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Analysis of Sign Damage and Failure: A Case Study in Utah
By
Wesley Boggs,
Graduate Research Assistant
Utah State University
SANT 002
Logan, UT, 84322
Email: [email protected]
Kevin Heaslip, Ph.D., P.E.*
Assistant Professor of Civil & Environmental Engineering
Utah State University
233 Engineering
Logan, UT 84322
Phone: 435-797-8289
Fax: 435-797-1185
Email: [email protected]
Chuck Louisell, Ph.D.
Senior Research Fellow
Utah State University
419 Barbadian Way
Mt Pleasant, SC
Phone: 843-327-7807
Email: [email protected]
(* = Corresponding Author)
Word Count: 4,671 words + 11(Tables and Figures)*250 = 7,424 words
TRB 2013 Annual Meeting
Submitted for Presentation and Publication
92nd Annual Meeting of the Transportation Research Board
Washington, D.C., 2013
Paper revised from original submittal.
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ABSTRACT
Since the establishment of minimum retroreflectivity levels, agencies and researchers have
focused on determining the service life of different sheeting type and color combinations. While
deterioration curves and measured retroreflectivity are viable methods for maintaining
retroreflectivity compliance they do not ensure the ability of the traffic sign to convey its
intended message. Retroreflectivity efficiency only ensures visibility but does not properly
describe the legibility of the sign. In 2011, a data collection effort was conducted by researchers
at Utah State University to assess the performance of traffic signs under the Utah Department of
Transportation’s (UDOT) jurisdiction. At its completion, 1,716 traffic signs were recorded. The
researchers determined that the sample sign population was 93 percent compliant with the
minimum retroreflectivity levels. Even though the majority of traffic signs were performing
above the minimum retroreflectivity levels, 28 percent were damaged to the degree that the
legibility of the sign could be questioned. Since signs under UDOTs jurisdiction had higher rates
of damage than rates of failure, analysis was conducted to determine the contributing factors that
lead to increased damage rates. Climate and location data were combined with the known
location of each traffic signs using geographic information system software. It was determined
that average annual precipitation, elevation, seasonal temperature swing and the exposure of the
sign were all contributing factors to higher damage rates.
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INTRODUCTION
On May 14, 2012, final revisions were adopted to the Manual on Uniform Traffic Control
Devices (MUTCD) that eliminated the three original target compliance dates for minimum
retroreflectivity levels. Two years from this effective date of revision the following provision
will take effect: “Implementation and continued use of an assessment or management method
that is designed to maintain regulatory and warning sign retroreflectivity at or above the
established minimum levels (1).” The coefficient of retroreflectivity, RA, which is commonly
referred to as retroreflectivity is the ratio of a signs luminance to the illuminance. After the
establishment of the minimum maintained retroreflectivity levels, agencies began to develop and
adopt assessment or management methods to maintain compliance with these minimum levels.
Many research studies have focused on developing deterioration curves to determine the service
life of different type and color combinations of traffic sign sheeting. While deterioration curves
and measured retroreflectivity are viable methods to maintain compliance they do not ensure the
legibility of the signs intended message. The desirable attributes for a traffic sign include high
legibility and visibility, and efficient retroreflectivity only ensures the latter half.
During a data collection effort for the Utah Department of Transportation (UDOT),
researchers from Utah State University observed a substantial amount of damage present in the
sign population. At the conclusion of the data collection effort, 28 percent of the 1,716 traffic
signs had major damage to the retroreflective sheeting. This percentage is far greater than the
seven percent of traffic signs that did not meet the minimum retroreflectivity levels.
Consequently, analysis was conducted to determine the causes of areas that have high damage
rates. For this report damage was organized into three categories: aging, environmental, and
vandalism. During the collection effort, major and minor damage severity was assessed for
damaged traffic signs where major damage caused substantial decline in the overall legibility of
the sign and minor damage had negligible affects on the legibility of the sign.
The purpose of this paper is to look past the minimum retroreflectivity levels and
determine different factors that contribute to increased damage rates. Climate and location data
was gathered from online sources and weather stations across the state in order to determine
which factors contributed to increased damage rates amongst the sign population. At the
conclusion of this analysis, the major contributing factors were determined to be average annual
precipitation, elevation, seasonal temperature swing, and the exposure of the sign.
