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S U M M A R Y This project was initiated to objectively determine and document how nighttime and day- time work zones affect traffic safety. More and more, agencies are doing roadway work on high-volume facilities at night to reduce adverse traffic impacts and complaints by the pub- lic that typically occur when the same work is being done during the day. Nighttime travel is commonly characterized by lower traffic volumes, a higher percentage of truck traffic, higher operating speeds, reduced visibility, and higher concentrations of drowsy and impaired drivers. Arguments exist on both sides of the question as to whether working at night is more or less safe than working during the day. Previous literature indicates that increased crash risks at a given project location are a combination of temporary changes in geometrics and influences due to work activity. Work activity influences can be from drivers distracted by work operations and equipment, tur- bulence created by work vehicle or equipment access to and from the work area, and tem- porary lane closures that increase traffic densities (possibly to the point of congestion) and require drivers to maneuver around the closure. However, efforts to better understand the relative contributions of work zone design and work activities to the increased crash risk are fairly limited in the literature. One of the key issues that had to be addressed early on in this research was the choice of appropriate measures of crash risk when assessing and comparing the safety implications of nighttime and daytime work. Traditional crash rates normalize crashes on the basis of vehicular-miles of travel or similar measure of exposure. This rate reflects a level of risk to an individual driver traversing that particular roadway segment. Percentage changes in this rate or similar indicators, such as the percentage change between actual and expected num- ber of crashes in a given time period, thus indicate how individual driver risk is affected by the presence of the work zone. Certainly, this indicator of motorist risk is an important consideration. However, from the practitionersâ perspective, when making the decision whether to work at night or during the day, they must also consider the consequences of increased crash risk to the driving population as a whole. Whereas the increase in crash risk to an individual motorist may, in theory, be greater at night than during the day, the much lower traffic volumes (and thus, vehicle exposure) that typically exist at a given location at night may more than offset this incrementally higher risk. Higher traffic volumes during the day mean that the same number of crashes will produce a much lower crash rate per million-vehicle-miles (mvm). If the day versus night decision is for a given work zone, then it would appear that the practitioner will want to minimize the number of crashes (assuming equal severity). Thus, crash rate per mile of work zone was deemed an important compar- ison metric. Certainly, differences in the severity of the increased crashes may also exist between daytime and nighttime work operations, which also must be considered in the Traffic Safety Evaluation of Nighttime and Daytime Work Zones 1
2analysis. Together, this implies that the use of additional crash costs, normalized on the basis of amount of work activity required at a given project location, most closely reflects the information that highway agencies must weigh in their decisions of whether or not to work at night. A two-pronged investigation was adopted for this research project. The first prong uti- lized the New York State Department of Transportation (NYSDOT) Work Zone Accident database. This database is a one-of-a-kind resource developed in the 1980s and expanded over the years, specifically for use in tracking all types of work-zone-related traffic crashes and worker accidents on NYSDOT construction projects statewide. For this study, relative differences were examined in the types and severities of traffic crashes and worker construction accidents during both daytime and nighttime work operations on New York freeway and expressway facilities. Even more importantly, the database included specific in- formation on various types of worker-involved traffic crashes and construction accidents sustained during both day and night work activities, something that is not available at the present time nationally in any other database. The second prong of the research effort was the collection and analysis of crash experi- ences of work zones performed in California, North Carolina, Ohio, and Washington. For some of the projects, work activities were done predominantly during the daytime; for other projects, work activities were done mainly at night. For other projects, some work activities were done during the day (mainly those activities that did not require the temporary closure of travel lanes), and other activities that required travel lanes to be closed for several hours were performed at night. The results of these investigations were very insightful. Overall, working at night does not result in a significantly greater crash risk for an individual motorist traveling through the work zone than does working during the day. The percentage increases in crash risk for work operations requiring the temporary closure of travel lanes were essentially identical when done at night or during the day. In addition, traffic crashes that occur in nighttime work zones were not necessarily more severe than those that occur in similar daytime work zones, again when compared across similar work operations. The implications of these findings are that work activities that require temporary lane closures have substantially lower total safety impacts to the motoring public if the work is done at night. The lower traffic volumes pres- ent at night result in a much lower number of crashes occurring over a work operation of a given duration. Although the increased risk of a crash is similar, differences do exist in the types of crashes that occur at nighttime and daytime work zones. For example, based on the NYSDOT work zone traffic crash and worker accident database, those traffic crashes involving workers, con- struction vehicles or equipment, and construction materials and debris (both intrusion and non-intrusion crashes) comprise a greater percentage of crashes at night than during the day. Although the relative percentage of these crashes was higher at night, it should be noted that they were only a small proportion of the total work zone crashes experienced in either time period. The recent National Cooperative Highway Research Program (NCHRP) 500 Report, Guidance for Implementation of the AASHTO Strategic Highway Safety Plan, Volume 17: A Guide for Reducing Work Zone Collisions, recommends a systematic process intended to reduce the frequency and severity of traffic crashes during roadway work zone operations. A number of specific strategies are named in that report that are believed to offer the potential to reduce work zone crashes, but information on the possible magnitude of such crash reductions was generally unavailable at that time. Using the findings from this research, a critique of those strategies was undertaken, and recommendations were made as to which strategies have the greatest potential to reduce work zone crashes. Strategies
that appeared to offer the greatest potential for crash cost reduction included the following items: ⢠Practices to reduce the number and duration of work zones required, ⢠Use of full directional roadway closures via median crossovers or detours onto adjacent frontage roads, ⢠Use of time-related contract provisions to reduce construction duration, ⢠Movement of appropriate work activities (i.e., those that require temporary lane closures) to nighttime hours, ⢠Use of demand management programs to reduce volumes through work zones, and ⢠Use of enhanced traffic law enforcement. Strategies that appeared to offer a moderate work zone crash reduction potential included the following: ⢠Design of adequate future work zone capacity into highways, ⢠Use of full roadway closures that require traffic detours onto adjacent surface streets, ⢠Use of intelligent transportation system (ITS) strategies to reduce congestion and improve safety, ⢠Improvement of work zone traffic control device visibility, ⢠Efforts to reduce flaggersâ exposure to traffic, and ⢠Efforts to reduce workspace intrusions and their consequences â primarily at long-term, high-volume work zones. Although these strategies appear capable of having positive impacts on work zone safety, determining the extent to which they meet these expectations can only be determined objectively through the improved collection and use of work zone crash data. Highway agen- cies have access to their state crash reporting databases and can usually develop some fairly basic metrics such as total work zone fatalities or injuries. Beyond that, however, the data are generally not sufficient to be useful for many of the potential applications. Although no work zone crash data system currently in use fully addresses the needs of effective work zone safety management, it appears that such a system can be developed by combining the desir- able features of the Model Minimum Uniform Crash Criteria (MMUCC) guidelines that have been developed nationally with an agency construction accident reporting program similar in concept to the one now in use in NYSDOT. However, revisions and improvements to both of these are considered essential to achieving the goal of providing comprehensive, timely, and consistent data for crashes, construction accidents, and other harmful events in and related to highway work zones. In addition to enhancing the actual crash data being col- lected, the collection of exposure data at work zones is particularly needed to improve process-level work zone crash analysis. 3
