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Using Global Positioning System Data to Inform Travel Survey Methods Stacey Bricka, NuStats Partners, LP Chandra R. Bhat, University of Texas at Austin While the transportation community continues to work GPS has been used in 12 regional travel surveys: Lexington, toward the long-term goal of using Global Positioning Kentucky (1996); Austin, Texas (1997); California (2001); System (GPS) technology to produce higher-quality trip Los Angeles, California (2001); Pittsburgh, Pennsylvania files, the reality is that the current method of random (2001); St. Louis, Missouri (2002); Ohio (2002); Laredo, samples, telephone surveys, and travel logs continues to Texas (2002); TylerLongview, Texas (2003); Kansas City, be used. Thus, for any given regional travel survey, trip Missouri (2004); Reno, Nevada (2005); and in a pilot test underreporting will occur at some level. The research for the upcoming Oregon statewide travel survey (2005). In question that forms the focus of this paper is whether an addition, other GPS studies not directly linked to regional analysis of GPS data collected as part of a regional travel travel surveys have employed GPS for speed studies and in survey can be used to minimize trip underreporting testing the development of trip tables solely from GPS data. through improved survey methods. The focus is on For purposes of this paper, references to GPS studies refer demographic characteristics, travel behavior characteris- to those conducted as part of regional travel surveys only. tics, and indicators of adherence to survey protocol that In the conduct of these studies, several important facts have potentially impact trip underreporting. The results sug- been gleaned: gest that, while more research into this subject is war- ranted, there are specific, low-cost changes to the survey Respondents who self-select to participate in GPS materials as well as to the interviewing process that can travel studies are different from those who do not elect be made immediately to reduce trip underreporting. to participate. As documented in several travel survey reports, GPS participants tend to report higher incomes and own their own homes compared with those who T en years ago, the transportation community began elect not to participate (see, for example, NuStats). Thus, in earnest an investigation into the application of most of the findings to date and conclusions about trip Global Positioning System (GPS) technology to the underreporting are based on a select group of respon- collection efforts for travel survey data. The immediate dents and not general populations of entire regions. focus of this technology application has been to improve The methods used to process the GPS data streams the quality of travel survey data, with a long-term goal of vary across the GPS studies conducted to date and influ- eventually replacing respondent-reported data with travel ence the degree of trip reporting detected. Some studies, details collected passively through GPS devices. The main such as the Los Angeles study, used in-vehicle devices to application of GPS in regional travel surveys to date has capture trip details for both drivers and passengers, while been for auditing trip reporting, to determine the level of others focused only on drivers. In addition, as shown in trip underreporting by vehicle drivers, and to develop an early analysis of the Austin data, the time thresholds appropriate correction factors for the data. Specifically, used in vehicle movement detection can cause the trip 89