Agenda - 05-19-1987 - Laserfiche WebLink

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Trip Distribution The trip distribution model is referred to as the Gravity Model because it distributes trips between zones in a manner analogous to Newton's Law of Gravity, i.e., the number of trips between any two zones is directly related to each's size (productions and attractions) and inversely related to the distance between them. The trip generation phase provides the size information. The distance information is provided by the development of an analysis network which is a computerized facsimile of the study area's street and highway system. From this network, the computer calculates the minimum path (and related travel time) between any two of the area's 600 zones and 45 stations. An important part of the process of calibrating the model includes developing travel impedance curves for each trip purpose which control the length of trips distributed. If the average trip length is too long, the network will be over-assigned with trips going too far and vice-versa. To achieve calibration, the average trip length was about ten minutes. The result of the trip distribution phase is a trip table which includes numbers of vehicle trips for all possible combinations of zones and/or stations for each of five trip purposes (Internal - HBW, ORB, & NHB, External-Internal, and Through). Trip Assignment The trips are assigned to the network in a variety of ways, some of which consider congestion and alternative routes and some of which don't. Both techniques provide useful information to the analyst. Knowing where travel desires to go if congestion were not a consideration emphasizes preferred routes whose integrity should be safe-guarded. The consequences of not providing sufficient capacity on a particular major route can be assessed by considering congestion in the assignment technique. Knowing to where traffic would shift to avoid congested facilities helps to identify measures to protect neighborhoods from cut-through traffic resulting from inadequate major thoroughfares. The model was calibrated to 1985 conditions by comparing the models synthesized traffic patterns to those actually measured and adjusting several of its parameters until these patterns sufficiently matched. The corresponding landuse data for the design years (2000 and 2010) were then similarly transferred into projected travel patterns for use in the later stages of this study. Chapter 4 discusses the development of these landuse projections. Chapter 5 discusses the related travel patterns and assesses the deficiencies in the area's thoroughfare system and current plans for its improvement. 3.2