Since the GPS trajectories are recorded at regular intervals of one second, the duration of each missing data portion can be readily calculated. If the duration of a missing data portion is less than ten seconds, the missing portion is compensated automatically using a straight line, with lost track points located uniformly on the line. Otherwise, we ask respondents to provide information on their travel routes where the track data is missing during the prompted recall survey. According to the experience accumulated in the survey, ten seconds is an appropriate threshold which balances the respondent burdens and accuracy requirements for GPS
trajectories. In total, 4685 segments were extracted from the aforementioned trips, all of which were confirmed by respondents.The number of segments with these five travelmodes is presented
in Fig. 1. Walk has the highest percentage by an overwhelming majority. This result is due to the fact that walk segments usually play a transitional role when inter-modal transfers are made. For example, one usually needs to walk to a subway station after getting off a bus when the bus and subway are both taken sequentially during a trip. Therefore, it is vitally important to effectively detect walk segments within the overall detection performance.