letters to the editor/AUTHORS' REPLY

Institute of Transportation Engineers. ITE Journal, May 2007 by Plourde, Rodney P, Vivian, Georgiena M

8. We do not agree that the author complied with ITE trip generation procedures as stated and, specifically, within Tables 3 and 4 of the article regarding the use of regression equations versus rates as referenced in the Trip Generation Handbook, 2nd Edition (page 9, Chapter 3). The handbook indicates that a regression equation should be used only if the independent variable (store size) is within the range of the data points of the equation (one sample, at 223,000 square feet, was clearly not); or alternatively, if there are more than 20 data points (there are only 10); the R squared is equal to or greater than 0.75 (it is only 0.57); and standard deviation is greater than 110 percent of the weighted average rate (it is not).

9. We also take issue with the author's statement that "free-standing discount superstores intuitively should have a higher trip generation rate than free-standing discount stores, which by definition do not contain a full-service grocery store but have most of the other amenities of the superstore." Are not shopping centers evidence that larger stores, with presumably more services or products in one location, result in documented lower trip rates, because customers shop longer and their shopping needs can be accommodated in fewer trips due to greater availability of goods and services? In fact, the author's argument is shown not to be the case in Table 1 of the article, where the author's own comparisons show that, as retail store sizes become larger and more services/products are offered, trip generation rates decrease. We also note that the number of samples for ITE free-standing discount store (47) and ITE shopping center (407) is large enough so as to make these land uses' rates statistically more reliable than ITE's rates for free-standing discount superstore (10 samples) or the author's study (five samples).

In conclusion, while the author's study and article adds to the body of knowledge on trip generation characteristics of superstores in excess of 200,000 square feet, its data and analysis of that data, we submit, are not rigorous or conclusive enough to support the article's recommendation that the rates derived from the author's analysis should be used as the future norm for 200,000 square-feet-plus superstores. Until such time that more samples are collected (we would recommend at least 20); preferably from various locations in the country, as she also recommends, to test geographic differences, if any; and are computed on common baselines first (separately, without, or with gas pumps) before combined (i.e., if not statistically different), we suggest that the jury is still out on the validity of this article's rates, conclusions and future use.

Rodney P. Plourde, Ph.D, P.E. (F)

President

for McMahon Associates, Inc.

Fort Washington, PA, USA

AUTHORS' REPlY

I would like to thank the commenter for his interest in the article, but we disagree with many of his opinions regarding the details of the analysis conducted by VRPA Technologies, Inc. In particular, we do not agree that the analysis is "flawed." The analysis was conducted according to ITE's published guidelines for trip generation studies. Responses to various comments are detailed below. In addition, we do not agree that the article will be utilized as "gospel." While some practitioners may utilize the results directly in their work, they will have full access to the required information to make an informed decision. The comments on the article and these responses should help in adding to the base of knowledge available on this topic.