National Academies Press: OpenBook

Statewide Travel Forecasting Models (2006)

Chapter: Chapter Four - Findings and Suggestions for Research

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Suggested Citation:"Chapter Four - Findings and Suggestions for Research." National Academies of Sciences, Engineering, and Medicine. 2006. Statewide Travel Forecasting Models. Washington, DC: The National Academies Press. doi: 10.17226/13958.
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Page 52
Suggested Citation:"Chapter Four - Findings and Suggestions for Research." National Academies of Sciences, Engineering, and Medicine. 2006. Statewide Travel Forecasting Models. Washington, DC: The National Academies Press. doi: 10.17226/13958.
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Page 52
Page 53
Suggested Citation:"Chapter Four - Findings and Suggestions for Research." National Academies of Sciences, Engineering, and Medicine. 2006. Statewide Travel Forecasting Models. Washington, DC: The National Academies Press. doi: 10.17226/13958.
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Page 53

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52 Statewide travel forecasting is becoming a more common activity in transportation planning. There is an increase in the number of states with models and many states are in the process of revising their models. The impetus for develop- ing a statewide model varies greatly from state to state. In some states models were created to address the needs of a specific large project; in other states models were created for general planning needs. Statewide models have become es- sential in some states for intercity corridor and statewide system planning. Most statewide models are similar in structure to four-step urban transportation planning models. Statewide models dif- fer from urban models primarily in how the steps are config- ured. There exists no well-accepted definition of best practice in statewide models. Models range greatly in cost, staffing re- quirements, development time frame, and capabilities. Special data collection efforts, apart from National Household Travel Survey (NHTS) add-ons, are sporadic. Most states are making efficient use of a wide variety of sec- ondary data sources. The following several distinct trends are apparent in re- cent statewide model development. • Many newer models have network detail at about the same level of precision as urban models. • There are more freight components that are commodity- based, rather than being truck-only. • There is a greater and more effective use of geographic information systems to manage and acquire model data. • There is more of a tendency to hold statewide models to the same standards of validation accuracy as urban models. • There has been a doubling of states (from one to two) that are pursuing models with integrated economic ac- tivity components. • Traffic assignments are less likely to be all-or-nothing and more likely to be equilibrium. • There is a greater emphasis on multiclass traffic as- signment for combining freight and passenger traffic forecasts. There are planning needs that have not been fully realized because of deficiencies in either data or algorithms. • The 1995 American Travel Survey (ATS), the last ma- jor source of information on long distance passenger travel, has not been updated. • Models tend to still have a time period of 24 h; none of the states have implemented the dynamic methods nec- essary for good forecasts of peak-hour travel in larger states. • There has been little progress in the creation and use of transferable parameters within any of the model steps. • With the exception of a few geographically small states, there has not been a full integration of statewide and ur- ban models. Integration is easier to achieve in small states where there are only a few metropolitan planning organizations in close proximity or one that spans the state. Statewide models defer to urban models within urban areas. • There has been little progress in integrating statewide models with national models, particularly the Freight Analysis Framework (FAF). This review identified two particular issues that are limit- ing progress in statewide model development. • Many databases are organized by county or other spa- tial units that are too coarse. • More experience is needed with modeling multiday, long distance trip making. The following innovations in both statewide and national modeling may lead to better planning practice. • Some states use nested zone structures to better tailor the level of spatial aggregation to the needs of a given model step. For example, several states have imple- mented subzones during traffic assignment to eliminate lumpy loadings. Other states have adopted dual sets of zones and networks to model both national and local travel effects. • The full integration of freight, passenger, and economic activity offers a worthwhile direction for the next gen- eration of statewide travel forecasting models. • Some states have implemented tour-based passenger components within their statewide models. • Proposed improvements to FHWA’s FAF may enable more rapid development of statewide freight compo- nents that are more accurate and more policy sensitive. CHAPTER FOUR FINDINGS AND SUGGESTIONS FOR RESEARCH

