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11 BACKGROUND The prediction of air passenger and airport employee mode choice decisions for their travel to and from the airport forms a key analytical component of airport landside planning, as well as airport system planning. There is currently however no well-accepted and validated process for modeling how airport users will change their access or egress mode in response to changes in the airport ground transportation sys- tem (such as changes in fares, rates, or service levels) or the introduction of new modes (such as the extension of a light rail system to the airport). The factors affecting airport travel are recognized as being significantly different from those affecting typical trips accounted for in regional transporta- tion models, as a result of both the characteristics of the travel party and the timing and duration of the trip, as well as the differences in available modes and services. Therefore, it is difficult to determine the economic feasibility of proposed projects to improve airport ground transportation or effec- tively manage the existing airport ground transportation sys- tem using traditional regional models. Travel forecasting models, particularly those developed to address airport ground access and egress trips, are highly specialized and not well understood by most airport man- agers and planners. With increasing emphasis on intermodal connections, there is a pressing need for more widely accepted and accessible reference material and information on such models. This synthesis project was undertaken to update and extend previous efforts to document the state of practice for airport ground access mode choice models. It also examines the characteristics of existing models and dis- cusses the issues involved in the development and use of such models to improve the understanding and acceptance of their role in airport planning and management. It is hoped that the report will also serve to focus research and develop- ment efforts on improving the state of the art of modeling air- port ground access mode choice by assembling a detailed database of comparative information on recent model devel- opment efforts and identifying technical issues that are not currently well handled. In contrast to the current state of practice of urban trans- portation modeling, which has been the subject of study and improvement by a large body of practitioners and researchers for many decades, airport ground access mode choice model- ing has received only limited attention and relatively little funding. As a result, no clearly recognized body or practice has emerged and relatively few people have worked in this area long enough to develop significant expertise. Typically, airport access mode choice models have been developed by transportation modeling specialists, airport planners, or acad- emic researchers who are mainly concerned with other issues. Part of the reason for this is that opportunities to develop such models occur relatively infrequently. At the same time, mod- eling airport ground access mode choice is arguably more complicated and challenging than general urban travel. First, airport ground access usually involves more transportation modes than are typically considered in general urban travel demand models, which are commonly restricted to single- occupant automobiles, carpools and vanpools with varying occupancy levels, and conventional transit services. The air- port ground access system includes all of these, as well as other modes that are rarely considered in general urban travel models, such as rental car, taxi, shared-ride van, dedicated express bus services, and hotel courtesy shuttles, all of which can have a significant mode share at large airports. Indeed, at many airports the rental car is the largest single mode after private vehicles, and the other shared-ride transportation modes typically carry more passengers than transit. Second, the circumstances surrounding airport access trips by air passengers are very different from other types of urban travel. A high proportion of travel is by visitors to the region; passengers travel in groups more often than for other urban trips, they make airport access trips far less frequently than other urban trips, they commonly have luggage, and they are often away for several days or even several weeks, which affects the cost and feasibility of parking a car at the airport while they are away. Picking up and dropping off air passengers at the airport with a private vehicle is one of the most common access and egress modes. This generates twice as many vehicle trips to and from the airport as air party trips owing to the round trip involved in either dropping off or picking up air travelers. These factors greatly complicate the issues that need to be considered in developing airport access mode choice models. METHODOLOGY A comprehensive review of the relevant literature in the field was undertaken. This was supplemented by an exten- sive survey of airport authorities, metropolitan planning CHAPTER ONE INTRODUCTION
organizations (MPOs), consulting firms and research orga- nizations, and other government agencies and industry organizations to document the extent of the recent use of airport ground access mode choice models and to identify sources of technical information on existing models. Based on the responses to the survey, technical documentation on existing models has been assembled and reviewed, supple- mented with follow-up communication by telephone and e-mail where necessary. The findings of this review were then assembled into a project database that was used to prepare summary descriptions of each model in a standard format. The survey of airport authorities and other organizations was undertaken to complement the review of published liter- ature as well as to obtain information on the extent to which each organization makes use of formal models of airport access travel and perceives the ease of use or reliability of such models. The survey was undertaken using Internet sur- vey software hosted on the TRB website. E-mails were sent to survey recipients requesting their participation and pro- viding a link to the appropriate survey. Four somewhat different survey questionnaires were developed reflecting the different concerns and needs of the various types of organi- zations included in the survey. Survey recipients were given the option of responding using a form that could be com- pleted off-line and returned by e-mail, fax, or mail. POTENTIAL USES OF AIRPORT ACCESS MODE CHOICE MODELS Airport access mode choice decisions by airport travelers, including both air passengers and airport employees, impact a wide range of airport planning and operational manage- ment decisions, including the development of landside