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2. chemicals industry
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Retroreflectivity Degradation Trend of
Preformed Patterned Tape under Dry,
Wet and Simulated Rain Conditions
To aid drivers in
maintaining the proper
position of the vehicle on
the road, longitudinal
pavement markings
need to be visible under
all driving conditions.
To enhance nighttime
visibility, markings are
retroreflective. The
objective of this research
is to evaluate and analyze
the retroreflectivity
characteristics of
preformed patterned
tapes under dry, wet,
and simulated rain
conditions.
By David B. Clarke, Ph.D. P.E.
and Xuedong Yan, Ph.D.
ITE Journal on the web / December 2009 INTRODUCTION
Longitudinal pavement markings provide a visual reference to aid drivers in
maintaining the proper position of the
vehicle on the road. Because guidance
is critical to safe performance of driving
task, markings need to be visible under all
weather conditions.
To improve nighttime visibility, pavement markings are retroreflective. The
markings incorporate tiny transparent
beads that reflect a portion of the light
from the vehicle headlamps back to the
driver’s eye. The more light the marking
reflects back to the driver, the greater its
nighttime visibility.
A marking’s retroreflectivity, defined
in units of millicandelas/m2/lux (mcd·m2·lx-1), is an indicator of its nighttime
visibility. Specialized retroreflectometers
enable the measurement of pavement
marking retroreflectivity. The American
Society of Testing and Materials (ASTM)
publishes a number of standards related
to pavement marking retroreflectivity, its
measurement, and retroreflective materials. Presently, there are no established
criteria for the minimum retroreflectivity
of pavement marking, but 100 mcd·m2·lx-1 is a target generally supported by
the literature.
The visibility of markings in wet
weather conditions is especially crucial for
driving at nighttime. However, a film of
water covering the beads will change the
refractive properties,
reducing the retroreflectivity.1 The depth
of submersion is a key
factor. Rain conditions that allow water to
completely submerse the marking can virtually preclude any retroreflectivity. Wet
conditions, where the marking surface is
not submerged, but retains a thin water
film, may permit a reduced level of ret-
roreflectivity versus dry conditions.
Suppliers offer several marking products that advertise improved rain and/
or wet retroreflectivity. One product–
preformed patterned tape–is textured
with raised sections to keep portions
of the retroreflective surface above the
water layer (see Figure 1). Schnell et al.
reported that the preformed patterned
tape incorporating 1.75 ceramic beads
can provide acceptable retroreflectivity
under wet conditions, but under simulated rain conditions its retroreflectivity
is as low as 15 mcd·m-2·lx-1—not better
than flat marking tape’s performance.2
Aktan and Schnell compared and evaluated the visibility of flat painted markings
with large-beads, patterned tapes with
high-index beads, and another patterned
tape with mixed high index beads.3 Under wet conditions, the patterned tape
with mixed-index beads performed best.
The retroreflectivity of the paint and large
beads was slightly lower on average than
the patterned tape with high index beads.
In simulated rain conditions, the paint
and large beads and the patterned tape
with high-index beads both yielded unsatisfactory retroreflectivity (lower than
50 mcd·m-2·lx-1), while the patterned tape
with mixed-index beads gave the best retroreflectivity (156.8 mcd·m-2·lx-1).
The objective of this research was to
evaluate and analyze the in-service retroreflectivity degradation trend of preformed
patterned tapes under dry, wet, and simulated rain conditions. Pavement marking
retroreflectivity generally degrades over
time because of abrasion by traffic, sun
and heat exposure, application methods,
and other factors.4 Since patterned tape
is still a relatively new product, there are
few published studies reporting its retroreflectivity degradation trend. All the
65
a. White marking
b. Yellow Marking
Figure 1. Patterned tape and marked sample locations.
previous studies on nighttime visibility
of patterned tape were conducted in experimental fields to measure and evaluate the initial retroreflectivity after marking application. It is not clear how long
patterned tape can provide acceptable
retroreflectivity under wet conditions as
traffic exposure accumulates. Preformed
tape has shown excellent durability in
retroreflective property studies.5, 6 It is
suggested for use in heavy traffic urban
areas with continuous roadway lighting.
