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Publication Number: FHWA RD-03-081
Date: June 2003
Updated Minimum Retroreflectivity Levels
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CHAPTER 5. ASSUMPTIONS AND LIMITATIONS
The recommended updated MR levels presented in this report represent the most recent results of dedicated research studies undertaken over the past two decades. They also represent the latest efforts in a long series of safety considerations related to providing safe and efficient roadways. FHWA will initiate MR rule making soon and it is likely that the recommendations presented here will accompany the rule. While it is impossible to predict the outcome of the rule-making process, it is possible and important to summarize the assumptions and limitations of the updated MR levels.
The key assumptions associated with the updated MR levels are described below.
The recommended updated MR levels are a function of the type of retroreflective sheeting material used on the sign face. Moreover, they depend on the ASTM classification scheme described in D-4956.(17) Several issues are created by referencing the ASTM classification scheme in the MR levels
One issue is that the committee in charge of maintaining ASTM D-4956 is currently debating the reorganization of the classification scheme. Much debate and controversy surrounded the initial development of the current scheme. Since the current scheme was approved, multiple proposals have been presented to ASTM suggesting different schemes. ASTM is currently considering the latest of these, which proposes that Types IV, VII, and VIII would no longer be described. A new type, Type X, would include Types VII and VIII. Type IV is no longer manufactured, so its removal is more maintenance than reclassification, although some DOT specifications still include Type IV as an approved material. It is likely that ASTM will develop a new classification scheme or at least modify the current one. The MR levels then will need to be modified to be current.
Another issue associated with the reference of the ASTM classification scheme is that it is based on measurement geometries that do not represent actual driving scenarios and therefore may not be the best criterion to use as a scheme. Perhaps the most revealing research related to actual viewing geometries versus standard specification geometries was completed by Brich in 2001.(60) In this research, Brich demonstrated the need to classify retroreflective sheeting materials on geometries that actually represent typical driving scenarios. Other countries are currently developing retroreflective sheeting material classification schemes based on such scenarios.(40) Fortunately, research is currently underway through the National Cooperative Highway Research Program in the United States that may provide a more practical classification scheme (Project 4-29).
Also, by using an ASTM Type designation rather than specific manufacturer and brand names, an assumption is introduced that inherently indicates that all manufacturer/brand products meeting a certain ASTM Type designation performance similarly. For example, according to FHWA's retroreflective sheeting material identification guide, at least nine products can be classified as ASTM Type III materials. (61) Not all of these perform equally, and a certain amount of error is introduced by collapsing them into one classification category. The amount of error is unknown but depends on various factors such as:
Several unresolved issues are associated with the measurement of retroreflectivity. This section describes these issues and indicates what is being done to address them.
One of the largest unknowns in terms of measuring retroreflective sheeting material is the repeatability and reproducibility of the equipment used to measure retroreflectivity. Several devices currently are available to make measurements; they can be described as either contact or non-contact devices. Some are portable and some are not. Regardless of which specific device is used to make retroreflectivity measurements, no information is currently available that describes the expected error associated with measurements.
One specific source of measurement error should be ASTM E-1709, Standard Test Method for Measurement of Retroreflective Signs Using a Portable Retroreflectometer. However, the precision and bias statement of E-1709 has not been completed. Preliminary estimates indicate error rates approaching 20 percent at the 95-percent confidence level. Fortunately, research is currently underway that will provide repeatability and reproducibility statistics for most currently available retroreflectometers.
According to ASTM E-1709, at least four measurements should be averaged when determining retroreflectivity of a specific sign. In a recent study, researchers measured retroreflective traffic signs in accordance with ASTM E-1709.(62) The signs had been removed from service by TxDOT maintenance personnel. Up to six retroreflectivity measurements were made on each sign. The retroreflective sheeting materials were limited to Type I and Type III (although most were Type I, because TxDOT switched from Type I to Type III in 1993). The results of the retroreflectivity measurements are summarized in table 28.
While the data used to generate the variability values shown in table 28 were not meant for such purposes and therefore are not statistically valid, they do indicate the level of imprecision that exists when measuring retroreflectivity on signs that have been in the field for a considerable length of time. Additional information is needed in standards, specifications, and practices in terms of how to measure retroreflectivity of used signs and how to use the data to get a representative retroreflectivity value.
Establishing a national standard for minimum levels of retroreflectivity as instructed by Congress requires accurate methods to measure retroreflectivity. Instruments are commercially available for these measures, and documented standards establish procedures for such measurements. However, there can be significant variability among instruments measuring the same object, and the standards do not ensure accuracy of the instruments. There are currently no traceable methods in the United States to determine the accuracy of measurements, because national calibration standards for retroreflectivity do not exist.
Research currently underway by the National Cooperative Highway Research Program is devoted to the development of a dedicated reference instrumentation suitable for calibration and characterization of retroreflective reference materials. This research is being performed by the National Institute of Standards and Technology, and will be complete in 2004.
ASTM E-1709 specifies two general types of sign retroreflectometers: point instruments and annular instruments, (62) and defines them as follows:
The instrument may be either a "point instrument" or an "annular instrument," depending on the shape of the receiver aperture. Point and annular instruments make geometrically different measurements of retroreflectivity, which may produce values differing on the order of 10 percent. Both measurements are valid for most purposes, but the user should learn the type of his instrument from its specifications sheet and be aware of certain differences in operation and interpretation. For both instrument types, the "up" position should be known. The point instrument makes a measurement virtually identical to a measurement made on a range instrument following the procedure of Test Method E 810...The annular instrument makes a measurement similar to an average of a great number of measurements on a range instrument with presentation angle (γ) varying between -180° and 180°.
