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Minimum Retroreflectivity Levels for Overhead Guide Signs and Street-Name Signs

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CHAPTER 2. PREVIOUS RESEARCH

There have been numerous studies related to many different aspects of overhead guide sign visibility, including a fair number of literature reviews summarizing previous research. Rather than repeating work that has already been performed, this literature review focused on research performed within the last 15 years. A particularly thorough review of the early research was published in 1984 and is used as a starting point for this review. However, where appropriate, earlier landmark research findings have also been referenced.

OVERHEAD GUIDE SIGNING RESEARCH

In 1984, Gordon summarized the nighttime visibility research performed on overhead signing.(6) Gordon reviewed more than 100 research studies concerned with various aspects of overhead guide sign effectiveness.

The results of Gordon's review were compiled and compared against recommendations put forth through a Caltrans (California Department of Transportation) experiment of nonreflective guide sign backgrounds without lighting. (FHWA granted permission for Caltrans to conduct this study in light of the noncompliance with the then-current 1978 MUTCD guidelines for overhead guide signs.) The Caltrans study included 43 porcelain enamel overhead guide signs with button-copy retroreflectors used in the legends and borders. Fourteen observers, mostly civil engineers, ages 32 to 60, were used to evaluate the detection and legibility of the signs while traveling at speeds of 60 miles per hour (mph). The Caltrans team evaluated five aspects of the experimental guide signs: (1) detection, (2) legibility, (3) impact of roadway geometry, (4) impact of background lighting, and (5) color coding.

Recommendations from the Caltrans review team included maintaining lighting on freeway off-ramps and on lane-assignment signs calling for immediate lane changes. It was also recommended that sign lighting be used where fog and dew are frequent occurrences.

The remainder of the literature reviewed by Gordon was compared to the Caltrans findings and recommendations. Gordon reviewed sign detectability and legibility, the effect of high- and low-beam headlamp patterns, traffic stream, angular position of overhead signing, sign maintenance, roadway geometry, and other factors. The relevant findings include (findings from Gordon unless otherwise referenced):

• A white legend of 3.4 candelas per square meter (cd/m²) should be taken as the lower limit of permitted sign luminance. Below this level, legibility rapidly decreases. Using this criterion, it was shown that illuminated button-copy legends on opaque backgrounds were adequate. However, nonilluminated signs, viewed with low-beam headlamps, were not always adequate. Legends of type III sheeting were considered marginal in that they only provided luminance values of 3.4 cd/m² or greater for distances of 450 to 900 ft. There was no distance where type I sheeting provided adequate luminance levels. Button copy was only sufficient at 450 to 600 ft.⁽⁷⁾
• While 3.4 cd/m² should be considered a minimum, 340 cd/m² should be the upper limit. Optimal legend luminance under most highway conditions is between 34 and 102 cd/m². A dark surround permits the use of lower legend luminance.^[1]
• Under high ambient illumination conditions, legend/background luminance ratios as low as 4:1 will provide satisfactory visibility. Under low ambient illumination conditions, where the sign background is almost black, the specific legend luminance is more meaningful than one of contrast.
• Overhead guide signs viewed under low-beam headlamp illumination and wet road conditions provide 3.5 to 5.0 times the amount of luminance as compared to dry conditions.
• Drivers tend to use low-beam headlamps under most driving conditions. Traffic volumes have to be as light as 30 vehicles per hour before high-beam use increases significantly. However, because traffic volumes on roadways with overhead guide signs are not typically this low, the sign is usually illuminated by more than one set of headlamps and can increase the luminance returned to any one vehicle approaching the sign. As a result of this common occurrence, some researchers have recommenced studying overhead guide sign performance under high-beam headlamp illumination.⁽⁷⁾ This thought is believed to counter the condition of research where there is usually one test vehicle compared to the field where multiple vehicles are usually illuminating guide signs.
• Some research has shown that the optimal sign position should be defined as within a 4° vertical and 6° horizontal displacement. However, other research shows that vertical displacements forward or back as much as 5 percent do not affect the luminance enough to have an impact on legibility.⁽⁸⁾
• Under most normal highway conditions, nonilluminated button-copy signs with opaque backgrounds will function satisfactorily if properly maintained, despite the absence of the color-coding redundancy feature that is built into other sign designs. However, auxiliary illumination may be required on curved roads, areas of high and spotted illumination, conditions of frequent fog and dew, and at action situations such as at highway exits.
• Color can be seen only at a luminance above 0.043 cd/m² and then just barely. Other research, subsequently documented, reports substantially higher luminance levels for color recognition.