METHOD
Site Selection
Between December 2005 and July
2007, the authors studied the retroreflectivity performance of 18 sites with
patterned tape longitudinal markings.
Those sites were distributed on roads in
5 counties in Tennessee. The sites varied
by pavement marking color (9 for white
and 9 for yellow), roadway type (8 for
interstate, 4 for off-ramp, and 6 for state
road), and Average Daily Traffic (ADT) (8
for lower ADT and 10 for higher ADT).
Initial retroreflectivity measurements were
taken soon after the application of the
pavement markings. At each site, yellow
centerlines or white/yellow edge lines
were selected for evaluation.
Apparatus and Data Measurement
Retroreflectivity of the in-service marking materials was measured under dry,
66
wet, and simulated rain conditions over
time in accordance with the American Society of Testing and Materials (ASTM):
•Dry condition: Measurements were
conducted according to ASTM
E-1710 (Standard Test Method for
Measurement of Retroreflective
Pavement Marking Materials with
CEN-Prescribed Geometry Using a
Portable Retroreflectometer, 2005);
•Wet condition: Measurements were
conducted using the wet recovery
test method, which is the definition
of condition of wetness according
to ASTM E-2177 (Standard Test
Method for Measuring the Coefficient
of Retroreflected Luminance (RL) of
Pavement Markings in a Standard
Condition of Wetness, 2005); and
•Simulated rain condition: Measurements were conducted using the
continuous wetting test method
according to ASTM E-2176 (Standard Test Method for Measuring the
Coefficient of Retroreflected Luminance (RL) of Pavement Markings in
a Standard Condition of Continuous
Wetting, 2005).
A hand held retroreflectometer using 30-meter geometry was used for the
measurements. Because data collection
on in-service markings required encroachment onto the traveled way, field
personnel worked within a short-term
work zone including, where necessary,
protection by arrow displays or vehicle
mounted attenuators. For safety reasons,
all data was collected during daytime
hours. Retroreflectometer tests showed
no significant difference in daytime and
nighttime readings.
To better compare white and yellow
marking retroreflectivity, eleven sample
locations of yellow and white markings
were spatially matched in each selected
highway segment. At each sample site,
the research team established eleven discrete measurement locations at five meter
intervals along the marking. Following
the initial site selection and measurement, the researchers visited the test sites
at three to six month intervals to measure marking retroreflectivity. The eleven
unique measurements taken during each
site visit were recorded; the mean value
represented the overall marking retroreflectivity for the site.
Pavement marking materials typically
do not exhibit homogeneous retroreflectivity, and only a few inches movement
of the test instrument can produce significantly different readings. For consistency, the exact same areas of pavement
marking material must be measured on
each visit. In order to place the instrument with sufficient precision to ensure
measurement repeatability, spray-painted
outlines shaped like the retroreflectometer
were used to mark all sample locations, as
ITE Journal on the web / December 2009
a. Dry condition
b. Wet condition
c. Rain condition
Figure 2. Degradation trend of pavement marking retroreflectivity.
illustrated in Figure 1. During each site
visit, field personnel also took digital photographs of the marking. These photos
provided a visual assessment of the marking condition to support data analysis.
Data Reduction
Examination of the final dataset revealed some anomalous values. If such values were explainable, they were removed
from the dataset. Recognized causes for
anomalies include tire marks on the test
sections, dirt or mud on the markings,
and possible human errors associated with
equipment operation or data recording.
Photos helped to confirm such issues.
The most typical time series pattern
resulting from dirty markings is that after marking application, retroreflectivity
increases, decreases, and increases again.
The reason is that during a certain site visit
the markings were relatively dirtier due to
continuous dry weather conditions, and as
a consequence, lower levels of retroreflectivity were observed. However, after that
site visit, heavy rain cleaned the markings,
resulting in large increases in measured
retroreflectivity at the next visit.
