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Publication Number: FHWA RD-03-081
Date: June 2003

Updated Minimum Retroreflectivity Levels

Final Report

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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.

GUIDE SIGNS

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.

Table 7 Initial MR Levels for Large Guide Signs

Position Speed Luminance Level ASTM Sheeting Type
I II III VII VIII IX
Overhead Any 55 290 290 250 230
65   400 350 320
Right Shoulder 55 115 115 110 95 100
65 160 160 155 135 140
Left Shoulder 55 210 210 190 160
65 290 295 260 225
•Retroreflectivity (cd/lx/m2) at observation angle = 0.2° and entrance angle = -4.0°.
•Represents only the white legend of white-on-green signs.
•Blank cells indicate that new sheeting will not provide sufficient levels of supply luminance to meet the demand luminance levels.

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.

Table 8. Initial MR Levels for Small Guide Signs

Position Speed Luminance Level ASTM Sheeting Type
I II III VII VIII IX
Right Shoulder Any 55 125 130 165 130 60
65 235 290 225 105
•   Retroreflectivity (cd/lx/m2) at observation angle = 0.2° and entrance angle = -4.0°.
•   Represents only the white legend of white-on-green signs.
•   Blank cells indicate that new sheeting will not provide sufficient levels of supply luminance to meet the demand luminance levels.

Street-Name Signs

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.

Table 9. Assumed Characteristics and Criteria for Street Name Signs

Position Ground Ground Ground Overhead
Speed (mph)
>40
30-40
< 25
Any
Letter height (in)
8
6
4
12
MRVD (ft)
320
240
160
480

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.

Table 10. Initial MR Levels for Street Name Signs

Position Speed Luminance Level ASTM Sheeting Type
I II III VII VIII IX
Ground > 40 55 140 145 180 140 70
65 255 315 245 120
30–40 55 240 290 285 80
65 170 210 255 250 70
< 25 55 710 660 135
65 240
Overhead Any 55 265 290 225 195
65 510 400 340
• Retroreflectivity (cd/lx/m2) at observation angle = 0.2° and entrance angle = -4.0°.
• Represents only the white legend of white-on-green signs.
• Blank cells indicate that new sheeting will not provide sufficient levels of supply luminance to meet the demand luminance levels.

WARNING SIGNS

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.

Table 11. Inventory of Standard Size Warning Signs

Letter Height (in) FHWA Alphabet (Primary Legend) N = 178 signs Row Cumulative Percent
Series C Series D Series E Percent
3 4 0 0 2.2 2.2
4 11 21 0 18 20.2
5 30 38 0 38.2 58.4
6 17 15 4 20.2 78.7
7 8 6 0 7.9 86.5
8 7 8 7 12.4 98.9
9 0 0 0 0 98.9
10 0 0 2 1.1 100
Column Percent 43.3 49.4 7.3

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.

Table 12. Criteria for Text-Based Warning Signs

Sign Size (in) [measured along an edge of the diamond] < 36 >36
Letter Height (inch) 6 8
Critical Distance (ft) 240 240
Effective LI (ft/inch) 40 30
Demand Luminance - 55 (cd/m 2) 1.4 1.0
Demand Luminance - 65 (cd/m 2) 1.9 1.0
Sign centroid height = 8.5 ft and offset from right edge line = 10.ft. One foot equals .31 meters.

 

Table 13. Initial Retroreflectivity Levels for Text Warning Signs

Position Sign Size Luminance
Level
ASTM Sheeting Type
(inch) I II III VII VIII IX
Right Shoulder
< or = 36
55
48
66
84
107
98
28
65
65
89
114
145
133
37
> 36
55
29
40
51
65
60
17
65
34
47
60
76
70
20
• Retroreflectivity (cd/lx/m2) at observation angle = 0.2° and entrance angle = -4.0°.

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.

Table 14. Initial Retroreflectivity Levels for Symbol Warning Signs

Position Sign Size Luminance Level ASTM Sheeting Type
(inch) I II III VII VIII IX
Right Shoulder Any Bold Symbol 34 47 60 76 70 20
Fine Symbol 151 192 244 224 65
• Retroreflectivity (cd/lx/m2) at observation angle = 0.2° and entrance angle = -4.0°.
• A blank cell indicates that new sheeting will not provide sufficient levels of supply luminance to meet the demand luminance levels.

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.

REGULATORY 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.

Table 15. Criteria for Speed Limit Signs

Sign Size (width) Luminance Level Speed (mph)
70 55 45 35 25
MRVD (ft)
513 393 308 227 145
24"
55
1.7
1.5
1.3
1
1
65
3.8
3.5
3.1
2.5
1.7
36"
55
1.4
1.2
1
1
1
65
3.4
2.9
2.5
1.9
1.1
48"
55
1.3
1
1
1
1
65
3.2
2.7
2.2
1.6
1
Sign centroid height = 8 ft and offset from right edge line = 10 ft.