In 1989, Stein et al., reported nighttime performance of overhead guide signs constructed from button copy and retroreflective sheeting on opaque and retroreflective backgrounds.(9-10) The project was a planned followup to Gordon's literature review and included:

• Investigation of then-current signing practices throughout the United States.
• Development of a set of in-use luminance values for guide sign materials.
• Development of life-cycle costs for signing material available then.
• Determination of driver response characteristics for these overhead guide sign systems.⁽⁶⁾

Stein et al., also tested new materials at the 3M™ test track in St. Paul, MN. The types of materials that were tested are listed below:

• Type III sheeting (white and green).
• Type I sheeting (white and green).
• Prismatic lens sheeting, green (two earlier versions of the prismatic materials available now).
• Prismatic lens sheeting, white (two earlier versions of the prismatic materials available now).
• Type III sheeting on a 12-year-old sign (white and green).
• Porcelain sign material (nonreflective).
• Button-copy legend.
• Reference light source.

Luminance measurements were taken twice, once with standard U.S. headlamps (200-millimeter (mm) sealed halogen beams) and again with standard European headlamps (165-mm H-4 halogen low beams). Of all the materials tested, button copy was the brightest. Type III sheeting was brighter than type I sheeting, which was brighter than the porcelain sign material. From the data reported, and assuming a minimum legend luminance of 3.4 cd/m² for legibility, the new type III sheeting performed adequately from 1500 to 500 ft; however, at 250 ft, the luminance fell to about 1.5 cd/m². Interestingly, the 12-year-old type III sheeting performed slightly better than the new type III sheeting for all distances, although it was still below the assumed 3.4-cd/m² threshold at 250 ft. The type I sheeting never reached a value of 3.4 cd/m². The maximum luminance for type I sheeting occurred at 750 ft, with a value of 2.7 cd/m². The European headlamps provided luminance values far below those reported from the standard U.S. headlamps. At distances of 1500, 1000, and 500 ft, the luminance provided by the European headlamps was 2.0, 3.0, and 5.5 percent of that provided by the U.S. headlamps.

The researchers also conducted a laboratory study to explore conspicuity issues. This study consisted of a static and a dynamic element. The static element included slide presentations of 120 different stimuli. The main factors under investigation were the impact of a driver's age, sign type, distance, color and luminance, and the level of obscurity of the sign. The same independent variables were used in the dynamic study. The following results were found to be statistically significant:

• Green signs provide greater detection distances than black or gray signs.
• As signs become brighter, detection distances increase.
• Increasing the amount of obscurity of a sign decreases its ability to be detected.
• More complex backgrounds compete with the signs for the driver's attention.
• As the driver's age increases, detection distance decreases.

• The differences between sign colors, luminance levels, and obscurity were found to be within one standard deviation. In other words, there is no practical difference between any of these findings.
• The older driver is not helped by any particular sign configuration. When reaction distance was compared for the color/luminance variable, the difference between the best and worst configurations was about 60 ft. Even at 45 mph, the best configuration allows the driver less than 1 second (s) of additional detection time.

• MLRL increase as the ratio of letter stroke width to letter height decreases.
• MLRL increase as the level of internal contrast decreases.
• Published data are inconsistent regarding the effects of sign luminance and ambient luminance.
• MLRL are not influenced by glare, unless the glare source is very bright and immediately adjacent to the sign.
• MLRL increase with the age of the observer.

In 1994, Mace performed another study that included research on guide signs.(15) He concluded that the driver's age had the greatest impact on conspicuity and legibility. Other factors that were determined to be significant were retroreflectivity, letter series, and letter height. For high-contrast signs, Mace found that a reduced stroke width improved legibility. Using letter spacing less than the standard spacing significantly reduced legibility.

