Mr. Shoup's article, "Truth in Transportation Planning," tends to view the Institute of Transportation Engineers' (ITE) Trip Generation, 6th edition and Parking Generation, 2nd edition reports as manuals to be followed step by step rather than as informational reports to be used to help guide transportation planning and development decisions. The intended purpose of the documents is stated in the reports. For example, page ix of the Trip Generation User's Guide contains the following:
ITE Informational Reports are prepared for informational purposes only and do not include ITE recommendations on the best course of action or the preferred application of the data.
It is important to note that Trip Generation does not represent a quick fix for transportation problems or a shortcut to planning procedures; rather, it serves as a foundation on which the professional engineer can build his or her own knowledge and experience and apply this knowledge to any given transportation-related situation. The intended users who estimate vehicle trip generation or parking demand are transportation professionals trained in mathematics, statistics, traffic engineering, and planning fundamentals and who possess engineering judgment.
ITE's reports provide a compilation of available data collected from numerous sources. In the sixth edition of Trip Generation, data are combined from more than 3,750 individual trip generation studies. This information is by no means all inclusive; however, it represents the best information available at the time of publication. ITE's Trip Generation report is updated regularly to include supplemental information as it becomes available.
Some of Shoup's commentary, examples, and assertions are directed to the fourth and fifth editions of Trip Generation. While many of these references are used to make a point, some of the discussion is not relevant as the data, assumptions, and reporting techniques are updated and improved from edition to edition. Further, we expect that transportation professionals will use the latest edition to obtain the most recent knowledge and data available.
In his article, Shoup correctly points out that reporting statistics with "extreme precision may suggest confidence in their accuracy." He also rightfully acknowledges that generation rates such as 623.12 could be reported as 623 and not affect the accuracy of the calculation. However, there are also many instances in Trip Generation where rates presented with two decimal places are appropriate at that level of precision (e.g., as a rate of 0.57 pm peak-hour trips per occupied room of a business hotel, or 7.27 weekday trips per occupied room). When developing the first edition of Trip Generation, the Trip Generation Committee wrestled with this issue of decimal placement and decided to be consistent in reporting all rates with two decimal places.
Shoup also notes that, from a statistical standpoint, some of the independent variables used are simply not related to trips (e.g., he points to an extremely low R2 value). This may be a valid point; however, in many instances the particular independent variable is chosen because it is the only information available in the early stages of development when these analyses are often undertaken. To that end, the Trip Generation User's Guide (vol. 3, p. 21) notes that: "Selecting an appropriate method for estimating trips requires use of engineering judgment and a thorough understanding of the three methodologies...."
In reference to Shoup's remarks regarding figure 4, the only independent variables available for this land use for measuring weekday trips were gross square feet and seats. We acknowledge that it is the customers and employees who make the trips, but these data were not available when the measurements were made and are rarely known when estimating proposed traffic impacts. Page 14 of the User's Guide addresses the variation in the statistics:
These variations may be due to the small sample size, the individual marketing of the site, economic conditions of the business market, the geographic location of sites studied, or the unique character of the specific site. Accordingly, judgment must be exercised in the use of the statistics in this report.
Shoup continues with a dialogue regarding ITE's advice to users to modify trip rates in response to special situations, such as the presence of public transportation service, ridesharing, and enhanced pedestrian facilities. We feel it is appropriate for ITE to point out potential cautions with the use of data without necessarily providing a solution if it cannot be supported by current research.
In Shoup's conclusion, he recommends that Trip Generation data be reported as ranges and not as precise point estimates. Current editions of Trip Generation and Parking Generation do provide ranges, average rates, and a data plot. This diversity in data presentation provides the user with a more comprehensive look at the data. Additionally, page 18 of the User's Guide provides a detailed description of a sample data page.
To produce resources supporting Trip Generation, ITE relies on the voluntary submittal of data from the transportation community. Calls for the submission of data have been ongoing over the years, with the intent to provide additional data to assist transportation professionals. ITE's openness about the availability of data can be seen on page one of the User's Guide:
In some cases, limited data were available; thus, the statistics presented may not be truly representative of the trip generation characteristics of a particular land use.
Such cautionary statements run throughout both the Trip Generation and the Parking Generation informational reports.
Trip Generation, 7th edition, and Parking Generation, 3rd edition, are slated for release in 2003. Data collected from various sources, as well as comments, including those provided by Shoup, are reviewed and taken into consideration during the revision process. ITE's intent is to provide a helpful resource that will guide transportation professionals in their decisionmaking.
Editor-in-Chief's Note: The discussants were chosen by the Institute of Transportation Engineers.
Author Addresses: Corresponding authorEugene Arnold, Senior Research Scientist, Virginia Transportation Research Council, 530 Edgemont Road, Charlottesville, VA 22903. Email: Gene.Arnold@VirginiaDOT.org.
Carl Buttke, Consulting Transportation Engineer, PO Box 2740, Hailey, ID 83333. Email: firstname.lastname@example.org.