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OCR for page 40
40
$70,000
Reduction in Crash Costs per 100
$60,000
$50,000
Hours per Mile
$40,000
$30,000
$20,000
$10,000
$0
0 50000 100000 150000 200000 250000
Roadway AADT
10% TDM Reduction - Daytime 10% TDM Reduction - Nighttime
20% TDM Reduction - Daytime 20% TDM Reduction - Nighttime
Figure 20. Example of reduction in crash costs by travel
demand management strategies during work activity
without temporary lane closures.
occurs, the situation is more complex, as was described earlier when lane closures are required can yield fairly substantial
regarding the impacts of the full roadway closure strategy. savings in excess crash costs during daytime hours but only
minimal reductions for nighttime hours. Of course, the like-
lihood of an agency actually performing work activity that
Designing Future Work Zone Capacity
requires lane closures during the day on higher-volume road-
into New or Reconstructed Highways
ways is fairly small. Consequently, it is probably more realistic
Another technique to reduce the impact of work zones is to compare costs when work activity during the day does not
to consider future work zone space needs in the design of new require temporary lane closures (because the roadway design
or reconstructed highways (52). Analyses that consider the allows it) to the costs when work activity is done at night with
potential impacts of work zone operations at various points temporary lane closures (because the roadway design did not
in the future can be incorporated into trade-off analyses of allow the work to be done without closing a lane). This com-
alternative designs during the highway planning process. parison is also illustrated in Figure 21 for the California data
In some instances, it may be better to acquire greater right- as an example. As the graph indicates, if an agency is willing
of-way widths and design a wider sub-base than is initially to do work at night that requires temporary lane closures, the
planned for a roadway segment in order to allow for future safety benefits associated with roadway designs that reduce
widening that will be faster and less challenging to accomplish the number of lane closures that are required will be fairly
than if the sub-base had not already been established. negligible, regardless of the AADT of the roadway segment.
Unfortunately, these types of design decisions and their In other words, an emphasis on design enhancements that re-
ramifications upon work zone safety are highly site specific. duce the frequency and duration of work zones has more of
No data are available upon which to base estimates of how a potential safety benefit than enhancements that reduce the
these types of decisions affect work zone duration or the fre- number of work hours that travel lanes need to be closed.
quency of future work zones. If such estimates were available,
it would theoretically be a simple process to assess the impacts
Strategies to Improve Work Zone
of such strategies using the roadway AADT and additional
Traffic Control Devices
work zone crash costs graphs that are illustrated in Figure 6
through Figure 8. Traffic control devices are used to communicate with
Whereas certain design decisions could ultimately reduce motorists in advance of and through work zones. Devices that
the frequency and duration of work zones, others could allow are used to inform the driver of desired actions and correct
those work zones that are still required to be accomplished travel paths through the work zone are especially important.
with fewer lane closures (i.e., a roadway with full shoulders Traffic control devices, especially those that are used to convey
could allow traffic to be shifted during pavement rehabilitation real-time information, can also significantly affect driver route
work and still maintain the same number of lanes). Roadway choice decisions. Taken together, these devices are believed to
designs that reduce the number of hours of work zone activity have a substantial impact on work zone safety.
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$40,000
Costs per 100 Hours per Mile
Difference in Excess Crash
$30,000
$20,000
$10,000
$0
0 50000 100000 150000 200000 250000
-$10,000
Roadway AADT
Lane Closure vs No Lane Closure: Daytime
Lane Closure vs No Lane Closure: Nighttime
No Lane Closure Daytime vs Lane Closure Nighttime
Figure 21. Reduction in crash costs by avoiding daytime
lane closures through roadway design enhancements
versus closing lanes at night.
The NCHRP guidance document identifies the following economically worthwhile. Unfortunately, it is not possible to
four main strategies under this particular safety improvement realistically assess the potential crash cost reduction or safety
objective (52): benefit associated with this particular strategy at this time.
Similarly, inadequately delineated work zone personnel
· Improvements in visibility of work zone traffic control and vehicles are believed to be at a higher crash risk than
devices, those that have been adequately delineated, although the
· Improvements in visibility of work zone personnel and extent of any changes in crash risk that are achieved by visibil-
vehicles, ity improvements has not been quantified. Because of these
· Reductions in flaggers' exposure to traffic, and constraints, it is not feasible to use the crash data from this
· Implementation of ITS strategies to improve safety. study to assess the potential safety benefits of this strategy.
