Pavement markings serve an important role in delineating the travel way and providing regulatory, warning and guidance information to drivers. The Manual on Uniform Traffic Control Devices (MUTCD) requires pavement markings to be retroreflective unless ambient illumination assures that the markings are adequately visible. Although retroreflectivity is an important property of many traffic materials, few transportation professionals receive formal training on retroreflectivity science. A basic understanding of this knowledge is necessary to appreciate the differences in retroreflective properties between wet and dry markings and the issues associated with measuring wet and dry marking retroreflectivity.
There can be a significant difference in the dry and wet retroreflective performance of markings. Most markings are intended to perform reasonably well when new and dry. But when water covers a marking, there are several factors that can significantly reduce a marking’s retroreflectivity. The major factors are a scattering of light due to specular reflection off the water’s surface and the change in refraction (bending) of light due to the light rays’ passing through the water before the bead.
Over the years, there have been advancements in pavement marking materials that have improved both durability and visibility. In particular, an increased emphasis on the nighttime visibility of pavement markings has occurred. In a recent rulemaking activity, the Federal Highway Administration (FHWA) indicated they plan to initiate rulemaking in the near future to revise the MUTCD to establish minimum levels of pavement marking retroreflectivity. Virtually all of the research supporting minimum retroreflectivity recommendations for pavement markings has focused on the performance of dry markings.
More recently, researchers and industry have focused increased attention on the nighttime visibility of wet pavement markings. This includes developing standard methods for measuring pavement marking retroreflectivity in various conditions of wetness. The Texas Transportation Institute (TTI) recently conducted research that expands the knowledge base on wet pavement marking visibility. A portion of that research effort included evaluations of issues related to measuring marking retroreflectivity in dry conditions and various conditions of wetness. This article briefly describes that research and some of its findings.
The spray and the bucket
ASTM has established numerous standards and procedures for measuring the retroreflectivity of pavement markings using the standard 30-meter geometry. ASTM E1710-05 is the current procedure for measuring the dry retroreflectivity. ASTM E2176-01 is the current procedure intended to represent the retroreflectivity of a marking material during a rain condition. This procedure also is referred to as the “spray” method. This spray rate equates to a rainfall rate of about 9.32 in./hr. However, the tolerances associated with various aspects of this standard procedure can result in a rainfall rate anywhere in the range of 5.78 to 14.39 in./hr. ASTM E2177-01 is the current procedure intended to represent the retroreflectivity of a marking material after rain has stopped and the marking is still wet. This procedure also is referred to as the “recovery” or “bucket” method.
Although the standard procedures for wet markings have been in place for several years, there is little practical experience with them. Therefore, a portion of the TTI research was intended to assess the effectiveness of the E2176 and E2177 methods and to identify critical aspects of each that should be considered for change or taken into consideration by the user when measuring the wet retroreflectivity of pavement markings.
Raining for 20 years
To assess the relationship between the rates used in the ASTM standard procedure with the rainfall rates that could be expected in real-world conditions, TTI researchers evaluated 20 years of Texas nighttime rainfall data acquired from the National Climatic Data Center. In analyzing the percentage of time when rain is present, researchers found that rain occurred during less than 1% of nighttime hours. This percentage only includes the time when rain is falling; it does not include the time when the road may still be wet from previous rain events.
The mean rainfall rate over the 20 years of Texas data was 0.40 in./hr. Approximately 87% of rainfall events produced average rainfall rates of less than 0.50 in./hr, and 93% of events produced average rates less than 0.75 in./hr. Approximately 79% of rainfall events produced maximum rainfall rates of less than 0.50 in./hr and 88% of events produced maximum rates less than 0.75 in./hr. It should be noted that the average rainfall rate is the sum of the observed rainfall divided by the duration of the event. It also should be noted that the maximum rainfall rate is the rate during the heaviest 15-minute interval of an event.
As previously stated, this analysis is limited to only rainfall events that occur at night in Texas. The results of this analysis though are similar to the results obtained by the Virginia Tech Transportation Institute (VTTI) when they analyzed Virginia rainfall events. VTTI found that 95% of rain events in Virginia had rainfall rates of 0.8 in./hr or less.
Soaked in data
A total of 14 measurement conditions (dry, recovery and 12 different continuous wetting conditions) were evaluated. The basic concept of the methodology was to measure the retroreflectivity of 18 samples of various pavement marking materials under the specified dry and wet conditions, then evaluate the results to identify trends and correlations. The marking materials represented a range that included many common materials and several newer materials specifically designed to provide wet-weather nighttime performance.
