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Guidelines for Network Representation of Transit Access - State of the Practice Summary


SG Associates, Inc. and Volpe National Transportation Systems Center




Determining the impedance for any possible travel choice involves creating a network--a schematic representation of the travel facilities available, associating an impedance with each segment of the network, finding reasonable paths through the network from trip origin points to trip destination points, summing the impedance on each link in a given path to determine the origin to destination impedance over that path, and in some methods allocating travel over a set of feasible paths to assess origin to destination impedance. The resulting impedances and, hence, the impedance values used to estimate and apply travel demand models, depend directly on the network structures--the ways in which real world transportation systems are represented for computational analysis. The methods used for network representation have evolved in response to the needs of travel forecasting models. The introduction of public transit issues into travel demand forecasting created a need for more complex network representation. The choice of transit access mode is influenced not only by travel time, but also by factors related to traveler characteristics, trip purpose, and available transit modes. These factors cannot be treated fully by network representation and must be incorporated in the broader travel choice modeling framework. Some of the methods developed and applied by analysts involved in transit planning and travel demand forecasting are documented in this report. The report is not intended to describe procedures applicable in all situations, but to identify and document methods that have been found useful in prior experience.

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Publication Date

June 1, 1988 (1988-06-01)

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