facilities, airport revenue from parking and other ground transportation services, and programs to reduce the growth in vehicle trips generated by the airport and the associated emissions. Potential uses of models that can predict the effect on access mode use of proposed changes to the system include sizing new planned facilities, evaluating the finan- cial implications of proposed changes in parking rates or other ground transportation fees, determining the expected air quality impacts of either planned new facilities or pro- posed mitigation measures, and assessing the feasibility of proposed projects to improve airport access. Airport acces- sibility is also a major determinant of airport choice in multi- airport regions or situations where air service competition exists between local and more distant airports. Airport access mode choice is often embedded within models of air- port choice, because the availability and use of different access modes affects the perceived accessibility of different airports, and improved representation of airport access mode choice behavior should benefit those applications as well. Several of these potential applications are discussed in more detail in chapter two. 12 REPORT AUDIENCE The information presented in this report may be of interest to a range of airport managers and planners, transportation planning professionals, researchers, and others interested in modeling airport ground access mode choice. Airport managers may be interested in obtaining a fairly high-level understanding of the current state of practice and an appreci- ation of where and how such models can be used, as well as what is involved in using them. Such readers may find the executive summary sufficient or they may prefer to go into more detail in selected sections of the report, particularly the description of the mode choice modeling process in chapter two. Airport planners and regional transportation planners who are more directly involved in airport landside planning or regional travel demand modeling but do not have exten- sive experience with airport access mode choice modeling may find the description of the mode choice modeling process in chapter two of interest, as well as the technical details of existing models in the following chapters. Regional transportation planners may find the discussion of the role of airport ground access mode choice modeling in the regional travel demand modeling process in chapter eight of particu- lar interest. Consultants and transportation modeling spe- cialists who have prior experience with airport access mode choice models may find the comparative discussion of the various existing models of interest, together with the review of their strengths and weaknesses contained in the detailed case studies in Appendix D (which is found in the online ver- sion only). Finally, researchers interested in issues involving airport ground access mode choice may be interested in the discussion of the aspects of the current state of practice that are not well understood and the associated suggestions for future research in this topic that appear throughout the report. REPORT ORGANIZATION The remainder of this report consists of eight chapters and four appendixes. Chapter two describes the airport access mode choice modeling process and discusses a range of issues that arise in modeling airport access mode choice. Chapter three reviews the published literature on air passen- ger airport access mode choice models and addresses airport employee mode choice for journey-to-work trips. Chapter four reviews the use of airport ground access mode choice models in airport planning based on the findings of the survey of airport authorities, regional and state planning agencies, and other organizations. The chapter examines the extent to which formal models of air passenger or airport employee ground access mode choice have been used in air- port planning studies, as well as how airport access trips are addressed in regional transportation planning models. Chapter five presents the current state of practice of air passenger ground access mode choice models and forms the core of this synthesis review. The chapter discusses the type
13 of planning issues to which the models have been applied, the technical approach adopted to represent air passenger choice behavior, the ground access modes included in the models, the explanatory variables used to model the transportation system and air passenger characteristics, and the market seg- mentation that has been used to improve the ability of the models to account for air passenger ground access behavior. It also discusses a number of considerations that arise in developing and applying these models, as well as issues of model performance. Chapter six reviews the corresponding state of practice with the analysis of airport employee journey-to-work travel. In contrast to air passenger ground access, the literature review and survey of airport authorities and other organiza- tions identified relatively few formal mode choice models explicitly developed to represent airport employee journey- to-work travel. Rather, analysis of airport employee travel in airport planning studies tends to be based on extrapolation from survey data of existing travel patterns, although regional planning studies typically use models designed for general urban travel. The next two chapters explore two issues that arise from the review of current practice described in the previous chapters. Chapter seven addresses the extent to which air passenger ground access mode choice models may be transferable to sit- uations different from the one for which they have been origi- nally developed, or whether it is necessary to custom build ground access mode choice models for every airport. Chapter eight discusses the issues that arise in integrating air passenger and airport employee ground access mode choice models into more general regional transportation planning models. Finally, chapter nine presents the conclusions of the syn- thesis of current practice and recommendations for further research. These address needed improvements in the current state of practice for air passenger ground access mode choice models as well as the development of appropriate models for airport employee mode choice for journey-to-work trips. The four appendixes provide more detailed information as background to the discussion in the body of the report. Appendix A presents the questionnaires used in the surveys of airport authorities, planning agencies, and other organiza- tions; Appendix B contains a detailed summary of the survey findings; Appendix C lists the organizations that responded to the survey; and Appendix D (provided only online) con- tains detailed technical summaries of nine of the mode choice models identified in the project.