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BACKGROUND
There has been limited previous research into the damage rates of traffic signs managed by an
agency. Several studies have been focused on determination of the service life of traffic signs,
but did not focus on the rate of sign damage. In 1991, a Federal Highway Administration
(FHWA) report stated that rural areas had a high frequency of vandalism damage (2). Another
report by McGee and Paniati, while it did not discuss damage rates, it concluded that the effects
of damage on a traffic sign should not be ignored (3). The report recommended that signs be
visually inspected in order to ensure legibility and visibility but was silent on the issue of the
frequency of inspection. This conclusion was reinforced by a report for the North Carolina
Department of Transportation (NCDOT) in 2002 (4).
From 2005 to 2010 researchers at North Carolina State University completed several
reports that discussed observed damage rates of NCDOT traffic signs (5) (6). A total of 1,057
traffics signs were measured by the completion of the collection effort. Damage was organized
into three categories: human caused, nature and non deliberate human damage. Of note is that the
majority of the sign population was made up of Type I and Type III sheeting with little
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evaluation of the damage sensitive prismatic sheeting. Within the sample, dominated by Type I
and Type III sheeting, researchers found that approximately four percent of all annual sign
replacements were the direct result of damage (6).
The NCDOT study contrasts the observations associated with this research in which the
sign population was primarily made up of signs that featured prismatic sheeting as a nighttime
visibility enhancement. The observations suggest that as agencies begin to increase the use of
more efficient, yet more vulnerable prismatic sheeting types within their sign population,
inclusion of a nighttime legibility criteria in a damage assessment program may substantially
increase the damage driven sign replacement rate. The observation of damage rates in excess of
25% when nighttime legibility was considered, led to definition of an extension of the research
effort to identify and characterize climate and location characteristics that are associated with
high damage rates. By identify locations where increased damage rates are expected agencies can
begin to fine-tune assessment intervals and to develop mitigation strategies in the continuing
effort to increase motorist safety. With the continued implementation of prismatic sheeting in
UDOTs sign population, maintaining the nighttime legibility of traffic signs is expected to
become more important than simply ensuring its visibility.
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DATA COLLECTION
In 2011, researchers from Utah State University completed a data collection effort for UDOT to
assess the current compliance of the traffic sign population. At the completion of the collection
effort, 1,716 traffic signs were surveyed (7). Although the goal of the collection effort was to
assess the compliance of UDOT maintained traffic signs, additional information was collected to
enable more in-depth analysis. During the collection effort, it was observed that traffic signs
exhibited a wide variety and severity of damaged to the face of the sign. It was determined that
seven percent of the sample sign population did not meet the minimum retroreflectivity levels,
while 28 percent had major damaged present on the sign face. This damage was divided into two
different severity categories. Major damaged severity was damage that caused substantial decline
in overall legibility, whereas minor damage severity had negligible affects on sign legibility.
Since minor damage had negligible effect on the shape, color, legend or illumination of traffic
signs, only signs with major damage are shown in this report. TABLE 1 displays the number of
sign failures by damage category. For this report, damage was organized into three categories:
aging, environmental, and vandalism.
TABLE 1 Retroreflective Performance of Damaged Signs
Retroreflective
Performance
Above
Below
Total
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Damage Category
Aging
109
79
188
Environmental
122
17
139
Vandalism
150
10
160
None
1,215
14
1,229
Total
1,596
120
1,716
Above and below refers to the minimum retroreflectivity level recorded via a portable
retroreflectometer. The damage categories have different relationships with the retroreflective
performance of the traffic signs. As shown above, aging damage is indicative of lower
performance but this does not hold true about other types of damage. Environmental and
vandalism typically passed the minimum retroreflectivity levels with failure rates of 12 and six
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percent, respectively. Environmental and vandalism damage accounted for over 61 percent of all
damaged traffic sign. As defined in the MUTCD, the basic requirements of a sign are, “that it be
legible to those for whom it is intended and that it be understandable in time to permit a proper
response (1).” Therefore, the effects that damage has on the legibility of a traffic sign should be
managed at the same importance as retroreflectivity is maintained. Simply ensuring that a traffic
sign will have adequate brightness during nighttime conditions does not guarantee message
conveyance. As shown in FIGURE 1, the S3-1 had measured retroreflectivity of 138 cd/lx/m2,
but the paintball damage across the legend has effectively rendered the sign useless to motorists.