Material-Based Research for Overhead Signs

One of the first field research efforts that documented different material types and their effect on legibility was conducted and published in 1966.(16) Using college-age subjects and 16-inch uppercase and 12-inch lowercase letters, the researchers evaluated the following six combinations of overhead guide sign material:

• Button copy on porcelain enamel background.
• Button copy on exposed-lens reflective background.
• "Signal" letters on reflective sheeting background.
• "Signal" letters on exposed-lens reflective background.
• Cutout reflective letters on reflective sheeting background.
• Internally illuminated sign.

Legibility distances greater than 70 ft/inch of letter height were obtained for all combinations except the cutout legend and the internally illuminated sign. The researchers concluded that satisfactory legibility might be achieved under many conditions without the use of overhead sign lighting fixtures. However, this finding is not surprising since it is based exclusively on the results from younger drivers.

Another study of signing materials was conducted 4 years later in 1970.(17) This study included a multidisciplinary team of six individuals observing overhead signs on various routes in various States. The recommendations stated that all overhead signs should be illuminated. However, at one location, the team observed an overhead sign with type III legend and background (type III sheeting was just introduced in the early 1970s). It was noted that this sign provided adequate visibility with low-beam headlamps. The researchers recommended additional research based on this observation.

Consequently, in 1976, the same researchers performed a study of the need for sign illumination when type III sheeting was used for the legend and background of overhead guide signs.(18) The researchers used previous evaluation techniques established by Forbes et al.(19-20) The study included three young subjects and two signs (with 16-inch letters). The first sign was externally lit and fabricated with button-copy legend and type I background. The second sign was unlit and fabricated with type III legend and background. The researchers evaluated sign height, angle of tilt, and approach speed.

The findings indicated that for the unlit type III on type III overhead guide sign, mounting height (from 18.5 to 22.5 ft), angle of tilt (from -5.0 to +5.0 degrees), and vehicle speed (from 35 to 55 mi/h) do not significantly contribute to differences in legibility distances. The average legibility distance for the unlit type III on type III sign was 19 percent less with low beams and 5 percent greater with high beams. The researchers concluded that unlit type III on type III overhead guide signs can be effectively used when background brightness is not excessive and when the minimum direct line of sight is at least 1500 ft.

In support of this conclusion, the Louisiana Department of Highways issued a directive that overhead signs constructed with type III on type III sheeting should not be externally illuminated. This decision was reached after a field test period of more than 3 years.(18)

Robertson conducted two research efforts directed at guide sign construction as it relates to retroreflective sheeting decisions.(21-22) At six sites, he compared two types of signs: one with illuminated type I sheeting and the other with nonilluminated type III sheeting. The luminance of the unlit type III sheeting was inferior to that of the illuminated type I sign when the signs were viewed from a single vehicle with low beams. However, Robertson believed that an individual vehicle was the atypical case. He recommended that external lighting be eliminated when overhead guide signs are constructed of type III sheeting and when the approach to the sign is straight. He suggested that overhead illumination be used on curves or where the lone driver is required to use low beams (e.g., narrow median).

The additional effectiveness of type III sheeting is also reported in Gordon's review of the literature. Two Dutch studies recommended type III sheeting for unlit overhead signs (except on curved sites) despite a decreased performance (when compared to illuminated signs with type I sheeting).(11-12) Additionally, the Dutch studies indicate that the decreased legibility of unlit signs with type III sheeting can be offset by increasing the letter height by 20 percent.

Another sheeting study summarized by Gordon was conducted for the Ohio Department of Transportation (DOT). This study included combinations of button-copy and type III legends on nonreflective, type I, and type III backgrounds.(23) In all cases, the findings show that button copy outperformed reflective cutout letters. It was also determined that the choice of legend material was more critical that the background material. Under high levels of illumination, the nonreflective background performed the worst. No significant difference was found between the type I and type III sheeting at high levels of illumination. At low levels of illumination, no advantage was found through the use of reflective backgrounds.

In 1987, McNees and Jones studied legibility distances for unlit overhead guide signs.(24) Using existing signs and disregarding the signs' age, retroreflectivity, and visual complexity, they found the legibility indices of various combinations of unlit legend/background materials to be as follows:

• Button copy on type I (59 ft/inch).
• Button copy on type III (55 ft/inch).
• Type III on type III (52 ft/inch).
• Button copy on opaque (50 ft/inch).
• Type III on opaque (48 ft/inch).
• Button copy on type II (46 ft/inch).
• Type III on type II (44 ft/inch).
• Type III on type I (40 ft/inch).

