Development of a procedure for the statewide distribution and assignment of truck commodity flows: a case study of Iowa
The purpose of this research is to develop a procedure for statewide planning of truck commodity flows and apply it to the State of Iowa. The methodology utilizes available relevant data sources at the state level and state-of-the-art freight transportation planning tools. A case study consisting of two manufacturing sectors and a simplistic transportation network is developed for Iowa to demonstrate the procedure. Difficulties encountered in the modeling process are identified and categorized by cause into modeling capability or data related. Data deficiencies having the greatest impact on the modeling process and the accuracy of the results are identified. Some methods to improve freight data are offered as are estimates of the effort entailed in improving and developing new data;The procedure for the statewide distribution and assignment of truck commodity flows provides a practical tool for state level freight transportation planning. The procedure examines major commodity movements on dense corridors. The general scheme of the methodology is to: (1) identify major commodities shipped in the state; (2) identify producing and attracting zones of the major commodities in the state; (3) estimate truck shipments between origin-destination pairs; and (4) assign estimated truck trips onto the primary highways within the state;Analysis zones within Iowa represent counties, while external analysis zones represent states. The total zonal freight tonnage generated is estimated using socioeconomic indicators, employment and population. Produced manufacturing freight is correlated with employment rates. Attracted freight is allocated to industrial inputs by employment and to consumption using population size. The truck freight share is estimated as the total freight generated less the freight tonnage shipped by rail. A gravity model is used to distribute the estimated truck tonnage among major origin-destination pairs. The impedance factor in the gravity model is equal to the inverse of travel time on links. Estimated truck tonnage is converted to vehicle trips using typical vehicle equivalent weights. Truck trips are assigned to shortest paths calculated using a tree building algorithm. Estimated truck trips are validated against truck counts on selected highway links.