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FIGURE 1 Daytime vs. Nighttime Visibility.
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Damage Categories
The first step was organizing damage types into damage categories. During the collection effort
the following damage types were used: bending, cracking, peeling, vandalism and other. Due to
similarities, the damaged types were organized into the following damage categories: aging,
environmental and vandalism, as shown in FIGURE 2.
Aging traffic signs are signs that exhibited cracking across the retroreflective sheeting or
peeling of the legend. This type of damage was most prevalent on UDOTs legacy Type I
sheeting. Although the exact installation dates for these signs are not known, this type of damage
is common on signs that have exceeded the manufacturer’s warranty.
Environmental damaged includes bending due to wind or snow thrown from snow plows,
damages attributed to vehicle knockdowns and damaged caused by tree sap, tree rubbing, et
cetera. The majority of environmental damage is considered inevitable, with the exception of
bending which can be mitigated via back bracing. Under close inspection a significant amount of
signs appeared to be damaged during the transportation and installation of the sign. The presence
of multiple cuts that penetrated one or more layers of the sheeting was identified as having
environmental damage since the damage was not deliberate.
Vandalism damage is defined as any deliberate damage to the face of a traffic sign.
Paintballs impacts, bullet holes, eggs, bumper stickers, spray paint were all categorized as
vandalism damage. This type of damage was found more frequent in Utah’s rural canyon areas
and is considered the most detrimental form of damage due to the difficulty to assess how it
affects the visibility of a sign at night.
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FIGURE 2 Damage Categories
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Weather Observation and Location Data
In order to determine the contributing factors to sign damage, weather observation and location
data were collected form sites across the state of Utah. Weather observation data was collected
from several sources in order to ensure completeness and accuracy. The two types of weather
observation data that were used during this report are average annual precipitation and seasonal
temperature swing.
The average annual precipitation data was obtained from the Parameter-elevation
Regressions on Independent Slopes Model (PRISM) climate mapping system (8). PRISM data
sets are recognized world-wide as the highest quality spatial climate data sets currently available.
For the analysis in this report, the thirty year average (1981-2010) data set was used.
The seasonal temperature swing data was collected via MesoWest databases using two
types of weather stations (9). The weather stations were a combination of National Weather
Service (NWS) and Bureau of Land Management remote automated weather stations (RAWS).
Hourly temperature data was downloaded for the last 10-years in order to represent temperatures
seen by a sign during its service life. In order to represent the temperature range seen by a sign,
seasonal highs and lows were averaged. For the summer months, the temperatures during the
hottest 12 hour period for each day were averaged. For the winter months, the coldest 12 hour
period was averaged. The difference between the summer and winter 12 hour averages was
defined as seasonal temperature swing. FIGURE 3 shows the location of the NWS and RAWS
weather stations along with the location of traffic signs recorded during the collection effort. The
MesoWest weather station databases also recorded hourly wind speeds and wind gust speeds.
Since the majority of the weather stations recorded similar average wind speeds, this variable
was considered negligible. Average wind gust speed was determined by taking the average gust
recorded by the station over the last 10-years.
Location data was organized into two categories: elevation and exposure. Both the
elevation and exposure information were recorded during the collection effort. The elevation of
each traffic sign was recorded by the portable data logger. The exposure of a sign was based on
the environment that that surrounded the sign and was categorized into four different groups:
canyon, mountain, rural, and urban. Routes that transitioned from rural to mountainous areas
were classified as having canyon exposure. The only distinction between mountain and rural
areas is that mountain areas had elevations greater than 6,000 ft. Urban exposure was latter
defined by the US Bureau of Census (BOC) urbanized area boundaries data set (10). The BOC
defines urban areas as having populations greater than 50,000. Traffic signs that were located
within these urban boundaries were classified as having urban exposure.
