Cover Image

Not for Sale

View/Hide Left Panel
Click for next page ( 30

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 29
29 and analyses. One of the better known firms is Global 6.2 The Origin-Destination Insights, formerly DRI/WEFA. Global Insights provides Factoring Method national, regional, state, Metropolitan Statistical Area (MSA), and county-level macroeconomic forecasts on a contract or Description subscription basis. Variables forecast include gross domestic As shown in Figure 6.2, the O-D factoring method can use product, employment, imports, exports, and interest rates. the conventional mode split and assignment model compo- Global Insights also produces short-term (two-and-one-half nents. The O-D factoring method uses an existing and factored to three-year) and long-term (20- to 25-year) industrial input O-D table of freight as input to mode split and assignment, and output forecasts for 250 industries (two-, three-, or four- rather than a table prepared by trip generation and trip distri- digit SIC code). Industrial inputs include employment, en- bution model components. ergy, and materials used in production. These input/output The acquisition and factoring of commodity O-D tables is forecasts are updated semiannually. Price and wage indices widespread. States and the FAF have generally used Reebie also are forecast for 650 different industries. Associates' TRANSEARCH database as the source O-D table. While some efforts have been made to use an O-D Matrix Estimation process, the required observations on links of Case Studies and References exclusively freight vehicles are rare and the tables produced Two case studies demonstrate the truck model: the Min- by this method are usually exclusively based on truck counts nesota Trunk Highway 10 Truck Trip Forecasting Model and that include freight and nonfreight trucks. The Bureau of the Heavy Truck Freight Model for Florida Ports. These are Transportation Statistics' CFS, while available publicly, does described in Sections 8.2 and 8.3, respectively. not provide geography below the BEA's Economic Areas, The Guidebook on Statewide Travel Forecasting discusses which are insufficiently detailed for statewide freight fore- time series methods for direct forecasts of vehicular volumes casting. Based on certain economic indicators, some states on highway and for forecasting the inputs to four-step mod- have successfully disaggregated Commodity Flow Survey data els.5 Major emphasis is on ARIMA models and on growth to the county level, but this process is costly and time con- factor methods. Examples are primarily for passenger car suming. By using the TRANSEARCH database and its rout- forecasting, but the methods are equally applicable to truck ing options, states can develop freight network assignments forecasting. The Guidebook also describes a linear regression even in the absence of an existing state model network. This model to forecast truck volumes on I-40 in New Mexico. is particularly useful given the lack of rail assignment models. Commercial truck traffic was found to be a linear function of Growth rates applied to the existing O-D tables can be the year, the U.S. disposable income, U.S. gasoline costs, and based on economic, employment, or other indicators of the New Mexico cost of residential construction. growth at the zonal level and are often developed by using Figure 6.2. The original destination factoring method.

OCR for page 29
30 simple economic models. These zonal growth factors are usu- volumes rarely provide information on the contents of the ally applied to tables through an iterative proportional fitting truck or purpose of its trip. Only in instances in which 1) all technique that balances production and attraction growth trucks can be assumed to be carrying freight, and 2) a break- rates. This technique, known as "Fratar factoring," is usually down by commodity is not desired can the method be used. available in travel demand model packages (TP+, EMME/2, In urban and suburban areas, freight trucks are only a portion TransCAD). Many states and the FAF have purchased freight of all observed trucks. According to the Federal Highway forecasts directly from consultants who produce the eco- Administration's FAF web site, the freight truck percentage nomic forecast and also update the O-D commodity tables. of VMT varies from 1% to 6% by urban area, and the total In most applications different growth factors are applied for truck percentage (including nonfreight trucks) ranges from each commodity. 5% to 18% by urban area.11 The choice of mode split depends on the availability of Due to the limitations of surveys and Origin Destination variables. Because mode split is usually most pronounced for Matrix Estimation techniques, most statewide O-D factoring distances over 500 miles, a source of impedances outside the methods acquire existing trip tables from public or private state is needed. If a national network is used, the utility of sources. travel between zones, such as times and costs, can be obtained from the model. Otherwise the national utilities must be acquired from other sources. Factoring the O-D Freight Table The network assignment component depends on the avail- The existing O-D freight trip table can be assigned to trans- ability of other data and is not limited by the O-D factoring portation networks to produce estimates of existing facility models. O-D factoring models can always use a rules-based flows. To produce future flows it is necessary to factor the assignment component, depending on the ability to convert table to obtain an estimate of O-D freight flows in a future a tonnage table to truck trips. They can also use a modal net- year. The factoring of O-D tables through an Iterative Pro- work assignment process that either excludes other automo- portional Fitting or Fratar process is an established practice bile or nonfreight traffic or recognizes this traffic only as in transportation planning. In this class of models the differ- preloaded volumes. Depending on the availability of com- ence is the source of the growth factors and the party that plete auto and nonfreight truck trip tables within a complete does the factoring. statewide model, the commodity freight trucks can be A state transportation agency that has obtained economic assigned simultaneously with these other tables to allow the growth factors that apply to specific industry or commodity analysis of congestion. origins and destinations may choose to factor the table itself. However, an agency that has obtained the factors from an Obtaining a Current O-D Freight Table economic model provided by a private firm may find it advantageous for that firm to factor the table as part of the In order to factor an O-D table of freight flows, there must economic model. be an existing table of freight flows. There are three means of obtaining existing freight O-D tables: Common Model Components Acquire a trip table from a public or commercial source; Mode Split Develop a trip table from a survey of freight shippers, receivers, and/or carriers; or The O-D freight table can be processed by a mode split Estimate a trip table from observed freight flows. equation as described in Section 4.4. If network information is available to provide utilities for movement between origins In practice, acquiring a trip table from public sources, such and destination by mode, and the coefficients of a sophisti- as the CFS, or from private sources, such as TRANSEARCH, cated mode split model have been developed or transferred are the most practical options. from another setting, then that mode split model can be used Given the diversity of geographic and commodity cover- as part of the O-D factoring class of models. age, the cost for a state to conduct sufficient surveys to develop a statistically reliable and sufficiently detailed O-D Network Assignment table would be prohibitive. Such surveys more often are con- ducted to develop the parameters in other model steps. The assignment of a trip table in an O-D factoring model Estimating a trip table from observed freight flows involves can use a variety of options as described in Section 4.5. Rail, the use of O-D Matrix Estimation techniques.10 The observed water, and air assignments typically follow the rules-based freight flows in most applications are truck volumes. Truck assignment process. The assignment of truck freight depends