OBSERVATION RESULT
AND DATA ANALYSIS
Retroreflectivity Degradation Trend
Figure 2 shows the retroreflectivity degradation trend of patterned tape based on
a 200-day marking service time interval.
The measurements reflect dry, wet, and
simulated rain conditions for both white
and yellow markings. Using 100 mcd·m2·lx-1 as the minimum threshold, both
colors of patterned tape markings provide
sufficient retroreflectivity under dry conditions during the observation period.
ITE Journal on the web / December 2009 a. White marking (dry)
b. Yellow marking (dry)
c. White marking (wet)
d. Yellow marking (wet)
Figure 3. Retroreflectivity changing trends for different levels of ADT per travel lane.
Under both dry and wet conditions,
the average retroreflectivity of white
markings is significantly higher than that
of yellow markings, and the difference
exists throughout the whole observation
period. This finding is consistent with a
number of previous observations for other
marking materials.7–9 The natural difference in initial retroreflectivity between
yellow and white markings indicates that
their degradation trends should be analyzed separately.
Under dry conditions, the white markings’ retroreflectivity decreased more rapidly over time than the yellow markings.
In fact the yellow marking retroreflectivity even increased during the observation
period (see Figure 2-a). One possible rea67
of ADT per lane (below 50 mcd·m-2·lx-1),
as illustrated in Figure 3-d.
a. White marking (dry)
c. White marking (wet)
b. Yellow marking (dry)
d. Yellow marking (wet)
Figure 4. Retroreflectivity changing trends for different highway types.
son is that white markings experienced a
higher level of traffic encroachment and
resulting wear than yellow markings.
Under wet conditions, the patterned
tape appeared to provide acceptable retroreflectivity only during the first 200
application days. Afterwards, the average
retroreflectivity fell to a maximum of 70
mcd·m-2·lx-1. Although the patterned tape
illustrates better wet-night visibility during
the initial application period than the flat
tape, its special function of wet-night visibility rapidly decreases with time, possibly
as the texture is worn by traffic.10
Under simulated rain conditions, the
patterned tape’s retroreflectivity is very low
(no more than 20 mcd·m-2·lx-1) throughout the measurement period. This finding
is also similar to the previous research
result done by Schnell et al.11 Thus, it is
not meaningful to further analyze the patterned tape’s retroreflectivity degradation
trend under rain conditions.
68
Effect of Traffic Abrasion
The average daily traffic per lane represents the potential traffic abrasion exposure of the pavement markings. Because
accurate ADT data are not available for
each site, the ADT per lane was classified
into 2 levels (level 1 = less than to equal to
6,000 veh/day; level 2 = more than 6,000
veh/day) to investigate the traffic abrasion
effect. 6,000 veh/day was chosen as break
point because it provides balanced observations between the lower and higher
traffic abrasion exposures.
Figure 3 depicts retroreflectivity trends
for white and yellow markings under dry
and wet conditions respectively. As expected, the higher volume generally resulted
in a higher rate of retroreflectivity decay.
Under the wet conditions during the first
200 application days, the yellow marking’s
retroreflectivity is acceptable for the lower
level of ADT per lane (over 130 mcd·m2·lx-1) but not sufficient for the higher level
Effect of Highway Type
Figure 4 compares retroreflectivity over
time among markings by types of roadways: interstate, off-ramp, and state road.
The marking retroreflectivity of interstates
is constantly lower than other roadways at
a given service time point, perhaps because
traffic volumes of interstates are higher
than those of off-ramps and most state
roads, resulting in accelerated wear. This
tendency is especially strong under wet
conditions (see Figures 4-c and 4-d). Comparison of markings of off-ramps and state
roads reveals no consistent trend, except
that yellow markings on off-ramps show
better wet condition visibility than similar
markings on state roads (see Figure 4-d).