Additionally, ASTM E-1709 includes the following information regarding rotational sensitivity. Glass bead sheetings tend to be rotationally insensitive. Therefore, point and annular instruments should produce similar retroreflectivity values for these sheetings. The values for prismatic sheeting are rotationally sensitive, and the values produced by point and annular instruments can differ on the order of 10 percent, with differences of up to 25 percent possible. Neither the magnitude nor the direction of difference can be predicted for unknown samples. Annular instruments cannot accurately gauge how the retroreflectivity of prismatic sheeting varies with rotation angle. Most prismatic retroreflectors are rotationally sensitive, having retroreflectivity values that vary significantly with rotation angle, even at small entrance angles.
A point instrument can gage the variation of retroreflectivity with rotation angle by placing it with different angular positions upon the sign face; variation of 5 percent for 5 degrees rotation is not unusual. Accordingly, repeatable retroreflectivity measurement of prismatic signs with a point instrument requires care in angular positioning.
To demonstrate the impacts of rotational sensitivity, consider the retroreflectivity measurements shown in figure 7. Here, retroreflectivity measurements were made with an annular retroreflectometer in 15-degree intervals from 0 degrees to 360 degrees. The measurements were made on various prismatic sheetings and on one beaded sheeting. Additionally, measurements were made on an unweathered control sample and a sample that had weathered approximately 3 years at a 45-degree orientation and facing south.
Figure 7 clearly demonstrates the rotational sensitivity of certain microprismatic retroreflectivity sheeting materials. The sample labeled 101 is beaded sheeting (white Type III encapsulated). The other materials are all microprismatic. Tables 29 and 30 were developed with the data from figure 7 to better demonstrate the actual rotational sensitivity of these materials.
The coefficient of variation (CV) is a measure of the dispersion and could be considered a method of normalizing the standard deviation. It is one of the best measures of rotational sensitivity. For example, a standard deviation of 50 could be considered small if the mean were 700 (CV = 0.07), but reasonably large if the mean were 250 (CV = 0.20). A low CV means that the material is rotational insensitive.
It is clear that, as indicated in ASTM E-1709, there is little rotational sensitivity with the beaded material (CV = 0.003). However, for the microprismatic materials, the CV value depends on the type of prismatic sheeting. Furthermore, the relative CV value changes depending on the type of microprismatic material and whether it was weathered. Additionally, the ratio between the minimum and maximum measured retroreflectivity is more than 200 percent for one of the samples. Obviously, this level of sensitivity will have implications when the updated MR levels are implemented.
From a practical point of view, researchers have demonstrated that the sensitivity of the orientation angle, s, was prominent only when vehicles were located 100 ft from the microprismatic retroreflective targets.(63) At 100 ft, when the datum axis of the microprismatic materials was changed (s not equal to 0) via the orientation angle, the performance of the sheeting degraded, in some instances significantly. However, at further distances of 300, 500, and 800 ft the degradation was small to negligible.
The study mentioned above is currently analyzing the impacts of rotational sensitivity of the currently available retroreflectometers. This study is also investigating how rotational sensitivity depends on the various types of retroreflective sheeting materials.
Figure 7 provides some early insight into the nonuniform degradation that some microprismatic retroreflective sheeting materials demonstrate. For instance, while there appears to be uniform degradation of samples 630 and 101, the other three samples show shifts in the peaks and valleys of the retroreflectivity measurements. One of the main assumptions related to the development of the updated MR levels is that all retroreflective sheeting materials degrade uniformly over time. In others, as a specific product weathers and becomes less efficient, its retroreflective properties degrade uniformly across a range of observation and entrance angles.
To test this assumption, the researchers obtained and measured 3-year Transportation Product Evaluation Program (NTPEP) panels (NTPEP weathers panels for a maximum of 3 years at a 45-degree orientation facing south). Table 31 includes a description of the panels and the measurements that were made at FHWA's Photometric/Visibility Lab.
The practice of weathering panels at a southern orientation and at 45 degrees doubles the degradation rate compared to a standard vertically mounted sign. Therefore, the weathered panels have been effectively weathered for 6 years.
The results of the measurements and subsequent analyses showed that, as expected, the retroreflective sheeting made with glass beads degrades uniformly, but sheetings made with microsized prisms does not. Although the extent of the variations depends on the sheeting type and weathering location, for all sheetings the variation of degradation rate increases with increased observation and entrance angles. Figure 8 shows an example of the observation angle profiles obtained from the panels weathered in Louisiana.
Figure 8b Percent Degradation of Louisiana Panels, Beta = 30°
Figure 8 shows that further weathering and subsequent analyses are needed. Because weathering takes time, an efficient way to obtain weathered data beyond 3 years would be to continue weathering the NTPEP panels (which have been stored in such a manner to prevent further degradation). However, several caveats are introduced by using the spent NTPEP samples. For instance, some panels may have been stored in ideal conditions while others may not. Because of the timing of when various materials were installed on the NTPEP panels, there may be a need to use some materials that have been in storage for a few years while other materials may be just coming off the NTPEP racks.
While significant progress has been made in the past 20 years regarding the nighttime visibility requirements of traffic signs, there is a need for additional research. The following research topics, which are based on the assumptions and limitations associated with the proposed MR levels, are recommended by the research team.
Long-term weathering research is needed to determine the validity of the uniform degradation assumption (over a practical range of observation angles). This research should also address the performance of retroreflective sheeting relative to the rotational aspects of retroreflectivity measurements made with point-source instruments.