A continuous wetting setup was constructed to produce rainfall intensities of 1.2, 2, 4, 6, 8, 9.5, 11.5, 14 in./hr and a flooding condition. These levels cover the range suggested by E2176 (6-14 in./hr) as well as rainfall rates higher and lower than this range. The lowest rainfall rate was the lowest possible rate that could be consistently produced by the spray setup; the highest rate was not consistently measurable as it was over 20 in./hr and thus deemed flooding. For rainfall rates less than 1 in./hr, the researchers made the measurements at the outdoor TTI rainfall tunnel. This tunnel provided the ability to create rainfall intensities of 0.28, 0.52 and 0.87 in./hr.
The researchers evaluated the effect of cross slope on the measurements by collecting recovery and continuous wetting retroreflectivity measurements on four sample materials. The researchers conducted several measurements of markings placed on 0, 2 and 4% cross slopes. As this was a preliminary setup prior to the formal data collection, the intensity rate for the continuous wetting condition was about 10 in./hr, which is slightly greater than the recommended rate in E2176, but well within the allowable limits.
Researchers found that an increase in cross slope resulted in an increase in retroreflectivity for the wet measurement conditions. Researchers found that the lack of slope on flat markings resulted in a flooded marking that significantly reduced the retroreflectivity measurement. In most cases, providing a 2% cross slope resulted in a retroreflectivity increase of 20% or more over the flat marking. Providing a 4% cross slope increased the retroreflectivity measurement by almost 50% or more. Based on these results, and the knowledge that the outdoor rain tunnel had a cross slope of about 2%, the researchers decided to position all markings at a cross slope of 2% for all wet measurements.
Dry isn’t the same as wet
Researchers found a consistent trend in the change of retroreflectivity values associated with the three ASTM standard procedures. The trend indicated that markings have the highest retroreflectivity when dry and lowest retroreflectivity during continuous wetting conditions, with recovery retroreflectivity values falling in between. Though the overall trend is consistent, for each marking type the results are not as consistent indicating that different marking types are not affected by the same magnitude during wet measurements.
The retroreflectivity values as a function of wetting rates between 0.28 and 20 in./hr (flood) indicates that almost all materials have noticeable changes in retroreflectivity as the wetting rate increases. In many cases, the change is substantial. Several of the markings were flooded before reaching even the lowest ASTM rainfall rate. The flooded markings displayed almost no retroreflective properties, and the recorded retroreflectivity values were attributed to measurement “noise.” After removing the markings that were not providing useful data, the retroreflectivity values for the 13 remaining samples were 12.8% less at a rate of 9.5 in./hr than at 6 in./hr. At a rate of 14 in./hr, the retroreflectivity values for the 13 samples were an average of 33.2% less than at 6 in./hr. This result indicates both a statistical and practical difference between retroreflectivity at various wetting rates.
Researchers also found that there was not a smooth transition between the measurements made at the rain tunnel and measurements made with the spray setup. Retroreflectivity values decreased as the rain tunnel rate increased, but then increased for the lowest wetting rate of the spray setup. The results showed a clear discrepancy between conditions that are more reflective of rain conditions (rain tunnel) from those that are more conducive to ease of measurement (spray setup).
After the study was complete, the following recommendations were made:
- Wet retroreflectivity values are less than dry retroreflectivity values. As continuous wetting intensities increase, retroreflectivity levels decrease. These decreases are not consistent across all material types;
- Dry retroreflectivity values are not a good predictor of wet-marking retroreflectivity;
- Wet weather performance is one of many factors that practitioners should consider when selecting pavement marking materials. Other factors that practitioners should consider include dry weather performance, costs, application capabilities, availability, service life, maintenance requirements and driver needs; and
- While wet-marking retroreflectivity provides a measure of performance for selected materials, by itself ASTM E2176 should not be used to establish performance criteria or requirements associated with installation or maintenance for the following reasons:
1. Markings are not viewed at night in rainy conditions very often;
2. The rainfall intensity recommended by E2176 is much greater than what can be expected during virtually all rainfall events;
3. The rainfall intensity range allowed by E2176 has a significant impact on retroreflectivity measurements, making it difficult to provide comparable and repeatable measurements between installations unless exactly identical setups and conditions are used; and
4. Variability in spray characteristics and marking longitudinal and lateral slope has a significant impact on wet retroreflectivity measurements.
