Following presentations related to the development of the current SEMCOG TDFM, Version E5, and the in progress updates that will lead to a new version of the SEMCOG TDFM, Version E6, in March 2012, the peer review moved on to presentations about the analytical needs of SEMCOG and model enhancement recommendations. This section of the report covers those presentations given by Rick Donnelly of Parsons Brinckerhoff.
The analytical needs of SEMCOG with respect to travel modeling were categorized into three groups: 1) national issues and trends, 2) Federal requirements (e.g. model certification by FHWA, and applications for FTA New Starts funding), and 3) Local requirements (comprising strategic uses, tactical uses, and performance measures).
The context for the discussion of analytical needs was set by referring TRB Special Report 288, Metropolitan Travel Forecasting: Current Practice and Future Direction (2007) and specifically this quote:
"The committee therefore recommends development and implementation of new modeling approaches to demand forecasting that are better suited to providing reliable information for such applications as multimodal investment analyses, operational analyses, environmental assessments, evaluations of a wide range of policy alternatives, toll-facility revenue forecasts, and freight forecasts, and to meeting federal and state regulatory requirements."
The presentation of analytical needs noted several important aspects of the Federal requirements, in terms of both the FHWA certification process and the use of the travel model to support FTA New Starts funding applications. FHWA has established a certification process and checklist for MPO models to ensure that they are capable of informing federally mandated air quality and transportation planning requirements. Other than certain requirements that affect the use of model outputs for air quality modeling, the certification checklist is not definitive about the structure or capabilities of travel demand models. FHWA acknowledges that analytical requirements vary from one MPO to another. However, the process is designed to ensure that the models that are in place are adequate for current and anticipated applications of the model. Moreover, they outline a series of questions to assess how well the modeling program addresses analytical risks, the agency's technical capabilities, and documentation. The documentation considered during certification includes, but is not necessarily limited to, three major areas:
While FTA does not approve models on a set schedule similar to FHWA's certification process, they do review models as part of their forecast review during consideration of New Starts applications. In general, FTA prefers to see a model with the following features:
Local requirements were presented in two main categories: strategic versus tactical uses of the model. Strategic uses include long-range transportation plans, transportation improvement plans, and analyses of region-wide impacts and opportunities. Tactical uses focus on specific projects or programs, whose effects are usually localized or concentrated in certain corridors or smaller study areas.
An additional local requirement that was discussed is the use of the TDFM to report performance measures. The transportation-related measures that the TDFM will likely be called upon to inform about include:
The presentation of local requirements assigned the strategic and tactical requirements (including the performance measures) to travel model capabilities that are traditional and those are non-traditional as shown in Figure 2.
Selected link analysis
Transit corridor or project analyses
Traffic impact studies
Time and cost savings of projects
Update RTP and TIP
Project prioritization and programming
Air quality conformity
Assessment of regional strategies
Improving regional mobility
Demand management vs. capacity
TOD and 4D analyses
Evaluating new transport modes
Construction and detour analyses
Work zone staging and management
Linkage with traffic operations models
Signal timing optimization
Traffic impact studies
Equity and environmental justice impacts
Capacity reduction strategies
Market accessibility and competitiveness
Economic impacts of transport
Understanding freight and supply chains
Impact of fuel price increases
Equity and environmental justice impacts
While the traditional travel model capabilities could be supported by a best practice trip-based model and would suggest that a continuation of SEMCOG's E Series models was sufficient, Parsons Brinckerhoff presentations identified that a dynamic traffic assignment (DTA) model would be required to support the non-traditional tactical requirements, while an integrated land use-transportation model would be required to support the non-traditional strategic requirements. Together, these two model requirements suggest the need for an activity based (AB) travel demand model.
Following on from the discussion of analytical needs, Rick Donnelly of Parsons Brinckerhoff presented a summary of model enhancement recommendations that would achieve those analytical needs. The recommendations were introduced with reference back to the three categories of analytical needs. While the current model meets FHWA certification requirements and could support FTA New Starts applications and many of the local requirements, advanced models are becoming the state of the practice and are needed for many of the local requirements. The concept of multi-scale travel modeling was introduced to describe the roles that a new SEMCOG advanced model would cover and its linkages to both larger and smaller scale models (Figure 3).
Figure 3: Parsons Brinckerhoff's vision for multi-scale travel modeling
The presentation described an incremental strategy for gradually moving from current trip-based models to advanced models. The incremental strategy would make use of shared components, such as travel behavior data (e.g. surveys), networks, and target and validation data. The gradual transition would allow for staff development, and maintain fully operational models at each stage. The following sections outline transition strategies for the demand side, supply side, and data programs. The model enhancement recommendations were summarized as:
Transitioning to an AB model for the demand side of the TDFM was described as providing several advantages, including deeper insights into travel behavior, more realistic representation of travel dynamics, making linkages to a DTA more straightforward, eliminating non-home based trips, and improving the ability to support equity and pricing studies. The following stages were laid out:
Transitioning to a DTA model for the supply side of the TDFM was described as providing several advantages, including enabling robust tactical solutions, and more realistic representation of travel dynamics. The following stages were laid out:
The following steps were laid out to transition to a data program that would support the development of advanced models: