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DRAFT November 21, 1995 C. Travel Demand Modeling • Travel demand modeling is the process by which socioeconomic data is changed to trip making dat in order to determine the ability of a roadway network to handle projected demands. The general practice is to follow a standard methodology of trip generation, trip distribution, modal split and trip assignment. Modal split analysis has a goal of estimating the effect of transit usage on the total number of vehicle trips placed on the network. In the case of Elk River, we conservatively estimated that transit usage would have a negligible effect - that by far the preferred means of transportation would remain the personal vehicle, as it is today. Many factors can influence the share of total person-trips that transit enjoys, including gas prices and fuel availability, transit routes offered, traffic congestion and "social desirability" of transit. By being conservative and estimating a limited impact of transit, we are not suggesting that it would not or should not play a significant role in Elk River. On the contrary, a good system will reduce energy dependence and traffic congestion. This is further discussed later in the report. As was shown in Figure 1, the travel demand modeling process was broken into two parts. First, the existing transportation system usage was replicated using existing socio-economic data. Trip generation, distribution and assignment calculations were adjusted as necessary to create a model that allocated traffic to the existing roadway network such that assigned volumes were as close as possible • to actual measured volumes. This was done to validate the travel behavior factors that would be used in the ultimate forecast. When enough adjustments had been made to arrive at a reasonably close replication, the socio-economic forecasts for ultimate development were used to create an ultimate travel demand forecast. The following sections describe the components of the process in greater detail. 1. Trip Generation In the trip generation process, generation rates are applied to the chosen socio-economic parameters to estimate the number of vehicle trips to and from each TAZ to be placed on the roadway network. A trip generated may be either a trip into or out of a TAZ; generally, no distinction is made when dealing with Average Daily Traffic (ADT) volumes. Rather, the reasonable assumption is made that in a given 24-hour period on a weekday, the total number of trips into and out of any TAZ is approximately equal. This is not the case in a peak hour analysis, where trips into and out of a TAZ can differ greatly depending on the types of generators it contains. The Elk River study was done using ADT volumes. The general reference used in determining trip generation rates was the 5th Edition of the Institute of Transportation Engineers Trip Generation • Manual, published in 1991. Information on generation rates for various land uses was obtained from this document and modified according to specific characteristics of the City of Elk River. CTP-230.156 -4- 230-156-80