When traffic engineers do a study for a new development, one of the first steps is forecasting how much traffic will come in and out of the development (jargon alert: this is termed trip generation). This forecasting is typically done by using data from the Institute of Transportation Engineers’ informational report, Trip Generation. This is the largest database of measured trip generation by land use type, with the current edition (7th) containing more than 4,250 trip generation studies. The data has been collected for several decades throughout the United States. Some land uses have a very robust data set, such as Single Family Detached Housing which has data from 350 studies. Other land uses have a minimal data set and should be used with extreme caution, such as a Synagogue which has data from 2 studies.
Although Trip Generation is the best database available, it is a tool that requires interpretation. A good example is the land use Pharmacy/Drugstore with Drive-Through Window. It contains data largely from stores with one drive through lane. There is data in the set from only one pharmacy that had two drive through lanes, which is a concern since nearly all pharmacies built today have two drive-through lanes. I performed a trip generation study for pharmacies with two drive through lanes in 2004 and found the trips generated by a pharmacy with two drive through lanes were nearly twice the trips calculated by using the average trip rate in Trip Generation (which was largely based on pharmacies with the single drive through lane). I submitted this data to the Institute of Transportation Engineers for their use in the upcoming 8th Edition of the Trip Generation report.
Traffic engineers must spend time reviewing the trip generation data sets at the beginning of any traffic study to make sure their forecasting is technically sound. As with any engineering study, "engineering judgment" is involved when interpreting the data set.