Another effort published in 1987 demonstrated the effect of different material types using compiled headlamp low-beam patterns that represent those in use circa 1975.(25) The researchers used the retroreflective properties of type I, type III, and prismatic sheeting (not defined) on two different overhead sign positions (directly above the vehicle and 12 ft left of the centerline of the vehicle). Using an assumed minimum luminance of 3.4 cd/m², the data show that type I sheeting does not provide adequate luminance levels for either sign position. For the centered signs, type III and prismatic sheeting appear to be adequate. For the left-side overhead sign, the type III sheeting results are marginal, while the prismatic sheeting results are adequate. The authors used Sivak and Olson's 75^th percentile value of 7.2 cd/m² as a criterion for inadequacy, admitting that this does not account for factors such as dirt, natural weathering, or the substitution of colors having lower retroreflectance values.⁽²⁶⁾ Using the 7.2 cd/m² criterion, only the prismatic sheeting produced adequate luminance values.

In 1993, Arizona DOT funded research in an attempt to determine MR requirements for signs on their State system.(27) Through an analysis of the literature and a survey of State policies, recommendations for the types of sheeting were made. For overhead signs, the recommendations included type III signs on freeways. The recommendations also included the use of type II signs where surround complexity is low and speeds are below 55 mi/h. When speeds are below 45 mi/h, the use of type I sheeting is recommended. It is unclear whether these recommendations are for the legend or the background or both.

STREET-NAME SIGNS

Compared to overhead signs, the research related to street-name signs is rather limited. Probably one of the earliest street-name sign research efforts was published in 1970.(28) Unfortunately, this research did not address the retroreflectivity of street-name signs. However, it did address color combinations and letter height. For example, the researchers determined that white-on-green street-name signs are the most appropriate colors in terms of satisfying drivers' needs. With respect to letter height, the researchers found that 6-inch letters are inappropriate for operating speeds of 35 mi/h or greater. When speeds are at this level, they recommend using advance street-name signs.

In 1992, the Institute for Transportation Engineers (ITE) summarized street-name sign practices in the United States and Canada.(29) A total of 638 questionnaires were sent out inquiring about details such as installation location, height, size of letters and panels, use of retroreflective sheeting, and color. While many of the results are listed and discussed, those pertaining to sheeting type are not. It is interesting to learn, however, that "most agencies are primarily concerned with traffic signing categories that are related to public safety. Street-name signing receives more casual attention."

The first retroreflective sheeting-based study was conducted in 1996.(30) The purpose was to compare legibility distance for street-name signs using types I, III, VII, and IX sheeting. Legibility distances were measured at three intersections in St. Paul, MN. The intersections were chosen to have varied background complexity.

The data were collected at night and with older drivers (nine males with an average age of 74 and nine females with an average age of 68). Street-names signs were placed on the departure side of the intersection and were randomly mounted on either the left or right side. Legibility distances, corrected for response times, were recorded as drivers approached the intersections and read the signs.

The findings show that the type VII and IX sheeting resulted in similar legibility distances. These distances were significantly greater than that for type III sheeting, which was significantly greater than that for type I sheeting. The findings also showed that the differences in sheeting type were more pronounced at intersections with greater background complexity.

A report on Toronto street-name signing was published in 1999 by Smiley.(31) The study was performed in the field with actual street-name signs. The study was focused on providing adequate conspicuity for detection in urban and suburban areas and adequate legibility for safe maneuvering. Consequently, various retroreflective materials and letter heights were evaluated. Subjects' responses were recorded as they drove predetermined test courses.

The recommendations included the use of 8-inch letters in urban areas and retroreflectorized signs. The type of material was not a primary focus of the study. However, it was reported that the signs were either weathered type III sheeting or new prismatic sheeting (the specific type is not reported). Informal analyses suggest that the prismatic sheeting appeared to perform better than the type III sheeting. The research also recommended the use of the Clearview™ uppercase/lowercase series for street-name signs, a practice that is developing momentum, but is still uncommon.