At this time there is no pressing need for best practice standards for statewide models. In states with integrated models, the state of the practice exceeds the curricular con- tent of transportation planning graduate programs. The review of current practice supports the four principal research suggestions of the Statewide Travel Demand Mod- els Peer Exchange. These research suggestions have been previously identified as being of high priority. • Rural Area Trip-Making Characteristics. Many urban models have benefited greatly from the existence of transferable parameters for forecasting travel within ur- ban areas. Notable sources of such parameters are NCHRP Report 187, NCHRP Report 365, and the Quick Response Freight Manual (QRFM). Similar data have not been compiled for intercity or rural travel. Some statewide models have used urban parameters for rural travel, perhaps introducing an unnecessary error to forecasts. Research is needed to define trip generation rates, trip distribution friction factors, vehicle occu- pancy rates, time-of-day factors, and mode-split model coefficients. This information is needed principally for passenger travel. A potential source of much of this in- formation is the NHTS. The QRFM should be updated to include rural commercial trip characteristics. • Development of a National Passenger Travel Model. The United States does not have a national model of pas- senger travel, although it does have a national freight model (FHWA’s FAF). Currently, most statewide mod- els have networks that extend well into neighboring states and beyond. A national passenger model would go a long way toward relieving statewide planners from the burden of modeling vast areas outside their borders to properly account for external travel. The main purpose of a national model would be to obtain reliable forecasts of passenger vehicle flows between states on major U.S. highways and passenger volumes through major airports and rail and bus terminals. Local detail in such a model would not be needed. • Development of Validation Performance Standards for Statewide Models. There are well-recognized quality standards for urban travel forecasting models, but none for statewide travel forecasting models. Because statewide models tend to be coarser than urban models and because statewide models are used to study a nar- rower range of policies and project options, there is a sentiment within some states that statewide models do not need to meet strict urban standards for validation. Research is needed in these areas, as identified by the Peer Exchange. – Acceptable ranges of parameters and values used as inputs to statewide models. – Key market segments that should be addressed in statewide models. – Suitable and unsuitable applications of statewide models. – Potential sources of data to support and evaluate statewide models. – Multimodal performance standards for trip genera- tion and activity, trip length and duration, mode choice, corridor assignment, low-volume roadway assignment, rural areas and facilities, multimodal de- mand, and multimodal assignment. – Comparison of urban and statewide planning model results and sensitivities. – Estimates of the time and costs for various options. • Long-Distance Travel Data Collection. Many states found the ATS to be an invaluable source of informa- tion on long distance travel within and across their borders. However, the latest data from the ATS is now 10 years old, and although the 2001 NHTS also con- tains data on long distance, infrequent trips, the data set is limited in the number of samples and the number of trips reported. It is suggested that the ATS be repeated or the NHTS be upgraded to a comparable level of detail for long distance trip making. In addition, the state of the practice suggests that addi- tional research be undertaken in the following areas. • Improvements in Traffic Assignment. As with urban models, traffic assignment is the step closest to the re- sults that influence decision making. However, produc- ing a traffic assignment is much more difficult in statewide models because of the larger sizes of the net- works, the distance between origins and destinations, and the coarseness of zone systems. In particular, there are three issues that need further investigation. – Peak periods and traffic dynamics. In states where in- tercity trip durations greatly exceed 1 h, static traffic assignment is incapable of directly performing peak- hour forecasts. Dynamic traffic assignment can track groups of vehicles in both time and space; therefore, it potentially can estimate traffic volumes and delays for short periods of time. Because no state is cur- rently using dynamic traffic assignment, its applica- bility should be tested on full-sized networks. – Spatial aggregation. To cover the full land area of a state, zone systems have been coarse. A few states have experimented with subzones during the assignment step to remove errors associated with large zones; how- ever, more experience is necessary. Research is needed to determine the best methods for establishing sub- zones and to ascertain the potential benefits. – Speed of execution. Some newer models have very large networks, causing very slow path building and traffic assignment. There is a need for faster algo- rithms, either by writing better algorithms or by fully exploiting computer hardware. Speed of execution will be of increased concern as states adopt dynamic and multiclass traffic assignment methods. • Intermodal Freight Networks. As with urban models, many statewide models have truck networks. Networks 53

54 for other freight modes are rare, and no state has re- ported networks capable of handling intermodal freight. Additional research and experience is necessary to determine the best way to build networks for handling freight that use more than one mode. • Cost Models for Freight and Freight Modal Choice Pa- rameters. Cost is the most important factor in freight mode choice. The knowledge of mode choice for long distance freight is inadequate, principally because the costs of transporting freight are not well understood. NCHRP Report 260 contains detailed methods for esti- mating the costs of freight; however, those methods are now outdated. New research is needed to ascertain the costs of moving one ton of a particular commodity from origin to destination by a variety of competing models. Additional research is needed to determine the sensi- tivity of cost relative to other factors within the modal choice process. The effect of changing logistics prac- tices on long distance freight movements needs to be quantified. • Innovative Methods of Estimating Origin–Destination Tables from Ground Counts. Developing very large origin–destination tables for specific purposes, modes, and commodities is currently difficult because of the amount of required information and the amount of com- putation time. Better methods, suitable for highly detailed and multiclass models, are needed to find accu- rate tables that use more then simple ground counts as inputs. Such methods need to be validated for accuracy. • Better Public Source Commodity Flow Information. The Commodity Flow Survey has been an invaluable source of information on freight shipments within the United States. However, the survey does not provide complete information. Innovative methods are needed for combining existing data sources and economic models for filling in the gaps in the Commodity Flow Study. These methods need to be expressed as simple procedures that can be executed by modeling staffs at state departments of transportation. The potential of FHWA’s FAF for providing better commodity flow in- formation could be explored. • Better Information on Non-Freight Commercial Vehicle Movements. Both urban and statewide models could benefit from a better understanding of commercial vehi- cle movements that are not transporting freight. A means of acquiring such information might be a Na- tional Business Travel Survey, which would be analo- gous to the NHTS. Such a survey would also be an op- portunity to learn more about business logistics practices and supplement the information from the Commodity Flow Survey. This information could also be helpful for developing default commercial trip making characteris- tics for an update of the QRFM. • Improved Curricula for Transportation Planning Graduate Programs. The emergence of integrated transportation/land use/economic activity models sug- gests that related topics might be elevated in importance within graduate program curricula.

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TRB’s National Cooperative Highway Research Program (NCHRP) Synthesis 358: Statewide Travel Forecasting Models examines statewide travel forecasting models designed to address planning needs and provide forecasts for statewide transportation, including passenger vehicle and freight movements. The report explores the types and purposes of models being used, integration of state and urban models, data requirements, computer needs, resources (including time, funding, training, and staff), limitations, and overall benefits. The report includes five case studies, two that focus on passenger components, two on freight components, and one on both passenger and freight.

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