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Publication Number: FHWA RD-03-081
Date: June 2003
Updated Minimum Retroreflectivity Levels
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CHAPTER 4. UPDATED MR LEVELS
This chapter describes analyses conducted to develop a set of preliminary updated MR levels. It includes the demand luminance criteria and other related conditions used to establish MR levels. The discussion is divided into three sections corresponding to the type of sign: guide, warning, and regulatory.
Several types of guide signs were considered for this analysis, all of which were assumed to have white legends on green backgrounds. Large and small guide signs and street-name signs were considered. This section describes how the updated MR levels were established for guide signs.
Large Guide Signs
The large guide sign category represents those used on freeways and expressways: used on high-speed facilities, have large letters, and are designed with redundancy. Overhead and right- and left-shoulder-mounted guide signs were considered.
A recent survey of transportation agencies showed that the combination of 16/12-inch uppercase/lowercase Series E (Modified) letters are the most commonly used legends for large guide signs.(46) Using the MUTCD legibility index criterion of 40 ft/in of letter height, it was assumed that overhead and shoulder-mounted guide signs need to be legible at 640 feet.
The signs were assumed to be located at fixed positions corresponding to typical State DOT practices. The overhead sign was positioned with a centroid 25 ft above the pavement surface and offset 18 ft to the left of the travel lane right edge line (i.e., centered above the left adjacent lane). Both the right- and left-shoulder-mounted guide signs were positioned with a centroid height of 14 ft above the pavement surface. The offsets used for these signs were 30 ft to the right and 42 ft to the left of travel lane right edge line.
Using the TTI demand luminance data for guide signs, MR levels for overhead and shoulder-mounted guide signs were based on demand luminance values of 2.3 and 3.2 cd/m2, for the 55-year-old and 65-year-old driver data sets, respectively (see figure 2). The corresponding MR levels needed to satisfy these demand luminance values are shown in table 7. The demand luminance values and therefore the MR levels shown in table 7 represent the white legend for white-on-green signs.
The MR levels for the green background were determined by first calculating the ratio of the levels shown in table 7 to the levels of ASTM D4956, (17) then multiplying the calculated ratio and the green levels of ASTM D4956. The white-to-green ratios of ASTM D4956 change as a function of sheeting type designation. Therefore, this process was completed by sheeting type. This was the same process that was used for all positive contrast signs.
Small Guide Signs
The small guide sign category of white-on-green signs was developed for guide signs much smaller than one would typically find on freeways; an example is destination and distance signs found along conventional highways. Only right-shoulder-mounted signs were included in this analysis.
These signs were assumed to have a legend made of Series D with letter heights of 8 inches. Therefore, using the MUTCD legibility index criterion of 40 ft/in of letter height, it was assumed that small guide signs need to be legible at 320 feet.
As with the large guide signs, the signs were assumed to be located at fixed positions corresponding to typical State DOT practices. The centroid height was assumed to be 8 ft above the pavement surface and offset 10 ft from the travel lane right edge line.
The TTI demand luminance data for street name signs was used to determine the initial MR levels for small guide signs. The street-name sign data were used instead of the guide sign data because the legends were assumed to be Series D, which is all uppercase letters, as is Series C, which was used on the street-name signs. To account for the legibility differences between Series C and D (because of the wider stroke width of Series D), the TTI demand luminance was lowered by 10 percent. Therefore, the demand luminance values for small guide signs were 3.5 and 6.2 cd/m2, for the 55-year-old and 65-year-old driver data sets, respectively. The MR levels associated with these demand luminance criteria are shown in table 8.
Because street-name signs are installed in somewhat unusual positions compared to other white- on-green signs, the researchers felt they warranted a dedicated analysis. Two street-name sign positions were analyzed. One was a right shoulder mounting and the other was an overhead mounting.
The size of the legends of the street-name signs varied depending on the speed limit on the roadway under consideration. In general, FHWA's proposed recommendations for Revision #2 of the Millennium MUTCD were used to select letter height as a function of speed. Table 9 provides a summary of the letter heights used for different speed ranges and the distances resulting from the application of the 40 ft/inch of letter height legibility concept.
Both types of street name signs were assumed to be located at positions corresponding to typical practices. The centroid height of the ground-mounted street-name sign was assumed to be 9 ft above the pavement surface (which is based on the assumption that it is located on top of a STOP sign) with an offset of 6 ft from the travel lane right edge line. The overhead street-name sign was assumed to be located on a signal mast arm or span wire and was therefore positioned 18 ft above the pavement surface and centered above the travel lane.