PERFORMANCE MEASURES

Retroreflectivity is not a measure that independently describes the legibility of highway signs; rather, it is a property of the sign material. Luminance is the photometric measurement that best relates to legibility. However, luminance is difficult to measure in the field and is dependent on illumination (from vehicle headlamps) and retroreflectivity (which is geometry-specific). If luminance were the basis for minimum end-of-service life for highway signs, a standard light source and specifically detailed measurement geometry would be required. Furthermore, the congressional mandate calls for retroreflectivity and not luminance.

Regardless, research has focused on both luminance and retroreflectivity recommendations for optimal and end-of-service lives for traffic signs. The following review includes both types of research related to overhead and street-name signs.

Alternative Performance Measures

Many studies have been conducted with a goal of determining minimum photometric requirements of traffic signs (usually in terms of luminance or retroreflectivity). In general, the relationship between legibility and luminance and/or retroreflectivity has been a function of surround complexity, luminance and/or retroreflectivity of the legend or background of the sign, or the internal contrast ratio between the legend and the background.

Research recommendations for MR levels are currently available for most signs. Minimum luminance values have also been proposed in the last couple of decades. However, the job of determining minimum photometric values that are commonly accepted is difficult for many reasons. First, there is an absence of conclusive performance data supporting minimal luminance standards. Second, there is no practical way of measuring overhead sign retroreflectivity or luminance in the field. One particularly difficult paradigm to consider is that luminance is needed for two distinct purposes: recognition and legibility. Extremely high values of luminance increase sign conspicuity, but degrade the legibility (this is not to say that the only factor related to conspicuity is luminance; in fact, many factors play a role). There are a host of other issues that make the job difficult.

According to Mace et al., there are at least three different approaches for determining minimum brightness levels.(14) The first is to use the 50-ft/inch rule that has been somewhat erroneously accepted as a standard; however, much of this standard is arbitrary. A second method is to provide enough luminance to accommodate 85 percent of the maximum nighttime legibility distance. A third method would be to identify the level of brightness needed for a given sign on the basis of the recognition or legibility distance requirement of that sign. Mace terms this the minimum required visibility distance (MRVD) and uses McGee's decision sight-distance model as a basis for MRVD. In other words, MRVD is computed using the distance needed by a driver to detect the sign, recognize or read its message, decide an appropriate course of action, initiate a control response, and complete the required maneuver. The luminance needed at the distance defined by MRVD has been used to derive the current research recommendations on MR levels. (2)

Minimum Levels

Probably the most referenced research effort related to recommended luminance requirements for highway signs was conducted by Sivak and Olson and published in 1985.(26) Computing the geometric mean of the findings of 18 previous research efforts, Sivak and Olson recommended optimal and minimal sign luminance values for low-beam U.S. and European headlamps. For optimal values, they used the crest of the derived inverted U-shaped luminance functions shown in the research findings. To determine the minimum sign luminance needed, Sivak and Olson used legibility indices of 50 and 40 ft/inch for younger and older drivers, respectively. Their recommended values are shown in table 4. The replacement values apply to signs in dark environments.

Table 4. Replacement Luminance Values

While the Sivak and Olson work included the review of 18 earlier studies, there are others that were not included in their effort and there have also been a few since. These studies are summarized below:

In 1983, Morales published work related to retroreflectivity requirements for STOP signs.(32) Morales developed a process where the overall retroreflectivity is the criterion and is dependent on the approach speed and the size of the sign. To determine the overall retroreflectivity, Morales recommended multiplying the red retroreflectivity value by 0.76 and the white retroreflectivity value by 0.24 and summing the two values. For a 30-inch STOP sign on roads with approach speeds greater than 50 mph, 40 candelas per lux per square meter (cd/lx/m²) is recommended as the MR value. Other values are reported for different speeds and sizes of STOP signs.

In 1985, Mace et al., investigated visual complexity and its impact on sign luminance.(33) The researchers used warning signs at three different luminance levels to determine detection and recognition distances. The major finding was that increases in visual complexity had a detrimental impact on recognition and no effect on legibility; however, brightness improved both recognition and legibility. Based on their findings, the researchers recommended warning sign retroreflectivity values of 18 cd/lx/m² for low-complexity areas and 36 cd/lx/m² for high-complexity areas.

