The Memphis MPO is currently developing a truck component of its overall travel demand model, both to fulfill their responsibility to conduct freight planning, and because truck demand and performance of all trucks has major impacts on all travel within the metropolitan areas. While freight trucks may represent a majority of the trucks traveling to, from and through a metropolitan area, service and other trucks are a majority of trucks travelling within metropolitan areas. The methodology being used by the MPO recognizes and takes advantage of this difference to develop models using different estimation datasets.
The trucks reported in the TnDOT’s commercial TRANSEARCH database will be used to estimate trip generation and distribution equations for I-E/E-I/E-E trucks. That database was assigned on a national highway network and windowed to the Memphis model boundary. The base and forecast year external station truck productions and attractions will be as reported from the windowed TRANSEARCH. Trip generation equations will be developed for the internal ends of those truck trips. The explanatory variables will be detailed NAICS employment, developed for Traffic Analysis Zones (TAZs) and aggregated to counties for estimation. The parameters of trip distribution equations will be estimated from the widowed TRANSEARCH database, and will be applied using impedances developed by the travel demand model.
The GPS truck records reported in the ATRI database, converted to a truck trip table for, and made available by, the TnDOT, will be used to estimate trip generation and distribution equations for I-I trucks. The trip ends for these trucks, which include trucks that serve other purposes in addition to carrying freight, will be estimated using the same explanatory variables as the urban trucks in the Quick Response Freight Manual (QRFM). The ATRI database will also be used to estimate parameters for truck trip distribution. Ratios between the resulting trip rates and parameters will be compared to values for heavy trucks in the QRFM and those ratios will be used to update the medium trucks rates in the QRFM.