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DATA ANALYSIS
Because the damage categories are affected by different weather and location factors, the
analysis portion of this paper is divided into two sections. The first section discusses the rates of
aging and environmental damaged with respects to average annual precipitation, elevation,
seasonal temperature swing, and wind gust speed. The second section will discuss the effects of
exposure on all categories of traffic sign damage.
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FIGURE 3 Locations of Traffic Signs and NWS/RAWS Weather Stations
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Aging and Environmental Damage Rates
This section discusses the effects that different weather and location factors had on traffic sign
aging and environmental damage rates. Analysis was conducted on the affects of average annual
precipitation, elevation, seasonal temperature swing, and wind gust speed on traffic sign damage
rates.
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Average Annual Precipitation
Measurements for average annual precipitation for each individual sign was extracted from the
average annual precipitation PRISM raster data using ArcGIS. The results of this extraction are
summarized in TABLE 2. As shown in TABLE 2 and in FIGURE 4, the majority of Utah’s
climate is classified as desert to semi-arid coupled with alpine mountains. From this data, it is
apparent that the average annual precipitation plays a role in damage rate of traffic signs. Both
damage and failure rates increased with an increase in average annual precipitation. Aging
damage was three times as likely for traffic signs that experience more that 16 inches of rainfall.
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Elevation
The elevation of individual traffic signs was recorded during the data collection effort via a
portable data logger. The effects of elevation on traffic sign damage rates are summarized in
TABLE 3. The GTOPO30 digital elevation model from the United States Geological Survey’s
EROS Data Center assisted in the creation of FIGURE 5 (11). Similar to the damage rates
observed with average annual precipitation, there is an observed increase in damage rates with
elevation.
With the increase in elevation comes an increase in UV radiation and snow frequency.
The increase in UV radiation can lead to rapid fading of darker background sheeting colors,
which caused a decrease in overall contrast of the sign. This is a particular concern for white on
red signs since they must maintain a minimum retroreflectivity level and legend to background
contrast ratio. As shown in both FIGURE 4 and FIGURE 5, in Utah an increase in elevation
typically equates to an increase in precipitation. Only 35 percent of the signs were located in
areas that had greater than 16 inches of precipitation and at an elevation of at least 6,000 ft.
Therefore, even though there is a correlation between precipitation and elevation, the majority of
signs do not have both high precipitation and elevation. As snow plows clear roadways a
significant amount of snow and roadway debris is thrown against the face of the traffic sign. This
causes environmental damage to signs in areas that do not frequently have high wind and gust
speeds.
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TABLE 2 Damage Rates by Precipitation
Damage Category
Precipitation (in)
<8
8-16
16-24
> 24
# of Traffic Signs Aging
165
610
786
155
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55
103
22
Environmental % Damage
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68
14
10.3%
16.9%
21.8%
23.2%
% Fail
6.7%
5.9%
7.5%
9.0%
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FIGURE 4 Average Annual Precipitation Map
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TABLE 3 Damage Rates by Elevation
Damage Category
Elevation (ft)
< 4,500
4,500-6,000
6,000-7,500
> 7,500
# of Traffic Signs Aging
527
45
836
95
258
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95
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Environmental
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67
30
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% Damage % Fail
15.0%
3.8%
19.4%
8.3%
23.6%
8.9%
26.3%
8.4%
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FIGURE 5 Elevation Map
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Seasonal Temperature Swing
To account for expected highs and lows in annual temperature, temperature data was collected
from weather stations across the state of Utah. For the summer months, the 12 highest hourly
temperatures for each day were averaged, whereas for the winter months, the 12 lowest were
averaged. By taking the difference of these measurements for the last 10-years, seasonal
temperature swings focused on signs that experience a wide range in temperature. As
summarized in TABLE 4, the majority of the sign population experience seasonal temperature
swings from 50 to 64 degrees. Sign locations that had lower seasonal temperature swings
experienced a lower rate of damage. In order to produce FIGURE 6, the seasonal temperature
swing data for areas in between weather stations was interpolated using ArcGIS. Values for
individual traffic signs were determined by extracting values from that raster file created by the
interpolation process.
