letters to the editor/AUTHORS' REPLY

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

In conducting the trip generation calculations, the independent variable used in the study was gross floor area of the store, including garden center and tire and lube shop, but excluding gas station square footage. Out of the total of five sites surveyed, one had a gas station on-site, one had an adjacent external gas station that was excluded from the trip generation analysis and three had no gas station. Therefore, the study allows for a consistent comparison of results.

This study allowed for the presence or absence of a gas station on the project site in order to be consistent with ITE Land Use Codes 813 (free-standing discount superstore) and 815 (free-standing discount store). Although gas station square footage is considered insignificant, it is recommended to be included in future studies to allow for a more precise definition of the independent variable.

We agree that gas station square footage is not a good choice of independent variable for a stand-alone gas station. However, when combined with a freestanding discount superstore, the square footage of the store is considered the most accurate independent variable, since the store is the primary trip generator.

Future trip generation studies may be able to add to the database to allow for separate analyses of free-standing discount superstores with and without gas stations. For now, there are not enough data available to make this distinction. Only one of the five sites included in this study had a gas station (site 3), and its gas station had only four pumps. This is not enough information to form any conclusions.

Although pass-by trips are an important consideration for traffic impact analyses of both free-standing discount superstores and gas stations, pass-by trips have no bearing on this study. All of the trips counted in this study were total trips entering and leaving the project site, including primary trips, diverted trips and pass-by trips.

There was no attempt to confirm whether the trip generation studies for ITE Land Use Code 813 included or excluded garden center square footage. However, if garden centers were excluded from previous trip generation studies, the addition of garden center square footage would increase the size of the surveyed stores and decrease the trip generation rate. This would result in an even greater discrepancy between the trip generation rates for ITE Land Use Code 813 and the recommended trip generation rates in the article.

Two of the 10 stores surveyed for ITE Land Use Code 813 had gross floor areas in excess of 200,000 square feet, even though the average of the 10 stores was 160,000 square feet. It would not be unreasonable to include the trip generation for the two stores over 200,000 square feet from ITE Land Use Code 813 with the five stores surveyed in this study to determine an average trip generation rate. This would result in an average p.m. peak-hour trip generation rate of around 5.1 to 5.2 trips per 1,000 square feet, which still is well above the average trip generation rate of 3.87 for ITE Land Use Code 813.