Revise transportation modeling methods
Action
Some states and most regional metropolitan planning organizations (MPOs) create and use sophisticated transportation models to estimate the effects of proposed future transportation projects. These models are often critically important to the development of regional long-range transportation plans that must be satisfactorily completed before state projects may receive federal transportation funding.
Increasingly, these models are being used to forecast the impacts of transportation projects on related issues, such as the release of greenhouse gases that contribute to climate change. To enhance the link between transportation and land use planning, it is important to remove barriers to building well-designed, mixed-use projects in accessible places. Therefore, states that do their own modeling, or that work with MPOs on modeling, should encourage the use of transportation modeling software that can help localities capture the traffic impact of development, including the impact of "smart growth" development. For example, methods for calculating the number of additional vehicle trips generated by new development often significantly overestimate the traffic impact of many infill projects, because the standard formulas are based on studies of existing sites in auto-dependent suburban locations. Applying these suburban standards to urban projects such as transit-oriented development projects can overestimate the number of vehicle trips generated by mixed-use, infill projects by 30 to 50 percent. VMT for more typical mixed-use infill projects can be overestimated by 10 to 20 percent.
The types of traffic models, assumptions, and internal structures, such as size of Transportation Analysis Zones (TAZs), have increasingly become a subject of debate. Robust growth assumptions, large TAZs, trend growth scenarios, and low fuel prices all served us well in the era when the Interstate Highway System was being constructed. Continuing to use these practices or parameters in the current era will return poor answers. Departments of transportation should ask:
- What are the growth assumptions for the model and how were they developed?
- Are these assumptions still valid in alternative scenarios that are of interest?
- How many TAZs were used in the modeling, and are they small enough to capture walking and internal trips?
- Did the model account for the fact that the additional transportation capacity that may be needed to keep up with anticipated growth may not actually be built? If that is the case, does the absence of that infrastructure restrain other growth assumptions? In other words, is the relationship between actual land use and transportation adequately represented in the modeling framework?
- Most of the MPO models do not account for the movement of freight and their impacts on infrastructure and congestion. If they are not explicitly accounted for, are the model outputs presented with sufficient qualifications?
The most important thing that a state department of transportation can do is reevaluate whether traffic modeling is needed in the first place. Too often, as a first step in the planning process, departments of transportation and communities run a traffic model that is designed for suburban development, and subsequently overestimate growth and necessary road size with no attention to the context and community that the transportation investment is seeking to support.
Process
To better understand the relationship between more compact development and traffic, states can use Smart Trip Generation Formulas to model traffic impacts. In urban areas, trip generation rates should be adjusted to account for transit availability, the amount of nearby activities that can be reached on foot, and the quality of the pedestrian environment. Departments of transportation should ensure that any future modeling can handle all multimodal trips, including pedestrian. This might require adding complexity to the model, reducing the size of the TAZs, or adding more pedestrian and transit links, which could add some costs to the modeling. However, this step will be necessary to avoid overestimation of projections, over-design of projects, and unnecessary damage to communities. States should use any model results with caution, by understanding the assumptions and inputs that underlie them. Pedestrian Environment Factors, which relate trip generation to characteristics of the built environment, can be used to adjust mode choice at a zonal level. Furthermore, departments of transportation should investigate the impact of freight on various transportation links and develop policies (along with the private sector, such as railroads and trucking companies) to enhance the results of MPO modeling efforts.
Examples
- Oregon’s Modeling Improvement Program
The state of Oregon has a well-integrated transportation, land use and economic model.
— Oregon’s Modeling Improvement Program
