the Amtrak service that connects New York to Boston, Baltimore, and Washington, DC).
The final step is trip assignment. This step takes all of the trips from mode choice (which it now knows if they are trips that will drive alone, share a ride, use the bus, or use another mode of transportation) and assigns them to a transportation network. Prior to the mid-1980s, these networks were largely text based, but with the advent of Geographic Information Systems and personal computers in the mid-1980s and moving forward, these networks became graphical.
These four steps represent the basic building blocks of most transportation models. These steps are a basic way to ask:
- How many people are going to travel?
- Where are they going to go?
- What transportation mode are they going to use to get there?
- What route will they take to get there?
Tags: four step model