In another effort documented in 1985, Schmidt-Clausen reported on the minimum luminance levels needed for sufficient and optimal performance.(34) The investigation was carried out on a 1:10 scale model and was compared to those values found in real-world situations. The study showed that a legend luminance of 3.5 to 10 cd/m² is sufficient. Luminance values between 10 and 35 cd/m² are optimal. The maximum luminance was determined to be about 60 cd/m².

In 1989, Olson reported on a study that included recommendations for minimum reflectivity for signs in urban, suburban, and rural areas.(35) His study consisted of laboratory and field evaluations. The goal was to determine the minimum luminance levels to ensure that the signs are detected and identified at adequate distances under nighttime driving conditions.

Olson made recommendations for several sign types, including overhead signs. To make his overhead signing recommendations, Olson had to make several assumptions as listed below:

• Green is equal in conspicuity to yellow in the same family of materials.
• The effect of a white border and legend on conspicuity is minimal.
• A correction for driver expectancy does not apply for guide signs. It was assumed that drivers are searching for guide signs and their emergence into the driver's field of view is expected.
• Olson used small roadside signs in the field study. Using results from his laboratory study, he assumed that a 2.4 multiplier is needed to account for the increased conspicuity of overhead signing. In other words, controlling for all factors other than location and size, overhead signs are 2.4 times more conspicuous than roadside signs.
• In an attempt to quantify the amount of headlamp illumination reaching overhead signs, Olson used the results from a computer model.
• Because of the difficulty associated with the angularity in reading overhead signs at relatively close distances and the rapid decrease in available illumination from headlamps at close distances, Olson assumed that drivers had to complete the reading task before passing 100 ft in front of the sign.
• Using the results from previous research, Olson assumed a reading time of three words per second.⁽³⁶⁾

Olson's recommended specific intensity per unit area (SIA)[2] values for overhead signing are included in table 5. The process used to derive these numbers is summarized below:

• The illumination reaching the overhead position was calculated using a simulation program. The resulting values were typically 10 percent of the roadside signs measured in the field study at the same distance.
• Using the 10-percent finding, overhead signs would need to be 10 times more efficient in terms of the amount of luminance developed with constant illumination levels.
• Olson assumed that the conspicuity of green was equal to yellow; however, the retroreflectivity of green is about 23 percent of yellow for the same family of material. This led to a reduction factor of 2.3.
• As mentioned, Olson determined that a correction factor for size was needed. Based on his laboratory studies, he determined that a factor of 2.4 would be most appropriate. This factor essentially cancels the 2.3 reduction factor for the decreased retroreflectivity of green as compared to yellow.
• Consequently, Olson used his derived values for 85^th percentile yellow warning sign identification distances (without correction for driver expectancy) to determine the proposed values for overhead signing.

The latest research on minimum luminance levels for highway signs was performed on yellow warning signs with two-digit, 6-inch Series E numbers used for stimuli. The findings suggest that a sign luminance greater than 40.2 cd/m² is needed to obtain at least 85 percent correct identification of the signs tested for a viewing distance of 90 meters (m), which correspond to the 50 ft/inch of letter height commonly used as a legibility index among traffic engineers. The recommended value was based on the results from subjects at least 65 years old (average age was 69).

Table 5. Recommended SIA Values for Green Background Areas of Overhead Guide Signs

A significant effort related to minimum luminance was conducted in Australia in 1991. The aim of this study was twofold: (1) to measure the retroreflectivity of road signs in the field and hence to establish their rate of degradation and the major influences affecting degradation; and (2) to establish a minimum performance criterion of retroreflectivity-a terminal value-below which a sign would become ineffective. This was determined by a literature review, a nighttime survey carried out by knowledgeable traffic engineers, and a laboratory experiment. The life performance curves of traffic signs throughout Australia were determined. The minimum luminance required of a traffic sign at night has been found from laboratory experiments to be 3.2 cd/m² for all signs other than warning and regulatory signs, where a higher value of 9.7 cd/m² is needed. The optimal luminance was found to be 18 cd/m² for all signs other than warning and regulatory signs, which were 23 cd/m². The researchers also found an internal contrast of 3:1 to be acceptable for fully reflectorized signs.

The current Australian standard for overhead signing includes the following statement: "lighting for overhead signs is usually avoided by using type III sheeting for the legend and, in some cases, the background." In other words, the Australians have concluded that the use of type III sheeting is adequate for unlit overhead signing.