The TTI demand luminance data for street-name signs was used to determine the initial MR levels for these signs. Therefore, the demand luminance values were 3.9 and 6.9 cd/m2, for the 55-year-old and 65-year-old driver data sets, respectively. Table 10 shows the set of preliminary updated MR levels associated with these criteria.
Warning signs include both black-on-yellow and black-on-orange signs. The analyses included two types of sign legends: symbol and text. For the symbol legends, a binary subclass was defined based on the symbol design, which included "fine" and "bold" classes. Symbol signs were initially categorized into these classes based on the initial analysis of Mercier et al. data. However, not all signs types were studied by Mercier et al. Therefore, Paniati's work on symbol sign legibility distances was used to classify the remainder of the symbol signs.(59)
For warning signs with text legends, an inventory of the Standard Highway Signs was completed where the sign size, letter size, and letter type were recorded.(33) The results of this inventory are shown in table 11.
Based on the inventory shown in table 11, the researchers selected a 6-inch letter to represent warning signs 36 inches or less. When the size of warning signs is increased to 48 inches, the legend size typically increases by 2 inches. Therefore, for warning signs larger than 36 inches, an 8-inch-tall letter was used.
Based on Mercier et al.'s study, (x) demand luminance curves were developed for Series C and Series D letters. However, because there is almost a 50-50 split between these two alphabet types (see table 11), the average results of both curves were used to establish a specific demand luminance requirement for the basis of MR. The criteria used to establish MR levels for text-based warning signs are shown in table 12. Table 13 shows the set of preliminary updated MR levels associated with these criteria.
For the bold symbol warning signs, a legibility distance of 240 ft was assumed, regardless of size or class of symbol. However, the analysis of Mercier et al.'s data showed a distinct demand luminance difference between fine and bold symbol signs. Further comparisons to Paniati's work related to symbol sign legibility distances confirmed this distinction.(59) Therefore, symbol signs were classified into the bold and fine classes, whose demand luminance values were 1.0 and 3.2 cd/m2, respectively. Table 14 shows the set of preliminary updated MR levels associated with these criteria.
A sensitivity analysis based on sign position indicated that warning signs installed on the left side of a roadway require approximately 50 percent more retroreflectivity than warning signs located on the right side of a roadway. This increase is also evident in table 7, which presents the initial set of updated MR levels for large guide signs.
There are two general types of regulatory signs with significant differences, so the analysis was split into two main headings: black-on-white and white-on-red regulatory signs. This section describes the analyses for each type.
Regulatory signs are almost always installed at the location where the specific regulation to which they refer begins. As described earlier, this practice can create a problem when using the legibility index of 40 feet per inch of letter height at high speeds. Therefore, the MRVD distances from CARTS were used for regulatory signs. By using the MRVD criteria, the updated MR levels consider the distance traveled from an initial speed to a final speed (depending on the sign) by serially summing the time required to detect a sign, recognize the message, decide an appropriate maneuver, initiate the response, and complete the response.
Four different black-on-white regulatory signs were analyzed. The SPEED LIMIT sign was analyzed to determine the MR levels needed to read the numbers on the sign. A KEEP RIGHT sign, ONE WAY sign, and a NO RIGHT TURN sign were also analyzed to determine the MR levels needed for the signs' symbolic message to be recognized.
The criteria used to establish the updated MR levels for SPEED LIMIT signs are shown in table 15. Table 16 shows the resulting updated MR levels.
The criteria used to establish the updated MR levels for the three symbol-based regulatory signs are shown in tables 17 through 19. The resulting updated MR levels are shown in tables 20 through 21.
Two sets of preliminary updated MR levels were developed for white-on-red signs. One set was for STOP signs and the other was for DO NOT ENTER signs. Again, it was assumed that the legends of these types of signs are not actually read but the signs are recognized through their unique design characteristics.(37)
The criteria used to establish the updated MR levels for STOP signs are shown in table 23. The resulting updated MR levels are shown in table 24. Tables 25 and 26 show the criteria and resulting updated MR levels, respectively for the DO NOT ENTER sign.
