Click for next page ( 52


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 51
APPENDIX C Analytical Tools for Corridor Analysis Current Practice States currently including corridor analysis in the development of statewide plans were surveyed to determine the tools being used to support planning decisions. Figure C-1 shows the tools currently in use in the sixteen states responding to this question, with some states using several of the tools listed. Ten states reported using statewide travel models. These models produce forecast year traffic volumes, vehicle-miles traveled, vehicle-hours traveled, levels of service (LOS), and other variables needed to drive many of the evaluation measures used in comparing candidate projects. They are intensive in terms of data requirements and resources required to develop and maintain them. Six states use GIS-based tools to overlay future development expectations on the existing transportation system and to identify corridors that can be expected to have capacity needs. Data to support such analyses are generally available from existing sources. Several states (three to seven, depending on the tool) reported using tools originally developed to support maintenance needs analysis (i.e., pavement management and bridge condition analysis, HERS, HPMS, current LOS/traffic counting/observed traffic growth trends) to provide information (primarily traffic forecasts) to evaluate corridor needs. Data to support these tools are for the most part collected as normal operating activities of state DOTs. A total of eight states use either project scoring or benefit/cost models to evaluate projects (e.g., Arizona uses both). Four states use a customized REMI model to estimate the economic impact of system improvements. Several conclusions might be reached from reviewing the state responses: To a large extent, corridor planning seems to be oriented to find the best ways of maintaining the existing system (and/or resolving capacity deficiencies that exist now). Analytical processes seem to be oriented towards finding the best solutions to existing problems (or expected physical deterioration of the existing system). States have developed individual analysis tools focused on meeting their own planning needs and priorities (e.g., pavement and bridge management, safety, project scoring, GIS-based mapping, and statewide forecasting models). While a toolkit of analysis techniques might be useful, it could be difficult to provide tools that meet the needs of all states. States also use existing government-sponsored analysis procedures (e.g., HPMS, HERS, MOBILE 6) to develop corridor evaluation measures. These techniques are cost effective but may not provide sufficiently corridor specific or detailed evaluation measures for use as the sole basis for corridor comparison and prioritization. With the possible exception of statewide traffic forecasting models, none of the currently used tools deal with intermodal issues. 51

OCR for page 51
52 A Guidebook for Corridor-Based Statewide Transportation Planning Analytical Tool Usage Benefit/Cost Analysis Bridge Condition Current LOS GIS HERS Highway Adequacy Model HPMS Mobile 6 Pavement Management Project Scoring Model REMI Safety/Accident Analysis Statewide Demand Model 0 2 4 6 8 10 12 Number of States Figure C-1. Planning tools. Potential Analytical Methods There appear to be several areas in which standardized methods might be developed that could Provide corridor-specific estimates of evaluation measures useful to corridor prioritization; Allow better assessment of intermodal freight issues and opportunities; Make use of available or easily obtainable data; and Optimize improvement programs with respect to available resources (considering multiple funding sources and constraints). Methods or procedures might be developed to provide the following capabilities: Traffic Growth Estimation--these could include such methods as Simple trend-based growth-factor calculations to GIS-based procedures for estimating traffic growth (from observed values) as a function of growth in selected socio-economic indicators adjacent to or served by transportation corridors; Simple procedures for developing a statewide network and passenger/freight vehicle trip tables (and growing base year demand to a future year); and A summary of issues, data needs, resource costs, expected performance, and other states' experience to date with full-blown statewide travel modeling. Roadway Operations Analysis--these would include standardized procedures for estimating individual roadway capacity; traffic distribution by time of day; vehicle speed; delay and operating costs (by vehicle mix and congestion levels that are estimated to exist by time of day); accident occurrence; and noxious emissions.

OCR for page 51
Analytical Tools for Corridor Analysis 53 User Benefit Estimation--reliable procedures for estimating the benefits accrued by improve- ments to existing roadways including vehicle-operating costs and travel times, accident reduction, and noxious emission impacts. Benefit/Cost Analysis--standardized procedures for collecting, organizing, and analyzing time-series benefits and costs associated with specific transportation improvements. Freight Movement Analysis--using federally compiled FAF-2 and or commercially available data, develop procedures for displaying and analyzing modal freight flows across a state (along specific corridors). Provide a means of examining and/or reporting freight flows by commodity type, mode, and origin/destination city pairs. Investment Program Optimization--funding sources of varying amounts and restrictions on their use may be available to pay for proposed transportation system improvements. Goals for spending funds by geographic area may also be important from an equitable-distribution-of- resources perspective. Candidate projects may be proposed having different benefits, costs, and other evaluation criteria. It may be useful to develop procedures to optimize expenditures (from a project selection, timing, and/or staging standpoint) in a way that will fully expend available resources, maximize user defined benefits, and meet other political and/or geographic constraints that may exist. Figure C-2 illustrates options that are available to states for conducting some level of corridor analysis. As shown, decision opportunities are offered to apply different strategies for analysis based on the presence or absence of Multimodal options, A statewide travel demand model, or Limited resources. The chart is followed by a description and discussion of some of the analysis tools. Statewide Models Statewide travel-demand models are the most frequently used tool for corridor study appli- cations. While the survey only reported 10 states using statewide models, NCHRP Synthesis of Highway Practice 358: Statewide Travel Forecasting Models reported that 26 states have developed a statewide model and that corridor studies are the most common application of this tool. Since this 2006 report was issued, several other states have begun developing this valuable tool. If a state has developed a statewide model, then traditional traffic forecasting and assignment procedures can be used to generate the various evaluation measures needed to estimate user and development benefits. If freight movements are included in the statewide model, then multi-modal analyses can be undertaken. These would include potential diversion between rail and highway modes and throughput assessment at intermodal terminals. Statewide models can be developed at varying levels of detail and complexity. However, even the simpler models (e.g., those using matrix estimation to generate base year road trip tables and FRATAR growth-factoring procedures) can be expected to provide reasonable relative performance measures for competing alternatives. The process of adding freight movements to a statewide highway-based private vehicle (or total vehicle) model does not need to be a hugely expensive undertaking. County-to-county flow data (for both base and future years) are available through private vendors (e.g., Transearch) and public sources (e.g., FAF-2). These flows can be converted to trip tables and assigned (as separate "vehicle classes") to the road network (and to the rail network, if one has been coded). Statewide model accuracy in a given corridor can be enhanced by focused data collection programs. For example, travelers might be surveyed on existing roads in the corridor. Survey

