letters to the editor/AUTHORS' REPLY

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

DEAR EDITOR:

As a transportation consultant who is involved in both the performance and the review of traffic studies, my colleagues and I at McMahon Associates, Inc. are extremely concerned that the August 2006ITE Journal article entitled "Trip Generation Characteristics of FreeStanding Discount Superstores" lacks the rigorous scientific analysis and thoroughness that we have come to expect in ITE Journal articles.

As such, although ITE Journal states: "Opinions expressed herein are those of the authors and do not reflect official ITE or magazine policy unless so stated," the article may be utilized by transportation professionals and others as "gospel" even though its analysis is flawed, in our opinion, in many respects.

Our review of the article has raised the following questions and concerns, which we believe must be answered by more scientific sampling and analysis before, as the author states: "Transportation professionals should consider using the trip generation rates documented in this study when analyzing free-standing discount superstores that most closely match the characteristics of the stores included in this study as compared to the stores used to supply data for ITE Land Use Code 813."

1. It appears that the author uses different baselines for store size in the five samples relative to inclusion or exclusion of gas station square footage and gas station trips.

2. Additionally, the square footage of a gas station is not a good choice for independent variable, as compared to the number of fueling positions, when determining its estimated trip generation; i.e., a 225-square-foot building could serve four fueling positions or 14 fueling positions.

3. Because gas pumps are significant traffic generators of themselves, the effects of internalization should be studied separately if these sites are to be included with sites without gas stations, or separate rates should be developed for the two baselines: a) without gas pumps and b) with gas pumps, until it is proven that the rates are statistically similar. As an example, if properly interpreting the authors samples 1, 2 and 4 as constituting "without gas pumps" and samples 3 and 5 as "with gas pumps," the author's p.m. peak-hour average rate of 5.50 trips per 1,000 would have broken out to 5.17 without gas pumps versus 6.00 with gas pumps. That difference alone (0.83), from the author's limited (five) samples, would result in a difference of 183 peak-hour trips for a 220,000 square foot store, which is not insignificant.

4. Additionally, the Trip Generation Handbook, 2nd Edition currently recommends a weekday street peak-hour pass-by trip rate of 28 percent for ITE Land Use Code 813 (free-standing discount superstores), while it recommends a higher, 42-percent pass-by trip rate for gasoline/service station, ITE Land Use Code 944. Therefore, superstore "new" trips would be computed differently without the gas pumps component versus with the gas pumps component. That difference needs to be explored and quantified so that appropriate pass-by trip rates can be applied to superstores over 200,000 square feet with gas pumps.

5. We also question whether the author confirmed, in her comparison to the ITE Land Use Code 813 rates, that the latter (ITE) square footage baselines are the same as she assumed, especially with regard to the garden center, which typically has significant (15,000 to 20,000 square feet) square footage. While we agree that the rates should be applied to "total" square footage, inclusive of a garden center, it is our understanding that the ITE samples were largely (or totally) based on building foundation square footage, not inclusive of outside garden centers. Our observations about baselines and "with and without gas pumps" are intended to reinforce our opinion that the author's analysis appears to be an "apples to oranges" comparison rather than "apples to apples."

6. As the primary basis for her conclusions, the author compares her results for her five samples over 200,000 square feet to the ITE Land Use Code 813 average rate of 3.87, which includes samples below the specified 200,000 square feet size. Yet two of ITE s data points are over 200,000 square feet in size: 202,000 square feet and 210,000 square feet, respectively. The average p.m. peak-hour rate for these two ITE data samples is approximately 4.18, regardless of the ITE baseline, which is much lower than the author's 5.50 average rate. If added to the author's samples, the author's average rate would have dropped to 5.12, which again attests to the need for more rigorous sampling and analysis because of the variability in results.

7. There is also a fairly large discrepancy between the number of vehicle trips collected between different days at some of the supercenter locations. Site 3 shows an increase of almost 17 percent in site traffic between the day 1 and day 2 counts. The increases in site traffic between the day 1 and day 2 counts at site 1 and site 5 are both about 10 percent. The fluctuation in these counts suggests that there could be flaws in the data or that other factors may have been involved in the traffic generation of the site on one or both days of the counts. These discrepancies may reflect seasonal variations, as the article indicates that the first weekday count was taken in July while the second count was taken in October.