Topics

• The Big Picture
  • Types of Models
    • Sketch Planning
    • Quick Response
    • Three Step
    • Four Step
    • Activity Based
    • Freight
    • Land Use
    • Air Quality
    • Integrated
  • Decisions on Approach
  • Transition Paths
  • End Users
• Model Input
• Model Development
• Model Application

Four Step

Variations on both the aggregate and disaggregate approaches (see discussion under Types of Models) can be developed and applied as stand-alone travel demand forecasting methods, appropriate for specific purposes. Alternatively, a set of methods can be applied in conjunction with each other to create a larger modeling framework. The four-step Urban Transportation Planning Process (UTPP) (Weiner, 1997), first developed in the 1950s to forecast automobile travel and now applied in urban areas throughout the world, is an example of such a framework. To predict how travel patterns will change as a result of future changes in land use patterns and the transportation system, this framework integrates models of various aspects of travel behavior (e.g., trip-making or mode choice) with spatial information on land use patterns and the transportation network.

The UTPP is important to understand because it is widely used in transportation planning. The basis for UTPP models is the division of the urban area into traffic analysis zones (TAZs), which may correspond to census tracts, and the definition of a network of transportation facilities connecting the zones (Figure 1). The network is described by the time and cost of travel, for each mode, between each pair of zones. Inputs include proposed future transportation networks and forecast population and employment characteristics by zone. A four-step process is then used to forecast travel:

1. Trip generation - Total trips generated by persons that start and end in each zone are predicted, based on the population, employment, household characteristics, etc., of the zone;
2. Trip distribution - The trips are distributed among pairs of zones, usually based on a gravity model which distributes trips in inverse proportion to the distance between zones;
3. Mode choice - The trips are allocated among the available travel modes, based on relative characteristics (usually time and cost) of the modes; and
4. Network assignment - The trips are assigned to specific links (road segments) in the transportation network, generally based on the shortest time path between two zones.

Figure 1: Structure of Regional Travel Model

The different stages of the process may include both aggregate and disaggregate behavior models. In addition, these models have sometimes been modified to incorporate additional travel behavior factors, such as feedback from later steps to earlier steps (e.g., congestion influencing trip generation and mode choice) or variations in travel by time of day.

The UTPP framework has primarily been applied to automobiles and transit but is increasingly being modified to include bicycles and pedestrians. Non-motorized modes can be incorporated in the models in various ways. For example, a bicycle or pedestrian network can be defined. Bicycling and walking can be included as modes in the mode choice model. [1] 

[1] Adapted from Guidebook on Methods to Estimate Non-Motorized Travel: Overview of Methods (July 1999)

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