Techniques that improve the visibility of flagger stations or
The extent to which improvements in traffic control device replace the flaggers entirely (i.e., temporary traffic signals or
visibility and work zone personnel and vehicle visibility can automated flagger technologies) are another identified strat-
result in reduced crash costs for a particular work zone depends egy that is believed to have the potential for improving safety.
both on the highway agency's current traffic control device Flaggers are not typically used at work zone operations on
standards (required grade of sheeting, whether fluorescent freeway or expressway facilities during the day or at night,
sheeting is used, types of pavement markings used, etc.) and and so the potential effects of this strategy upon safety cannot
work zone inspection practices (frequency, level of diligence be assessed with the data collected and analyzed for this study.
applied, etc.) to ensure that the devices are adequately main- The final strategy listed is the use of ITS which allows for
tained. A work zone that has high-quality devices that are improved real-time information about conditions in and
positioned properly and maintained during the project may around a work zone to be collected, collated, analyzed, and
not experience any safety benefits through the installation of then disseminated to drivers. This information can improve
additional devices (in fact, too many devices or even brighter safety by alerting drivers to the presence of the work zone as
devices may have a detrimental effect if an information over- well as providing information that can be used to make
load situation is created). On the other hand, work zones realtime decisions regarding speed or travel route choices.
where the traffic control devices are worn, have poor Consequently, these systems allow agencies to better target
retroreflectivity at night, are misaligned or otherwise out of those work zone crashes that are congestion related or are the
position, etc., may experience substantial improvements in result of other violations of driver expectancy, namely rear-end
safety by improving those devices. Devices in poorer condi- collisions and sideswipes (52).
tion at night, confounded with higher percentages of impaired From the data illustrated in Figure 9 and Table 18, the
drivers, a lack of other visual cues, etc., could result in percentage of crashes that involve rear-end collisions increases
higher crash costs, making efforts to improve those devices as roadway AADT increases to a point, whereas sideswipe
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$15,000
Sideswipe Crash Costs per
Reduction in Rear-End and
100 Hours per Mile
$10,000
$5,000
$0
0 50000 100000 150000 200000 250000
Roadway AADT
Work Zone Active with Temporary Lane Closures
Work Zone Active without Temporary Lane Closures
Work Zone Inactive
Figure 22. Estimated reduction in crash costs due
to a 10 percent reduction in rear-end and sideswipe
collisions: daytime conditions.
collisions tend to comprise a fairly constant percentage of the numbers since an ITS deployment could potentially re-
work zone crashes across a wide range of AADTs. Therefore, duce some of those crashes that would have occurred even if
the crash costs that are expected to occur because of rear-end the work zone were not present. As a result, the effects of
and sideswipe collisions combined also increase as a function work activity (with or without lane closures) are not as sub-
of AADT. Although the actual crash reduction potential of a stantial upon crash costs as they are for other strategies. In fact,
work zone ITS deployment is currently not known, it is pos- the expected reduction that would be achieved during times
sible to assess what crash cost savings could be achieved if the of work inactivity could serve as a conservative estimate of
system were able to reduce these types of crashes by some the potential crash cost savings during the work zone, re-
amount. Again using California data as an example, Figure 22 gardless of whether or not work activity and lane closures
illustrates the estimated reductions in crash costs that would were present. The reduction in crash costs when the work
be achieved if the system were able to reduce rear-end and zone is inactive ranges from about $1,000 per 100 hours per
sideswipe collisions by 10 percent during daytime conditions mile during the day on 5,000 vpd roadways to about $11,000
(the crash costs during nighttime conditions are shown in per 100 hours per mile on 250,000 vpd roadways.
Figure 23). All rear-end and collision crashes, not just the At night, the values range from as little as $500 per 100 hours
additional crashes due to work zone presence, are included in per mile at 5,000 vpd up to approximately $3,000 per 100 hours
$15,000
Sideswipe Crash Costs per
Reduction in Rear-End and
100 Hours per Mile
$10,000
$5,000
$0
0 50000 100000 150000 200000 250000
Roadway AADT
Work Zone Active with Temporary Lane Closures
Work Zone Active without Temporary Lane Closures
Work Zone Inactive
Figure 23. Estimated reduction in crash costs due
to a 10 percent reduction in rear-end and sideswipe
collisions: nighttime conditions.
