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51 it may introduce, and how factors and policies external to its Prior to development of the models discussed here, the Port control or authority, such as vehicle traffic congestion and of Seattle had developed a parking choice model with price transit service frequency provided to PDX by TriMet (the elasticity curves to analyze the parking market shares of its one regional transit authority) influences airline passenger choices. public parking facility, the main garage, versus off-airport lots To summarize, the model results indicate that increased in the vicinity of SEA. In 2008, several analytical model steps parking rates primarily result in shifts to pickup and drop-off that led into, and interfaced with, the parking choice model modes, which double vehicle trips to PDX compared to park- were added. The sequence and components of the expanded ing for the duration of the airline passenger's trip. Increasing model set, referred to as the Composite Parking Demand parking rates would reduce the demand for parking, but it Model, are presented below. would also increase vehicle traffic congestion and emissions generated by airline passenger access and egress trips. Even a 1. Enplanement Model--The Enplanement Model and its tripling of parking rates would only increase the transit mode two submodels for originating and returning passengers share from 6% to 8%. Increasing transit service and offering were developed to provide an analytical tool for forecasting transit service free of charge, options that are not within the future volumes of these two specific groups of passengers control of the Port of Portland, would only increase the tran- that form the basis for demand for ground transportation sit mode share from 6% to 7% (this scenario is not shown in access to SEA. the table, but it was tested by the Port of Portland). Therefore, 2. Main Garage Demand Model--The Main Garage Demand improving transit options as a stand-alone strategy to relieve Model and its three submodels used to predict parking constrained parking would not appear to be a successful strat- demand for weekly, daily, and hourly parking transactions egy for managing constrained parking. (19) were developed to provide a tool for forecasting the future parking activity of three distinct groups of parking cus- Airport Parking Models tomers driven by different sets of economic and travel con- siderations. This set of three submodels was also designed to An airport operator also may develop its own predictive support estimation of the capacity and revenue impacts in tool (i.e., a model) for parking demand or for all airline pas- the main garage of different pricing scenarios and of fluc- senger modes. The model may be developed as part of the tuations in the local economy. These submodels add to parking management program, the airport ground access pro- the Port of Seattle's analytical toolkit for evaluating policy gram, or as part of a larger project, such as a master plan. The options related to the main garage. Combining enplanement Port of Seattle developed such a model to predict parking forecasts from the Enplanement Model with forecasts of the behavior under a variety of scenarios that consider pricing, various factors included in the main garage submodels sup- capacity, and other factors. ports both parking transaction forecasting efforts and sensi- tivity analysis of the effects of future scenarios involving the Seattle-Tacoma International Airport's sets of influential variables in the three submodel equations. Composite Parking Demand Model 3. Monthly Transactions and Duration Conversion Model-- In late 2008, the Port of Seattle initiated an effort to gain a The Monthly Transactions and Duration Conversion Model better understanding of the underlying causes of fluctuations is a submodel of the Composite Parking Demand Model. Its in parking demand at Seattle-Tacoma International Airport purpose is to translate the parking transactions forecast (SEA). Various multiple regression econometric models were information from the three main garage submodels into a constructed to help forecast enplanements and parking trans- detailed format conformable to the Parking Choice Model. actions in the main garage at SEA. The econometric models 4. Parking Choice Model--This model was developed to ana- included independent variables, such as per capita income, lyze data on the main garage versus off-airport parking lot population, airfare, and employment, while also taking into shares of overall airline passengers parking in the Seattle- account seasonal variations in demand and major shifts in Tacoma area. It includes detail on the main garage shares of demand due to one-time events, such as the terrorist attacks the market for parking transactions of different durations, on September 11, 2001. provides information on the main garage versus off-airport The effort concluded with the development of various mod- prices for parking transactions of different durations, and els to help forecast and strategize garage parking pricing, park- derives price response coefficients based on how much the ing demand and capacity requirements, and new program main garage share declines for longer stays as its price pre- implementation. By using the models, the Port of Seattle has mium over off-airport options becomes greater. Originally been able to gain a better understanding of the relationship calibrated for 20062007 transactions, the Parking Choice between key economic conditions and the demand for airport Model is structured so that alternative transaction data, such garage parking. as that produced by the Monthly Transactions and Duration