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