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14 CHAPTER 3 Analysis of Traffic Crashes during Nighttime and Daytime Work Zone Operations For the daytime work periods, the effect of working during Study Methodology the day at higher-volume locations would have to be extrap- Researchers designed the experimental plan for this com- olated beyond the limits of the data; for nighttime work peri- ponent of the research study to best answer the previously ods, an extrapolation to lower-volume freeway types would identified question, "How does doing a project task at night be required. This analysis approach is depicted graphically affect traffic safety relative to doing this same task during the in Figure 1. The relationships are depicted as nonlinear, re- day?" Implied in this question is the recognition that the flecting expectations that the effects of congestion and queues comparison should be based on the same work task being significantly add to the crash risk as volumes increase. The performed (at least from a traffic control perspective) and on potential for the increased crash costs at night work operations doing the work at the same location. Also, implied in this to exceed those at daytime work operations is also implied at question is the assumption that the comparison is being made very high traffic volume levels since temporary lane closures on the basis of total additional crash costs being incurred over on very high-volume facilities can still create traffic and con- the duration of the project task that needs to be completed gestion even at night. (since the severity of the additional crashes occurring during Similar relationships could likewise be developed for other the day and at night may be different). comparable work conditions. For example, daytime versus Because directly comparing traffic crash experiences for nighttime crash cost increase comparisons during periods of similar temporary traffic control setups used during night- work activity without temporary lane closures would be of in- time and daytime work operations at a given location would terest to assess the relative effects of work distractions, vehicle be too limited to allow proper statistical analyses, researchers and equipment access impacts, and other non-lane closure developed an experimental plan to make use of a large num- influences on traffic safety. In addition, daytime versus night- ber of projects on freeway facilities that involved frequent or time comparisons of crash cost increases during periods of intermittent temporary lane closures during either daytime work inactivity could provide an indication of the relative or nighttime work hours. Researchers expected that daytime effects of work zone design features (reduced lane and shoul- temporary lane closures would exist at project sites on lower- der widths, lane shifts, etc.) on traffic safety. volume facilities where the closure of travel lanes would not Researchers initially contemplated stratifying the data on generate significant congestion and delay. Similarly, researchers the basis of roadway type. One would expect the decision of anticipated that projects where temporary lane closures are whether or not to work at night to be influenced by different done at night would occur predominantly on higher-volume reasons for an urban arterial work zone versus a work zone facilities because of desires to avoid creating significant con- on a freeway or expressway segment, for example. Differences gestion and delays during daytime hours. There would be some in driver demographics, traffic volumes and speeds, and crash overlap of these ranges, depending on the specific criteria rate increases between daytime and nighttime conditions are used by the highway agency having jurisdiction over each likely not the same for different roadway types. Unfortunately, project, roadway and work task characteristics, etc. From this the sample sizes required to properly evaluate nighttime and dataset, statistical techniques would be used to establish daytime work operations on multiple roadway types were separate relationships of the additional crash costs per period beyond the budget limitations of this study. Consequently, of work activity involving temporary lane closures during researchers focused their efforts in this study on freeway and daytime work periods and during nighttime work periods. expressway facilities. Although the results of the study do