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SUMMARY
Traffic Safety Evaluation of
Nighttime and Daytime Work Zones
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 practitioners' 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
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analysis. 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
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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 flaggers' exposure to traffic, and
· Efforts to reduce workspace intrusions and their consequences 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.