 

Table 16 Initial Retroreflectivity Levels for Speed Limit Signs

Sign Size Speed Luminance
Level
ASTM Sheeting Type
I II III VII VIII IX
24" Sign 10" E
70
55
33
33
33
34
29
25
65
73
74
73
76
62
56
55
55
39
43
43
52
38
26
65
90
100
101
101
88
61
45
55
63
80
84
107
84
38
65
200
255
200
91
35
55
31
45
62
77
74
19
65
78
113
155
192
185
48
25
55
42
46
88
337
200
41
65
72
78
150
574
340
69
36" Sign 12"E
70
55
27
27
27
28
23
21
65
65
66
65
68
56
50
55
55
31
34
35
41
30
21
65
75
83
83
100
73
50
45
55
49
61
65
82
64
29
65
153
161
205
161
73
35
55
31
45
62
77
74
19
65
59
86
118
146
140
36
25
55
42
46
88
337
200
41
65
46
50
97
371
220
45
48" Sign 16"E
70
55
25
25
25
26
21
19
65
61
62
61
64
52
47
55
55
26
29
29
34
25
17
65
69
77
78
93
68
47
45
55
49
61
65
82
64
29
65
135
142
181
142
65
35
55
31
45
62
77
74
19
65
50
72
99
123
118
31
25
55
42
46
88
337
200
41
65
42
46
88
337
200
41
• Retroreflectivity (cd/lx/m2) at observation angle = 0.2° and entrance angle = -4.0°.
• A blank cell indicates that new sheeting will not provide sufficient levels of supply luminance to meet the demand luminance 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.

Table 17. Criteria for KEEP RIGHT Signs

Sign Size (width) Luminance Level Speed (mph)
70 55 45 35 25
MRVD (ft)
273 245 218 196 174
24" 55 1 1 1 1 1
65 1.73 1.32 1.06 1 1
36" 55 1 1 1 1 1
65 1 1 1 1 1
Sign centroid height = 8 ft and offset from right edge line = 10 ft.

 

Table 18. Criteria for ONE WAY Signs

Sign Size (width) Luminance Level Speed (mph)
70 55 45 35 25
MRVD (ft)
225 203 188 173 159
36" 55 1.1 1 1 1 1
65 3 2.4 2.1 1.7 1.3
54" 55 1 1 1 1 1
65 1.1 1 1 1 1
Sign centroid height = 9 ft and offset from right edge line = 10 ft.

 

Table 19. Criteria for NO RIGHT TURN Signs

Sign Size (width) Luminance Level Speed (mph)
70 55 45 35 25
MRVD (ft)
273 245 218 196 174
24" 55 1.00 1.00 1.00 1.00 1.00
65 2.30 2.10 1.70 1.50 1.20
36" 55 1.00 1.00 1.00 1.00 1.00
65 1.10 1.00 1.00 1.00 1.00
Sign centroid height = 8 ft and offset from right edge line = 10 ft.

Table 20. Initial MR Levels for KEEP RIGHT Signs

Sign Size
(width)
Speed
(mph)
Luminance Level ASTM Sheeting Type
I II III VII VIII IX
24" 70 55 24 33 37 46 38 14
65 41 57 65 80 66 25
55 55 29 40 49 63 57 22
65 38 52 65 83 75 20
45 55 33 48 69 93 81 20
65 33 48 69 93 81 20
35 55 35 47 77 127 93 22
65 35 47 77 127 93 22
25 55 38 48 89 165 130 28
65 38 48 89 165 130 28
36"
70 55 24 33 37 46 38 15
65 24 33 37 46 38 15
55 55 29 40 49 63 57 17
65 29 40 49 63 57 17
45 55 33 48 69 93 81 20
65 33 48 69 93 81 20
35 55 35 47 77 127 93 22
65 35 47 77 127 93 22
25 55 38 48 89 165 130 28
65 38 48 89 165 130 28
• Retroreflectivity (cd/lx/m2) at observation angle = 0.2° and entrance angle = -4.0°.