OCR for page 51
54 A Guidebook for Corridor-Based Statewide Transportation Planning Figure C-2. Corridor analysis options.

OCR for page 51
Analytical Tools for Corridor Analysis 55 data could be expanded and used to replace portions of the model estimated base year trip table. These trips could then be grown to a future year using existing model procedures. Having an actual representation of origin-destination movement magnitudes would improve both base and forecast year analyses. States may also arrange for add-on samples to the U.S. Bureau of Census National Household Travel Survey (NHTS). These can be focused geographically to provide additional detail within a given area. The opportunity to collect this data comes at roughly 5-year intervals. Statewide models also provide planners with at least two other analysis tools: 1. The statewide trip tables can be used to estimate external travel at urban model area boundaries (or at least provide another estimate for comparison to existing external traffic estimation procedures) and 2. The models also provide the framework for determining the impact of tolls or other user fees and for examining the feasibility of new transportation modes (e.g., providing intercity passenger rail service) within a corridor. Alternatives to Statewide Modeling Many states do not currently have a statewide traffic forecasting model due to financial con- siderations and/or concerns about its utility and/or accuracy. However, there are also analytical techniques that may be employed to evaluate alternative single corridor improvements or compare corridor-versus-corridor improvements. Traffic growth estimation often is accomplished as a simple trend extrapolation from prior counts. More sophisticated procedures are sometimes used to provide additional sensitivity to adjacent area development and regional growth expectations. The combination of HPMS data and HERS analytical software provides a means of conducting several performance analyses on a state's road system. These may include Traffic growth and roadway LOS, Pavement deterioration impacts and maintenance cost estimation, Improvement cost estimation, User benefit estimation, Roadway deficiency analysis, and Benefit/cost ratio estimation and ranking. The HERS user may also conduct analyses focused on particular corridors or regions. This may be accomplished by selecting portions of the HPMS data base for inclusion. HPMS traffic growth estimates may be supplemented with truck movement data from FAF-2. Base and forecast flows of heavy trucks are available along individual roadways. These may be used to identify corridors of large truck traffic growth that may need capacity increases, require additional maintenance, or identify opportunities for modal diversion. Relatively simple GIS applications may also be used: first, to organize and display development data (e.g., population and employment, major industrial developments, major tourist areas, or intermodal transfer facilities) superimposed on the corridor, regional, or statewide transporta- tion system and second, to identify areas underserved by multi-lane, high-quality roadways or alternative modes. If underserved areas are forecast to have large existing development densities or growth in development, corridor improvements may be warranted. In the 1980s, FHWA developed a planning tool called the Highway Investment Analysis Package (HIAP). The system was a FORTRANbased series of mainframe computer programs that might

OCR for page 51
56 A Guidebook for Corridor-Based Statewide Transportation Planning be considered quite primitive given today's computer/user interaction possibilities. However, the system contained components dealing with most of the above analysis needs (i.e., traffic growth, roadway operations, benefit estimation, benefit/cost analysis and investment program optimization). The underlying procedures used in HIAP might be used to form the basis for a new group of analysis tools, as set out above. Transit Models A number of mode-choice models have been developed for transit planning purposes, partic- ularly in major urbanized areas. Such models are usually relatively expensive, largely due to the high costs of data collection and the time and effort for model validation. There are no national sources of location-specific transit data, so all data has to be gathered locally--for example, origin- destination data gathered by onboard surveys of existing transit riders. However, there are some relatively low-cost planning tools available that utilize readily available data. Emerging Tool: National Travel-Demand Model Scoping work has begun on a national model under the auspices of NCHRP Project 8-36/ Task 70, "Scoping Study for Statewide Travel Forecasting National Model." The objective of this research is to carry out a scoping study to estimate potential usefulness, scope, purposes, costs, and administration of a national travel-forecasting model that can assist states in estimating external trips. Although just the first phase of this study is underway, the national model will be very useful in providing uniform traffic flows from state to state as well as provide a large building block for developing a statewide model. States opting to forego a statewide model can still use the national model as a sketch planning tool with an available network and traffic flows that have gone through a quality-assurance process. Ideally, this tool will incorporate freight flows so that it can be used for multi-modal analysis.