Site-by-Site Retroreflectivity Decay Trend
To better investigate the retroreflectivity decay patterns, a site by site analysis of
marking retroreflectivity was conducted
under dry conditions. The plots of retroreflectivity versus time for the selected
sites revealed two basic patterns for dry
conditions. Pattern #1, shown in Figures
5-a, is a parabolic trend. After marking
application, reflectivity first increases, and
once it peaks, it gradually reduces over
time, presumably because the markings
wear further. Pattern #2, seen in Figures
5-b, is a monotone decreasing trend: after
marking application, reflectivity gradually decreases. Under wet conditions, only
pattern #2 was observed. As explained before, the tape’s wet-night visibility declines
as the texture is worn by traffic.
Under dry conditions, patterns #1 and
2 were each observed at 9 sites. The occurrence likelihoods for both patterns are
therefore equivalent. According to previous research results, retroreflectivity is dependent on the embedment depth of the
bead in the pavement marking material.12
Either lower or higher embedment depth
of the bead may affect the longevity of the
beads and marking retroreflectivity. The
optimum retroreflectivity occurs at 5060% bead embedment. Generally, a new
marking has 70% of the beads completely
buried in marking materials. So, for new
tapes, reflectivity increases initially because the glass beads become exposed
ITE Journal on the web / December 2009
a. Pattern #1
b. Pattern #2
Figure 5. Typical marking retroreflectivity changing trends under dry conditions.
after some amount of traffic wear. Once
the depth of the exposed beads reaches
40-50%, retroreflectivity peaks and then
gradually reduces over time as the markings wear further. This process explains
the mechanism of pattern #1. Several previous studies on marking retroreflectivity
also reported this phenomenon.13–15 On
the other hand, when traffic volume is
quite heavy and vehicles frequently run
over the markings, pattern #2 is more
likely to be observed since the beads can
be quickly abraded from the material.
Figure 6 illustrates how the two patterns are associated with marking color,
ADT per lane, and highway type. Yellow
markings, lower ADT per lane, and offramps and state roads are more likely associated with the pattern #1, while white
markings, higher traffic volume, and interstates are associated with pattern #2.
CONCLUSIONS
This study was focused on the retroreflectivity degradation trend of patterned
tape markings under dry, wet, and simulated rain conditions during the markings’
early service life (800 days). Based on data
observation and analysis, the following
ITE Journal on the web / December 2009 conclusions can be drawn:
•Under dry conditions, the patterned
tape markings are durable materials
and can continuously provide acceptable retroreflectivity, even in heavy
traffic highways.
•Under wet conditions, the initial retroreflectivity of the patterned tapes is
acceptable, but wet retroreflectivity
falls below acceptable levels after the
first 200 application days, perhaps as
the texture is worn out by traffic.
•Under simulated rain conditions,
patterned tape retroreflectivity is
too low and to provide acceptable
visibility. Therefore, patterned tapes
are helpful but not essential solution
for improving marking visibility in
heavy rain areas.
•The initial retroreflectivity of white
markings is significantly higher than
that of yellow markings. However, the
white markings illustrated a more apparent degredation trend in retroreflectivity compared to the yellow markings. Presumably, the white markings
experienced a higher level of traffic
encroachment and wear than the yellow markings during the service time.
Figure 6. Association of retroreflectivity changing
trends with marking color, traffic volume and highway type.
•Patterned tapes are sensitive to the
level of ADT per lane. When applied
at heavy traffic highways such as interstates, their retroreflectivity is apparently lower than those applied at
relatively light traffic roadways. ADT
per lane appears a useful independent
variable to predict patterned tape’s
service life under dry conditions.
•Patterned tape markings exhibit two
types of retroreflectivity degradation
trends: parabolic and monotone decreasing. The form of the two trends
appears correlated with the intensity
of traffic exposure and with marking characteristics such as bead size,
embedment depth, and distribution. The patterned tape’s production quality is much better controlled
than that of thermoplastic markings
which retroreflectivity quality could
be affected by many application factors. Further studies are suggested
to explore the relationship between
production factors and the retroreflectivity life of patterned tape.
