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92 I N N O VAT I O N S I N T R AV E L D E M A N D M O D E L I N G , V O L U M E 2 more likely to miss trips than those who use the travel decreases with age. Thus the worst trip reporters are diary, and proxy reporting leads to more missed trips. those respondents under the age of 30. The authors rec- The model results can be used to identify specific ommend that future travel surveys consider the funds to improvements in the methods to conduct future travel conduct cognitive interviews or focus groups targeted surveys. These improvements may include (a) the use of specifically toward younger drivers. The purpose of this special survey materials for respondents who travel more qualitative research would be to identify specific than usual or who are under the age of 30 and (b) devel- methodological improvements to the survey instruments oping better probes in telephone interviews when col- that would result in better capture of travel from this age lecting information from unemployed individuals, proxy group. It may be possible, for example, that this group is reporters, and individuals who travel longer-than- more impatient with the telephone interview format and average distances. Each of these potential improvements more receptive to self-reporting their travel via an is discussed below. Internet-based retrieval tool or simply being encouraged to return their logs by mail, with telephone follow-up as needed. Use of Special Survey Materials Finally, most travel survey materials are designed for persons with an eighth-grade education. However, this The empirical results from this study indicate that an study found that respondents with less than a high school important predictor of trip underreporting is the extent education are very likely to underreport their travel. This to which a respondent travels. Those who travel more finding is independent of the age effect (i.e., a continued have a higher propensity to underreport trips. This reflection of being under age 30). While most of the empirically supports the findings of prior studies, most respondents reporting the lowest education level were of which related the increased travel to heavier respon- under the age of 30 and still in high school (67%), one- dent burden (and thus suggested missed trips were the third reported ages from 32 to 82. Further investigation respondent's way of ending the survey interview early). is warranted to identify improvements in survey materi- While the relationship between respondent burden and als so that individuals with a low education level can trip underreporting is well accepted, there is another understand what travel to report and how to record the component to this relationship that should be consid- travel as part of the survey. Different approaches may ered: the design of the travel log. likely be needed based on whether the respondent is still The travel logs used in the Kansas City study allowed in high school or in a later stage of life. space for recording up to 10 trips and instructed respon- dents to record additional travel on paper. The limit of 10 trips was based on the fact that most people report an Developing Better Probes average of five person trips in a day. In addition, it allows for a portable-sized log when printed. It works well for On the basis of the findings of the earlier GPS studies, it normal or light travelers who typically have room in has become standard procedure to probe workers about their travel diaries at the conclusion of the travel day. It potential stops made during their commutes. In addi- is possible that the heavy travelers record only up to the tion, as a form of validation, respondents who report no space in the log and nothing more (while the GPS unit travel are subjected to a series of questions to confirm continues to detect trips for the remainder of the travel the legitimacy of the reporting. The results of this study day). The problem may be further compounded if the suggest that additional probes as part of the travel data are then reported by proxy: the person reporting retrieval interview may be warranted for all travelers, travel for the heavy traveler may read the 10 trips from not just workers or those who report no travel. the log, and, not knowing what other travel was made Specifically, this study indicates that there is a high that day, end the travel day prematurely. Additional propensity to underreport travel if the driver is unem- study is warranted to determine the characteristics of ployed, has his or her travel data reported by proxy, or heavy travelers such that they can be preidentified in the travels long distances. The finding that unemployed driv- recruitment interview and provided a special log with ers have a higher tendency to underreport trips is a new either additional pages or a special insert for recording correlate to be considered. In the past, the modeling additional trips (similar to the way special instructions focus on the work trip (and how discretionary travel may about transit-trip recording are provided to zero-vehicle be incorporated into the work commute) has led to an households). This is a relatively low-cost solution that emphasis on collecting travel activities that occur during would help to minimize trip underreporting from the the lunch break or during the commute to or from the heavy-traveler group of respondents. workplace. Drivers who are unemployed do not receive A second important driver characteristic is age. This similar levels of scrutiny or probes but, according to the study reveals that the propensity to underreport travel findings of this study, should.