Through observation made by researchers during the collection effort, aging damage was
affected by the sheeting type of the traffic sign. For UDOTs legacy Type I sheeting, aging
damage commonly resulted in cracking across the sign face that penetrated down to the
aluminum backing. On the oldest Type I signs, the retroreflective beading became very powdery
and could be easily removed. The presence of aging damage on Type I sheeting proved to be a
valid indicator that the traffic sign would not meet the minimum retroreflectivity levels. At the
completion of collection effort, over 87 percent of aging damaged Type I traffic signs were
performing below the minimum retroreflectivity levels. This did not hold true for multi-layer
sheeting types. Of the observed 83 Type III signs with aging damage, 95 percent were
performing above the minimum standards. Even though the vast majority of these signs retained
enough retroreflectivity efficiency, other issues began to present themselves. Once a multi-layer
sign is cut, cracked, or punctured it allows water to begin to collect within the layers of the sign
sheeting. Over several seasons, the aging damage worsens via the freeze-thaw cycle causing the
damage to fan out across the face of the sign. Not only does this begin to expose the
retroreflective under layer to the elements, it also diminishes the contrast required for adequate
legibility and visibility.
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Wind Gusts
In order to determine if wind gust speed was a contributing factor to increased damage rates, data
was analyzed form the MesoWest database. Of the different contributing factors analyzed in this
report, this is the only one that had a counterintuitive damage rate trend. As wind gust speed
increased there was a decreased in the rate of damage. After further inspection, it was determined
that UDOT has installed a significant amount of back bracing on traffic signs with average wind
gust speeds above 20 miles per hour. For areas that averaged wind gust speeds greater than 25
mile per hour, over 64 percent of traffic signs had back bracing. Continuation of this
maintenance practice will reduce the number of signs that are bent from both wind and snow
plow spray.
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TABLE 4 Damage Rates by Seasonal Temperature Swing
Damage Category
STS (°F)
<50
50-57
57-64
>64
# of Traffic Signs
152
880
630
54
Aging Environmental
12
5
84
77
83
52
9
5
% Damage % Fail
11.2%
2.0%
18.3%
6.8%
21.4%
8.3%
25.9%
9.3%
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FIGURE 6 Seasonal Temperature Swing Map
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Vandalism Damage Rates
The affect that vandalism damage has on the legibility of a traffic sign depends greatly on the
type of vandalism. Paintball and egg damage limits the available amount of light that can be
retroreflected, but during the day it has little effect on the overall legibility of the sign. Compared
to bullet holes, bumper stickers, and spray paint that can be seen during both day and nighttime
conditions. In order to determine areas that exhibited high rates of vandalism damage, the traffic
signs were organized into different exposure categories. Urban areas were determined by 2010
BOC urbanized area boundaries data. Using ArcGIS, traffic signs that intersected these areas
were defined as having urban exposure. The remaining traffic signs were designated as having
canyon, mountain, or rural exposures. Because vandalism damage is solely the result of humans
it was excluded from the previous analysis section.
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Exposure
During a preliminary collection, it was quickly observed by the researches that the damage rate
for rural signs was greater than urban signs. Therefore, exposure was added to the collection
attributes for each traffic sign. By the completion of the collection effort, this trend held true for
signs across the state of Utah. As shown in TABLE 5, canyon areas had the highest rate of
damage, while the urban sign population had the lowest observed damage rate.
Organizing the signs by exposure illustrates how much higher the rate of damage is
compared to the minimum retroreflectivity failure rate. For all exposures, the damage rate was at
least three times greater than the rate of failure. Canyon exposure had the lowest percentage of
aging damage signs coupled with the highest rate of vandalized signs. A particular area of
concern are dead end canyon routes which had the highest rate of damage observed during the
collection effort. Unlike signs with canyon exposure, the majority of mountain exposed sign
damage was aging damaged. In addition to the aging damage, mountain exposed signs had the
highest rate of environmental damage. Rural exposed signs had the most evenly distributed sign
damage of any of the exposures. Unexpectedly, urban exposure had the lowest damage rate with
the majority of its damaged signs being from aging instead of vandalism damage.