CONTRAST RATIO RESEARCH

For fully reflectorized signs with almost no background complexity (i.e., values up to 0.4 cd/m²), Sivak and Olson recommended a contrast ratio of 12:1 for optimal performance. For a background complexity greater than 0.4 cd/m², the retroreflectivity needs and corresponding contrast ratio become dependent on the amount of background complexity. The values reported in their literature review range from 3:1 to 45:1. Other reported minimum contrast ratios for white-on-green signs have ranged from 3:1 to 7:1. The Australian research recommended a value of 3:1. However, their guidelines call for a minimum of 7:1, but prefer 10:1.

A 1988 report examining fully retroreflective signs suggest a contrast ratio range from 4:1 to 15:1 as being appropriate for most conditions. For example, if the luminance of the green background is 5 cd/m², the luminance of the legend should be at least 20 cd/m². Lower contrast ratios reduce legibility and may not be acceptable, and contrast ratios as high as 50:1 may reduce legibility, but could be quite adequate under certain conditions.

The initially proposed FHWA sign retroreflectivity values suggest a minimum contrast ratio of 4:1, but no recommendation for maximum contrast. This 4:1 minimum contrast ratio was initially recommended for both white-on-red and white-on-green signs.

For red-and-white signs that have been screened, the minimum contrast ratio may be more difficult to maintain than the absolute MR values. According to outdoor weathering data from Arizona, the 4:1 ratio can only be maintained for 4 to 5 years with ASTM type I and type II sheeting.

Like the red-and-white signs, the initially proposed FHWA minimum contrast ratio of 4:1 was also required for white-on-green signs. However, the screening issues of white-on-red signs are not prevalent with white-on-green signs since these signs are not typically screened. In fact, FHWA later revised the initially proposed MR values and minimum contrast ratios, dropping the minimum contrast ratio for white-on-green signs.

PROPOSED MINIMUM RETROREFLECTIVITY LEVELS

When the original set of research-developed MR levels were introduced in 1993, the levels were included for overhead signs (see tables 6 and 7).⁽²⁾ However, in a 1998 report, the values were removed. The following explanation was provided: "Given the many unresolved issues with vehicle headlamp performance specifications and the difficulty in measuring overhead sign retroreflectivity, at this time, the FHWA is not recommending that minimum levels be established for overhead-mounted signs."⁽³⁾

An examination of the initially proposed overhead levels reveals that minimum values for type I sheeting are at a level that may exclude its use on high-speed roadways. Type II sheeting becomes marginal when degradation is considered.^(41-43) Other more efficient sheeting appears to perform adequately in comparison to the initially proposed levels.

Table 6. MR for White Signs

As mentioned, the initially proposed retroreflectivity levels included overhead signs. An investigation of the Computer Analysis of Retroreflectance of Traffic Signs (CARTS) software used to develop the initially proposed levels shows that three different overhead signs were included for evaluation. Because there is no standard guide sign design, three generic signs were developed for CARTS modeling purposes. The three signs have one, two, and three lines of text.

The MRVD submodel of the CARTS model is made up of five time-based components: detection, reading, decision, response, and maneuver. These components incorporate many assumptions and previously developed models. While these assumptions and models are generally accepted as reasonable, they were designed to accommodate the drivers' need for roadside signs and were not specifically designed for overhead signing. Consequently, the number of assumptions related to overhead signing is increased to make up for the submodel caveats. The results, after proceeding through the CARTS assumptions for overhead signing, oversimplify the driver's task related to detecting and reading overhead signs.

Once CARTS calculates the MRVD needed for the situation entered by the user, it uses another submodel, PCDETECT, to determine the needed luminance and, ultimately, the MR. PCDETECT has been used for years and its strengths and weaknesses are well documented in the literature. It is believed to be a reasonable model for the task at hand except that it is a "cyclops" model. In other words, the model assumes that there is one illumination source and that the observer's eye is in the same plane as the illumination source. This is of particular concern when the full retroreflection system is used or needed, such as when prismatic sheeting is being considered (as will be discussed later).