 

Table 21. Initial MR Levels for ONE WAY Signs

Sign Size Speed
(mph)
Luminance Level ASTM Sheeting Type
I II III VII VIII IX
36" Sign 4" D 70 55 39 55 78 96 91 23
65 151 212 263 247 64
55 55 40 56 89 138 103 24
65 96 134 214 331 247 58
45 55 43 58 101 190 128 29
65 91 122 213 400 270 61
35 55 45 57 107 200 162 34
65 76 97 182 340 275 58
25 55 48 56 107 275 200 40
65 62 73 140 358 260 52
54" Sign
6" D
70 55 35 50 71 88 82 21
65 39 55 78 96 91 23
55 55 40 56 89 138 103 24
65 40 56 89 138 103 24
45 55 43 58 101 190 128 29
65 43 58 101 190 128 29
35 55 45 57 107 200 162 34
65 45 57 107 200 162 34
25 55 48 56 107 275 200 40
65 48 56 107 275 200 40
• Retroreflectivity (cd/lx/m2) at observation angle = 0.2° and entrance angle = -4.0°.
•A blank cell indicates that new sheeting will not provide sufficient levels of supply luminance to meet the demand luminance levels.

 

Table 22. Initial MR Levels for NO RIGHT TURN Signs

Sign Size
(width)
Speed
(mph)
Luminance
Level
ASTM Sheeting Type
I II III VII VIII IX
24" 70 55 24 33 37 46 38 14
65 54 76 86 107 88 33
55 55 29 40 49 63 57 17
65 60 83 103 132 120 35
45 55 33 48 69 93 81 20
65 57 82 117 158 138 35
35 55 35 47 77 127 93 22
65 52 70 116 191 139 33
25 55 38 48 89 165 130 28
65 45 58 107 198 156 33
36" 70 55 24 33 37 46 38 14
65 26 36 41 51 42 16
55 55 29 40 49 63 57 17
65 29 40 49 63 57 17
45 55 33 48 69 93 81 20
65 33 48 69 93 81 20
35 55 35 47 77 127 93 22
65 35 47 77 127 93 22
25 55 38 48 89 165 130 28
65 38 48 89 165 130 28
• Retroreflectivity (cd/lx/m2) at observation angle = 0.2° and entrance angle = -4.0°.

White-on-Red Signs

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.

Table 23. Criteria for STOP Signs

Sign Size (width) Luminance Level Speed (mph)
70 55 45 35 25
MRVD (ft)
915 608 437 293 176
30" 55 1.4 1 1 1 1
65 11.5 6.8 3.7 1.3 1
36" 55 1.2 1 1 1 1
65 9.3 5 2.4 1 1
48" 55 1 1 1 1 1
65 6.2 2.7 1 1 1
Sign centroid height = 8 ft and offset from right edge line = 10 ft.

 

Table 24. Initial MR Levels for STOP Signs

Sign Size
(width)
Speed
(mph)
Luminance Level ASTM Sheeting Type
I II III VII VIII IX
30" Sign 10" C 70 55 56 53 52 52 43 46
65 424 352 376
55 55 40 39 38 37 33 31
65 261 254 224 211
45 55 22 23 23 28 20 16
65 80 86 85 99 73 59
35 55 22 30 32 42 33 14
65 28 37 40 51 40 17
25 55 and 65 38 48 88 163 125 27
36" Sign
12" C
70 55 48 46 45 44 37 39
65 343 284 304
55 55 40 39 38 37 33 31
65 192 187 164 155
45 55 22 23 23 28 20 16
65 52 56 55 64 48 39
35 55 and 65 22 30 32 42 33 14
25 55 and 65 38 48 88 163 125 27
48" Sign
16" C
70 55 38 36 36 35 29 32
65 232 228 190 203
55 55 40 39 38 37 33 31
65 106 104 101 89 84
45 55 and 65 22 23 23 28 20 16
35 55 and 65 22 30 32 42 33 14
25 55 and 65 38 48 88 163 125 27
• Retroreflectivity (cd/lx/m2) at observation angle = 0.2° and entrance angle = -4.0°.
• Represents only the white legend of white-on-red signs.
• Blank cells indicate that new sheeting will not provide sufficient levels of supply luminance to meet the demand luminance levels.

 

Table 25. Criteria for DO NOT ENTER Signs

Sign Size
(width)
Luminance
Level
Speed (mph)
70 55 35 25
MRVD (ft)
915 608 293 176
30" 55 3.1 2.2 1 1
65 16.7 11.3 3.2 1
36" 55 2.6 1.7 1 1
65 13.8 8.4 1.7 1
Sign centroid height = 8 ft and offset from right edge line = 10 ft.

Table 26. Initial Retroreflectivity Levels for DO NOT ENTER Signs

Sign Size
(width)
Speed
(mph)
Luminance Level ASTM Sheeting Type
I II III VII VIII IX
30" Sign
4" D
70 55 118 116 114 95 101
65 615 511  
55 55 87 86 84 82 72 68
65 422 372 351
35 55 22 30 32 42 33 14
65 17 23 25 31 25 11
25 55 38 48 88 163 125 27
65 38 48 88 163 125 27
36" Sign
5" D
70 55 99 97 96 79 85
65 508 422  
55 55 67 67 65 63 56 53
65 314 276 261
35 55 22 30 32 42 33 14
65 37 48 52 66 53 22
25 55 38 48 88 163 125 27
65 38 48 88 163 125 27
• Retroreflectivity (cd/lx/m2) at observation angle = 0.2° and entrance angle = -4.0°.
• Represents only the white legend of white-on-red signs.
• Blank cells indicate that new sheeting will not provide sufficient levels of supply luminance to meet the demand luminance levels.

