Agenda - 02-16-1988 - Laserfiche WebLink

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The trip distribution model is referred to as the Gravity model because it distributes trips between zones in manner analogous to Newton's Law of Gravity, • i.e., the number of trips between any two zones is directly related to the size (in number of trips) of each and inversely related to the distance between them. The result of the trip distribution phase is a trip table which includes numbers of vehicle trips between all 645 zones and stations for each of three internal trip purposes (Home-based Work, Other Home-Based, and Non-Home-Based) and two external trip purposes (External-Internal and Through). The trip assignment model assigns the trips from the gravity model to the street and highway simulation network, which is comprised of 3000 individual links for which speed, distance, capacity, and other significant characteristics are identified. Minimum time paths are calculated for all of over 415,000 possible combinations of zones and stations. Although a variety of assignment techniques are possible, the most efficient method loads all zone-to-zone traffic on the minimum time paths, thereby reflecting where traffic would likely go if congestion were not a consideration. Automated capacity-restrained assignment techniques, while more accurate, are very difficult --- requiring all 415,000 time paths to be recalculated several times, as traffic is diverted from preferred routes to less-preferred routes. Manual traffic reassignment techniques efficiently address the shortcomings associated with the model's enormity in those few areas (such as Chapel Hill/Carrboro's Town Center and near Duke Medical Center) where non- capacity restrained techniques are problematic, though raw model output must be adjusted before comparison to actual traffic. By calibrating the model's synthesized traffic patterns to actual 1985 traffic, various parameters of the model are adjusted until predictive accuracy is within plus or minus ten percent. The corresponding land use data for the design years (2000 and 2010) are then similarly transferred into the model to arrive at projected travel patterns and volumes on various networks. Networks tested with 1985, 2000, and 2010 traffic include; the Existing Plus Committed (E+C, the existing road system plus projects currently programmed for construction), the currently adopted thoroughfare plans (as fully implemented), and the adopted plans plus potentially beneficial improvements. The resulting deficiency analyses of the E+C and Adopted Plan networks were discussed in Report #1. In the latter alternative networks, each area of deficiency in the adopted plan network was analyzed individually to determine what new or improved facility would reduce or eliminate the anticipated traffic problems. -The alignment for each new or improved facility was analyzed to determine environmental and social impacts, physical constraints and costs. Also, the potential impact of demand reduction strategies such as alternative modes of transportation was considered. These alternatives are discussed in Chapter 4. 3.2