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20 Ground Access to Major Airports by Public Transportation a reasonable definition. However, for the analysis of public transportation patterns, it is critical to separate publicly available buses and vans from limousine service not available for shared-ride purposes. Similarly, the question "What mode do you usually take to the airport?" gets a differ- ent response from that of the preferred formation, "On your last trip to the airport--and only that trip--what mode did you take?" A survey bias towards socially desired behavior patterns occurs on the first question. Data Collection to Monitor the Performance of the System The measurement of performance of the system is a very important output from the data collection process. A classic example of a commitment to measurement exists in the contrac- tual relationship between the Massachusetts Port Authority (Massport) and the Conservation Law Foundation, a non-profit environmental organization. The simplest, and most basic, commitment is to the continual monitoring and measurement of mode share to the airport and to the volume of vehicle miles traveled (VMT) associated with airport access. For such a program, it is critical to monitor the actual vehicle volumes throughout the airport roadway system; the accurate calculation of VMT will require both traffic counts by vehicle classifica- tion and the kind of origin-destination information only made available by the Ground Access Survey. A very basic example of a system of performance measurement was developed by Massport in the mid-1980s. Table 1-1 shows the number of vehicle trips on the roadway created by one air traveler gaining access to the Boston airport on the ground system by various modes. Each of the values was calculated empirically from observed occupancy and load factors for each of the modes. In the evaluation of the performance of the system, a given strategy was considered to be beneficial if it moved the traveler to a more efficient mode (i.e., down the rows of the table) rather than to a less efficient mode (i.e., up the rows of the table). The implications of some changes in travel behavior are intuitively obvious; a new express bus service that diverts a traveler away from his/her former drivepark mode is a more efficient mode and is evaluated positively. But not all implications of mode changes are intuitively obvi- ous. If, for example, on-airport parking rates are set extremely high to discourage the use of drivepark, the implications of the resulting mode change are not so clear. If that trip is diverted to pick-up/drop-off mode, the implications for vehicle miles traveled are highly negative, and the candidate practice is evaluated negatively. The program of monitoring performance must be designed to record such subtle changes in travel behavior. Table 1-1. Measures of effectiveness in Massport program: ground access vehicle trips per air traveler trip. Mode Vehicle trips per air traveler trips Pick-up/Drop-off 1.29 Taxi 1.09 Drivepark 0.74 Rental car 0.69 Door-to-door van 0.33 Scheduled bus 0.10 Rapid transit 0.00 SOURCE: Massachusetts Port Authority, "Logan International Airport, Ground Access Non-Pricing Study, Second Report to the Conservation Law Foundation," 1991.