At least in theory, nearly every individual driver may need a unique set of MR levels that address the different signs she or he may encounter. In addition to covering all the various signs, each set of driver-specific minimum levels would vary depending on factors such as the vehicle and even the driving environment (i.e., rural, suburban, and urban). However, from a practical point of view, the MR levels need to be easy to manage and implement, and thus be consolidated into a straightforward format. This was one of the most consistent and frequently heard comments during the four national MR workshops held over the summer of 2002.(14) To consolidate the MR levels, certain decisions were made regarding the resolution of the levels. The consolidation efforts ultimately resulted in some degree of compromise between the precision of the minimum levels and their brevity.
The research team proposed the first step toward consolidation by suggesting the elimination of MR levels associated with the demand luminance levels representing the 50th percentile of drivers 65 and older. This early decision reduced the total number of specific numeric values by 50 percent, leaving only MR levels associated with demand luminance levels representing the 50th percentile of drivers 55 and older (i.e., a 62-year-old driver). The researchers based this suggestion on several factors, including:
The suggestions were submitted to FHWA as Working Paper #2. (31) The researchers then met with the Retroreflectivity Technical Working Group (RTWG) of FHWA to discuss the recommendation of eliminating MR levels associated with demand luminance levels representing the 50th percentile of drivers 65 and older. The group reached a consensus to drop the higher levels. To receive additional feedback regarding this decision, the RTWG of FHWA decided to present the preliminary levels to the participants of the first of four national MR workshops, which was held in Lakewood, CO, in July 2002 (the handout materials, including the then- current research recommendations regarding MR levels, are available on the Web at http://tcd.tamu.edu.
In general, the comments received from the Colorado participants were positive. They noted that retroreflectivity alone would not define nighttime sign visibility. Other factors such as color, uniformity, and sight distance are also important. The participants were also concerned about the precision of the MR levels. For instance, if a sign has a measured retroreflectivity of 38 cd/lx/m2 and the minimum level for that sign is 40 cd/lx/m2, then the sign would technically fail to meet the minimum levels although the difference would not likely be noticeable from a driver's perspective. An example of a resolution was to use a band of retroreflectivity levels representing desired and minimums.
After the Colorado workshop, the RTWG and the researchers met several times throughout the summer of 2002 to discuss additional consolidation efforts. During the remaining three workshops, the most current form of the MR levels was presented so that researchers could receive outside feedback (again, the levels presented to each workshop are available on the Web at http://tcd.tamu.edu.
During the consolidation process, the RTWG and the researchers made several assumptions regarding the amount of consolidation that could be performed without compromising the use of any one particular type of retroreflective sheeting material. The assumptions were based on various data and information that either FHWA or the researchers had available. For instance, for the majority of the minimum levels proposed herein (i.e., 25 - 75 cd/lx/m2),* it was assumed that there is no perceivable difference in sign luminance when retroreflectivity values are within 15 cd/lx/m2. It was also assumed that for microprismatic retroreflective sheeting materials the practical difference between relatively low retroreflectivity levels (at least in terms of the typical levels represented by microprismatic retroreflective sheeting materials) such as 100 cd/lx/m2 and 50 cd/lx/m2 is insignificant. This assumption was based on currently available weathering data, which indicate that it is doubtful that these types of sheeting materials will ever reach such low levels without catastrophic failure (such as delaminating).
There were also some exceptions. For instance, using the similar ratio method to determine MR levels for backgrounds would have eliminated all sheeting materials except microprismatic materials for overhead guide signs. However, several States are using a combination of microprismatic sheeting materials for the legends of white-on-green signs with beaded sheeting materials for the background. This practice appears to be catching on well. Therefore, exceptions were made to allow any type of retroreflective sheeting to be used for the background, as long as it was maintained to a level that produced enough luminance for adequate color coding at night.
The results of the consolidation efforts are presented in table 27. The MR levels represent the most current research recommendations, but are subject to change as additional research is performed and implemented. A list of research needs is presented later in this report.
It is important to note that the level of complexity of the MR levels of 1993 and 1998 was a particularly significant issue as seen by the AASHTO Retroreflectivity Task Force. As the research to update the MR levels was nearing completion, the researchers focused on consolidating the recommendations into an easy-to-use format. In consolidating the MR levels, certain decisions were made as described above. The consolidation efforts ultimately resulted in some degree of compromise between the precision and the brevity of the MR levels.
*Retroreflectivity levels discussed without reference to a specific measurement geometry should be assumed to have the standard measurement geometry, which includes an observation angle of 0.2 degrees and an entrance angle of -4.0 degrees.