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6 closures were done at night, and other types of work activities dependent. Such variability in studies to date is not surprising; off the travel lanes occurred during the daytime (39). Two work zones themselves are highly variable entities. other smaller projects that involved nighttime lane clo- The literature does imply that the amount of increased crash sures for pavement resurfacing activities were also examined. risk at a given project location is a combination of temporary Researchers found that the major reconstruction projects ex- changes in geometrics and influences due to work activity. perienced some increase in crashes during both daytime and Work activity influences can be drivers distracted by work nighttime periods, indicating that temporary geometric de- operations and equipment, turbulence created by work vehicle gradations (closure of shoulders, reduced acceleration lane or equipment access to and from the work area, and tempo- lengths at ramps, concrete barriers immediately adjacent to rary lane closures that increase traffic densities (possibly to the travel lanes, etc.) had an effect on crash likelihood. Crash in- point of congestion) and require drivers to maneuver around creases were even larger during periods of work activity (day or the closure. However, efforts to better understand the relative night), on average, with the increase in crashes during night contributions of work zone design and work activities to the work activities somewhat higher than during daytime work increased crash risk are fairly limited in the literature. activities. Given that the daytime work activities at these proj- An even more critical factor that has not been previously ects did not involve lane closures whereas the nighttime work evaluated in the literature with any degree of success is the activities did, this finding was expected. Unfortunately, as actual difference in safety between performing a particular with the previous study, these results do not provide any work activity or project at night versus doing that same indication of how crashes might have been affected had the activity or project at the same location during the day. Rela- lane closures and work that was done at night actually been tive safety is one of the key recommended considerations that instead performed at each site during the day. practitioners face when assessing whether or not to do a par- ticular project or project task at night (35). A few studies have provided some insight into the amount by which the normal Nighttime versus Daytime Worker Safety nighttime crash rate increases if work is performed at night. Although not all worker accidents involve traffic crashes, However, the increase that would have occurred in the day- working at night does significantly impact the lives of high- time crash rate at those locations if the work had been done way workers and so is of at least some relevance to the goals during the day instead has not been quantified. Unfortunately, of this study. Overall, the safety impacts of performing road- opportunities to evaluate this question directly at individual work at night (relative to daytime operations) on highway project sites are almost nonexistent. In most cases, the pri- workers have not previously been examined in detail. Work- mary reason that an agency and highway contractor work at ers generally perceive traffic speeds past the work site to be night at a location is that there is a need to close one or more higher at night and so also perceive their level of safety to be travel lanes for a temporary period to complete the work, and diminished (40). The limited amount of accident data avail- doing so during daytime hours would generate unacceptable able on highway workers has not necessarily confirmed this traffic delays and queues as well as severely limit the contrac- perception, however. A study by the National Institute of tor's ability to move work vehicles and materials into and out Occupational Safety and Health (NIOSH) examined fatal of the workspace. Unacceptable impacts on adjoining prop- occupational injuries for highway construction workers be- erties may also occur from temporary lane closures during tween the years 1992 and 2000 (41). Based on their assess- daytime hours. ments, the NIOSH researchers concluded that "working at Related to this comparison of nighttime-daytime crash risk night is not responsible for the overall increase in highway increase is the choice of appropriate measures of risk to use, worker deaths." However, it should be noted that there were i.e., should it be crash rate per mile of work zone or crash insufficient data to actually compare highway worker accident rate per mvm? Traditional crash rates normalize crashes on rates at night versus during the day. the basis of vehicular-miles of travel or a similar measure of exposure. This rate reflects a level of risk to an individual driver traversing that particular roadway segment. Percent- Implications for This Study age changes in this rate or similar indicators, such as the There is a strong general consensus in the literature that percentage change between actual and expected number of work zones increase the likelihood of crashes that occur on a crashes in a given time period, thus indicate how individual particular segment of roadway. Less agreement exists as to driver risk is affected by the presence of the work zone. Cer- whether work zones result in more severe, less severe, or tainly, this indicator of motorist risk is an important consid- equally severe crashes as before work began. Certain types of eration. However, from the practitioners' perspective, the crashes appear to be more affected by work zone presence decision whether to work at night or during the day must also than others, but again, the amount of influence is heavily site consider the consequences of increased crash risk to the