TABLE 5 Damage Rates by Exposure
Damage Category
Exposure
Canyon
Mountain
Rural
Urban
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# of Traffic Signs
197
262
778
479
Aging Environmental
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81
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Vandalism
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% Damage % Fail
37.6%
7.1%
32.8%
9.5%
28.3%
6.4%
22.3%
6.5%
Summary of Analysis Results
At the completion of the data collection effort, it was determined that over 28 percent of the
recorded traffic sign population had major damage to the retroreflective sheeting. With the
location of 1,716 traffic signs spread across UDOTs maintenance regions analysis was carried
out to determine the effects of precipitation, elevation, temperature, and exposure had on traffic
sign damage rates. The results of this analysis determined that there is a higher rate of damage
associated with areas that receive more annual precipitation. It was also determined that for
every 1,500 ft above 4,500 ft the rate of damage increased by 3.8 percent. A damage rate
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increase of 4.9 percent was determined for every 7 degrees increase in seasonal temperature
swing. When the signs were segregated by exposure, it was determined the rural canyon routes
had vandalism damage rates of 17.8 percent with an overall damage rate of 37.6 percent. Wind
gust speed data was also analyzed, but due to the bracing practices implemented by UDOT the
damaging effects of high wind gust speed were negligible.
It should be recognized that the different damage categories are not completely
independent of each other. In service, traffic signs often had a combination of damage types, and
only the most prevalent damage type was recorded as the major damage for the sign. It was
observed that vandalism or environmental damage that cut, cracked, or punctured the layers of a
sign would often lead to aging damage over seasons of service. Although the initial damage was
caused by bullet holes, cracking due to bending, etc. since the overlay had begun to delaminate
across the face of the sign it was categorized as having aging damage.
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CONCLUSIONS
With an increased emphasis being placed on formalizing maintenance procedures, agencies have
begun to reevaluate the management of roadside assets. Currently, pressure has been applied by
FHWA on traffic sign retroreflectivity. In the summer of 2011, researchers from Utah State
University collected 1,716 traffic signs with the goal of assessing UDOTs compliance rate with
the minimum retroreflectivity levels. The researchers determined that 93 percent of UDOT
maintained traffic signs were performing above the minimum retroreflectivity levels. Even
though UDOTs signs were performing above the minimum levels the observed rate of damage
was a concern to both the researchers and UDOT officials. By the completion of the data
collection effort, it had been determined that over 28 percent of traffic signs display major
damage, which reduced the legibility of the traffic signs. The MUTCD provides a support
statement which states “the basic requirements of a sign are that it be legible to those for whom it
is intend and that it be understandable in a time to permit a proper response (1).” Since agencies
have two years from May 14, 2012 to implement an assessment or management method, an
emphasis should be placed on ensuring both legibility and visibility of traffic signs. As shown in
TABLE 1, less than one percent of the traffic signs did not pass the minimum levels that would
not have been replaced due to damage. This report determined that the major contributing factors
that resulted in increased damage rates were average annual precipitation, elevation, seasonal
temperature swing, and the exposure of the sign. By determining the contributing climate and
location factors that lead to increased damage rates, agencies can identify routes that need more
frequent assessment of sign legibility and visibility and can explore damage mitigation strategies.
Acknowledgements
This work was sponsored by UDOT and the authors are grateful for their support and assistance.
Accurate climate data was obtained with the assistance of Leigh Sturges, Manager of UDOT
Weather Information System and Dr. David Tarboton. In addition, the authors would like to
acknowledge PRISM Climate Group, U.S. Geological Survey’s EROS, and the Bureau of the
Census and Innovative Technology Administration’s for the access to data that was vital to this
research. This assistance is gratefully acknowledged.
14
TRB 2013 Annual Meeting
Paper revised from original submittal.
Boggs, Heaslip, & Louisell
1
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TRB 2013 Annual Meeting
Paper revised from original submittal.