69
ACKNOWLEDGMENT
The authors would like to acknowledge the Tennessee Department of Transportation (TDOT) for its sponsorship
of this research project. Appreciation is
also extended to TDOT staff for providing assistance with data collection. The
recommendations of this study are those
of the authors, and do not represent the
views of TDOT. n
References
1. Schnell, T., Aktan, F. and Lee, Y.C., 2003.
Nighttime Visibility and Retroreflectance of
Pavement Markings in Dry, Wet, and Rainy
Conditions. Transportation Research Record 1824,
pp. 144–155.
2. Ibid.
3. Aktan, F. and Schnell, T., 2004. Performance Evaluation of Pavement Markings under
Dry, Wet, and Rainy Conditions in the Field.
Transportation Research Record 1877, pp. 38–49.
4. Migletz, J., Graham, J.L., Harwood, D.W.,
and Bauer, K.M., 2001. Service Life of Durable
Pavement Markings. Transportation Research Record 1749, pp. 13–21.
5. Attaway, R.W., 1989. In-Service Evaluation of Thermoplastic and Tape Pavement Markings Using a Portable Retroreflectometer. Transportation Research Record 1230, pp. 45–55.
6. Lee, J., T.L. Maleck, and W.C. Taylor, 1999.
Pavement Marking Material Evaluation Study in
Michigan. ITE Journal, vol. 69, no. 7, pp. 44–51.
70
7. Scheuer, M., Maleck, T.L., and Lighthizer, D.R., 1997. Paint-Line Retroreflectivity
over Time. Transportation Research Record 1585,
D.C., 1997, pp. 53–63.
8. Zwahlen, H.T. and Schnell, T., 1995.
Visibility of New Pavement Markings at Night
Under Low-Beam Illumination. Transportation
Research Record 1495, pp. 117–127.
9. Zwahlen, H. T. and Schnell, T., 1997.
Visibility of New Centerline and Edge Line Pavement Markings. Transportation Research Record
1605, pp. 49–61.
10. Schnell 2003
11. Ibid.
12. VDOT, 2008. www.vdot.virginia.gov/
business/resources/bu-mat-PaveMarkCh2.pdf,
Access date: April 9, 2008.
13. Gates, T.J., Hawkins, H.G., and Rose,
E.R., 2003. Effective Pavement Marking Practices
for Sealcoat and Hot-Mix Asphalt Pavements. Research Report No. 0-4150-4. Texas Transportation Institute, the Texas A&M University.
14. Kopf, J., 2004. Reflectivity of Pavement
Markings: Analysis of Retroreflectivity Degradation
Curves. Report WA-RD 592.1. Washington State
Transportation Center, University of Washington, Seattle.
15. Thamizharasan, A., Sarasua, W.A.,
Clarke, D.B., and Davis, W.J., 2003. A Methodology for Estimating the Lifecycle of Interstate
Highway Pavement Marking Retroreflectivity.
Paper presented at the 83rd Annual Meeting of
the Transportation Research Board.
David B. Clarke,
Ph.D., P.E., presently serves
as Director of the University of Tennessee Center for
Transportation Research.
He also directs the Tennessee
Transportation Assistance
Program (TTAP), a federally funded Local Technical Assistance Program center providing training,
technical assistance, and technology transfer services
to local highway agencies throughout Tennessee.
Dr. Clarke’s research focus areas include highway
and railroad safety, materials performance, transportation facility operations, and the development
of planning and analytical models.
Xuedong Yan,
Ph.D., achieved his Ph.D.
in civil engineering at
the University of Central
Florida. Dr. Yan serves as a
research assistant professor
in the Civil & Environmental Engineering Department at the University
of Tennessee and also serves as a research director in
the Southeastern Transportation Center (STC) to
technically coordinate the expanding comprehensive
transportation safety research efforts of the STC. Dr.
Yan’s research focus areas include database analysis,
highway design and operation, driving simulation,
and intelligent transportation systems.
ITE Journal on the web / December 2009