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63 estimated workload that it would not be cost-effective to hire Sampling Plan. The special study subsystem would typically an additional investigator per participating PSU for this sup- select cases from both within and outside the NASS CDS sam- plemental field data collection. ple. The sampling plan should take into account key consider- Coding of additional information and reconstruction of ations as those for the selection of PSUs including number of the cases as well as the performance assessment would be eligible crashes, the number and experience of the investigators, handled by the outside contractor so as to minimize the time geographical location, work load, etc. Again, it is critical to required of the PSU and Zone Center personnel. develop the sampling plan in conjunction with NHTSA and the two Zone Centers. Every effort should be made to avoid inter- ference with the regular NASS CDS work of the selected PSUs. 5.6 Data Collection Protocol-- Notification System. A special notification system is needed Special Studies Subsystem for cases to be selected from outside the NASS CDS sample. Under the special study subsystem, single-vehicle, ran-off- Care should be taken to make sure that the notification sys- road crashes involving specific roadside safety features or tem for the special study would not interfere with the CDS or devices would be selected from both within and outside the add too much work to the cooperating agencies. Depending CDS sample. The data to be collected under this special study on the nature of the special study, another consideration is subsystem would include: the time lag from the time the crash occurred to the time the PSU is notified of the crash. For certain types of crashes, the 1. Selected CDS data time lag may need to be relatively short in order to gather the 2. Supplemental highway data for the continuous sampling needed scene evidence. In such cases, the notification system subsystem would have to be devised to reduce the time lag to an accept- 3. Detailed information on the roadside feature or device able level. under study 5.6.2 Data Collection Forms The special study cases would then be reconstructed to estimate impact conditions and vehicle trajectories, and the The general structure of the data collection forms and pro- impact performance of the specific roadside feature/device cedures for the special study subsystem would be similar to under study will be assessed. those used with the continuous sampling subsystem, includ- The specific data collection protocol will differ from spe- ing but not limited to: cial study to special study. Thus, the discussions will be more general in nature to cover the key considerations. Supplemental Highway Data Collection Form Object Struck Data Collection Form Reconstruction Coding Form 5.6.1 Sampling Plan Performance Assessment Form As mentioned previously, it is impossible to devise a spe- Scene Photographs cific sampling plan that works for all special studies. Thus, the discussions will be more general in nature to cover the key However, the forms would be tailored to the specific road- considerations in developing the sampling plan. side feature/object under study. The Supplemental Highway Sample Size. The number of cases to be investigated Data Collection Form would likely remain mostly unchanged. would first have to be determined. This is usually determined The other data collection forms would have to be modified to by study/analysis requirements and the available funding. address the specific roadside feature/object with more specific Study Location. PSUs with the most eligible cases would and greater details. first be identified. The most appropriate PSUs would be selected for participation in the special study, based on the 5.6.3 Organization required sample size and factors such as: number of eligible crashes, the number and experience of the investigators, geo- The sponsorship and organization of a special study data col- graphical location, work load, etc. It is critical that the PSUs lection effort would be similar to those of the continuous sam- are selected in conjunction with NHTSA and the two Zone pling subsystem. The program is best sponsored by AASHTO Centers. Every effort should be made to avoid interference and administered through NCHRP. The conduct of the study with the regular NASS CDS work of the selected PSUs. would be handled by an outside contractor and coordinated Study Period. Again, this is a function of the required with NASS while a project panel or a technical advisory com- sample size and the number of eligible cases from the partic- mittee would provide guidance and direction to the contractor ipating PSUs. and review the study progress and results.