A traffic assignment planning model: The load-node concept

Abstract

The use of the traffic assignment model in urban transportation planning is universal; however, traffic assignment in its present form is expensive, cumbersome, and has serious limitations. In this research the authors have developed and tested the load-node concept of traffic assignment which reduces many of these problems. In the load-node concept the corners of zone boundaries are used for points of trip origins and destinations. In this manner the zone centroids and associated connector linkages of the network are removed. This results in a substantial reduction in network coding. In this study the total number of nodes required to define the network decreased by almost 3000 to 995 and the number of links in the network decreased by two-thirds. The forecasted 1990 Phoenix metropolitan area was used to test this approach and the average link volumes of all-or-nothing capacity restraint assignments using both the load-node and zone centroid concepts formed the basis of the comparison. The analyses indicated the average link volumes of the two concepts are comparable. A probable reduction in computer time of 40 + per cent was obtained when the load-node concept was used. The authors suggest that this approach is primarily adaptable for planning purposes and not design. With respect to the design use of traffic assignment results, the travel between adjacent zones is seldom included in the major arterial street and freeway traffic volumes obtained from zone centroid assignment. This may result in a substantial underestimation of future traffic volume and turning movements particularly if stepped down 24-hr assignments are used for design purposes. The load-node approach does assign travel between adjacent zones.