Consolidation

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:

  • It is very possible to preselect conditions that, once analyzed, reveal that no current retroreflective sheeting material can produce supply luminance levels at least equal to the assumed demand luminance levels. For instance, it is possible to consider a 65-year-old driving an 18-wheeler in the right lane of an 8-lane freeway (4 lanes per direction). If the sign position is assumed to be left-mounted and overhead and the headlamps are VOL style, then it is quite possible that no amount of retroreflectivity would result in enough sign luminance for that driver's legibility at an adequate distance. In other words, while the criteria associated with the worst-case scenario ensure that all other conditions are satisfied, they also can produce unrealistic demands.

  • The researchers compared the preliminary sets of updated MR levels to the levels found in ASTM D4956, which represents the minimum levels for new retroreflective sheeting materials.(17) This comparison revealed that many updated minimum retroreflective levels were actually higher than the minimum levels for new sheeting, especially when the higher luminance demand criteria were used. Based on their engineering judgment, and the lack of a scientific link between higher grades of retroreflective sheeting and safety, the researchers felt that the minimum levels based on the lower of the two luminance demand-based levels were the most reasonable in terms of practicality and acceptability.

  • It is possible that modeling parameters could be selected so that the luminance demand criteria are so high that only microprismatic retroreflective sheeting materials would meet the assumed luminance demand. However, while this may not be reasonable, it may also unnecessarily eliminate some retroreflective sheeting materials that perform well.

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.

Table 27. Updated MR Levels for Traffic Signs

Sign Color Criteria Sheeting Type (ASTM D4956-01a)
I II III VII VIII IX
White on Red See note 1 35 //7
Black on Orange or Yellow See note 2 *
50
See note 3 *
75
Black on White   50
White on Green Overhead *// 7 *// 15 *// 25 250 // 25
Shoulder * // 7 120 //15
NOTE: Levels in cells represent legend retroreflectivity * background retroreflectivity (for positive contrast signs). Units are cd/lx/m2 measured at an observation angle of 0.2° and an entrance angle of -4.0°
1. Minimum Contrast Ratio > or = 3:1 (white retroreflectivity ÷ red retroreflectivity).
2. For all bold symbol signs and text signs measuring 48 inches or more.
3. For all fine symbol signs and text signs measuring less than 48 inches.
* (Sheeting Type should not be used.)
 
Bold Symbol Signs
  • W1-1 - Turn
  • W1-2 - Curve
  • W1-3 - Reverse Turn
  • W1-5 - Winding Road
  • W1-6 - Large Arrow (One Directions)
  • W1-7 - Large Arrow (Two Directions)
  • W1-8 - Chevron
  • W1-9 - Turn & Advisory Speed
  • W1-10 - Horizontal Alignment & Intersection
  • W2-1 - Cross Road
  • W2-2, W2-3 - Side Road
  • W2-4 - T Intersection
  • W2-5 - Y Intersection
  • W2-6 - Circular Intersection
  • W3-1a - Stop Ahead
  • W3-2a - Yield Ahead
  • W3-3 - Signal Ahead
  • W4-3 - Added Lane
  • W6-1 - Divided Highway Begins
  • W6-2 - Divided Highway Ends
  • W6-3 - Two-Way Traffic
  • W10-1, -2, -3, -4 - Highway-Railroad Intersection Advance Warning
  • W11-2 - Pedestrian Crossing
  • W11-3 - Deer Crossing
  • W11-4 - Cattle Crossing
  • W11-5 - Farm Equipment
  • W11-5p, -6p, -7p - Pointing Arrow Plaques
  • W11-8 - Fire Station
  • W11-10 - Truck Crossing
  • W12-1 - Double Arrow
All symbol signs not listed in the bold category are considered fine symbol signs.
Special Case Signs
  • W3-1a - Stop Ahead
  • Red retroreflectivity > or = 7, White retroreflectivity > or = 35
  • W3-2a - Yield Ahead
  • Red retroreflectivity > or = 7, White retroreflectivity > or = 35
  • W14-3 - No Passing Zone, W4-4p - Cross Traffic Does Not Stop, or
  • W13-2, -3, -1, -5 - Ramp & Curve Speed Advisory Plaques
  • Use largest dimension

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United States Department of Transportation - Federal Highway Administration