Developing Better Probes. Based on the findings of the earlier GPS studies, it has become
standard procedure to probe workers about potential stops made during their commutes. In addition, 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 under-report travel if the driver is
unemployed, has his/her travel data reported by proxy, or travels long distances. The finding that
unemployed drivers have a higher tendency to under-report 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/from the workplace. Drivers who are unemployed do not receive similar levels of
scrutiny or probes, but should according to the findings of this study.
Unlike employment status, the finding that proxy-reported travel is associated with higher
propensities of under-reported travel is well documented. While the most obvious solution is not to allow
any proxy reporting, the cost implications of such a decision are tremendous and may introduce more bias
into the survey data than that introduced by allowing proxy reporting. A second, but also costly, approach
is to only allow proxy interviews if the travel log is used. The better solution here may be to strengthen
the telephone interview in a manner similar to the recommendation above for strengthening the travel of
unemployed persons.
In summary, this paper has examined the driver demographics, driver travel characteristics, and driver
adherence to survey protocol considerations that impact the likelihood of under-reporting as well as the
level of trip under-reporting. These results can be used to adjust for under-reporting in traditional
household travel surveys and/or to improve travel survey data collection procedures. Although we do
plan to replicate this analysis on future travel surveys with GPS components, we believe that the survey
method improvements identified in this study will enhance the collection of complete trip information in
any household travel survey.
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