In summary, while the CARTS model has been built to accommodate many different factors and may work well for small roadside signs, the overhead guide sign assumptions raise questions that decrease confidence in the overhead guide sign MR levels derived from CARTS.

VEHICLE HEADLAMPS

Headlamp placement, illumination, and intensity are all significant factors in the development of MR for overhead signs. They are related to the geometry of the viewing system (which incorporates the signing and the driver's eye position), which can be somewhat sensitive depending on the sign location and the sheeting used to construct the sign. There are also significant changes underway in terms of headlamp standards that could potentially impact the amount of light available to be retroreflected.

Standards

FMVSS 108 provides the requirements for lighting equipment and its placement on motor vehicles. This standard requires that headlamps be no lower than 22 inches (1.83 ft) and no higher than 54 inches (4.5 ft). It also requires that the headlamps be located on either side of the vertical centerline of the vehicle as far apart as practicable.

Fambro et al., collected driver's eye height and headlamp height for several thousand vehicles around the United States. Table 8 summarizes their efforts:

Table 8. Headlamp and Driver's Eye Height

The Society of Automotive Engineers (SAE) specification for headlamps was J579; however, this has been cancelled in lieu of an effort to harmonize headlamp design worldwide.⁽⁴⁵⁾ The SAE standards and FMVSS 108 apply to all vehicles registered in the United States, regardless of the design of the headlamp filament or light source. The output of two- and four-headlamp systems in the United States is limited by these specifications to the following:

• Type 2 or 2A Sealed Beam.
  • Upper beam (each lamp): 20,000 to 75,000 cd.
  • Lower beam (each lamp): 15,000 to 20,000 cd.
• Type 1 or 1A Sealed Beam.
  • Upper beam (each lamp): 18,000 to 60,000 cd.

The early efforts of headlamp design harmonization are summarized in SAE J1735.⁽⁴⁶⁾ The goal of the harmonization efforts is to develop specifications for one headlamp pattern that satisfies worldwide illumination criteria. In general terms, the U.S. pattern has traditionally provided substantially more light above the horizon than the European and Japanese patterns. However, attempts to harmonize these headlamp patterns have resulted in several compromises among all three patterns. For the U.S. pattern, one of the more significant compromises has been the decreased amount of light above the horizon. In fact, with the 1997 revision to FMVSS 108 allowing visually/optically-aimed (VOA) headlamps (including both visually/optically left-aimed (VOL) and visually/optically right-aimed (VOR) designs) and a global 1999 agreement concerning harmonized headlamps (a drastic compromise between the U.S. philosophy of maximizing visibility versus the European philosophy of minimizing glare), the amount of light above the horizon will continue to decrease. A recent report shows comparisons between the U.S. conventional headlamps and the VOL, VOR, and harmonized headlamps. For overhead signs at approximately 500 ft, there are consistent trends showing decreased illumination above the horizon. Compared to the conventional U.S. headlamps, the VOL headlamp reduces overhead illumination by 28 percent, the VOR headlamp by 18 percent, and the harmonized headlamp by 33 percent.

One of the more recent headlamp research projects was published in 1999 and sponsored by FHWA. Funded because of a concern about changes in the headlamp performance of the present U.S. vehicle fleet in terms of adequately illuminating traffic signs, especially overhead guide signs, the research was charged with determining the minimum luminance requirements needed for overhead guide signs and then establishing whether the current vehicle fleet was providing enough illumination to create such minimum luminance levels.

The literature review determined that the minimum threshold luminance value for the nighttime visibility of guide signs is about 3.2 cd/m², while the optimal values are on the order of 75 cd/m². A laboratory experiment conducted as part of the project found minimum luminance values to be about 13.2 cd/m² for white-on-green signs with a contrast ratio of 8:1.

Field experiments were conducted with 50 different vehicles having a variety of different headlamp types. Based on an assumed minimum luminance of 3.2 cd/m² for the legend of overhead signs, the researchers concluded that certain cars in the vehicle fleet do not provide adequate illumination unless type III or brighter sheeting is used. The following general conclusions are based on illumination data from more than 1500 headlamp distributions:

• Right-shoulder-mounted signs receive sufficient illumination (more than 99 percent satisfaction).
• Left-shoulder-mounted signs receive barely sufficient illumination (more than 90 percent).
• Overhead signs receive marginally sufficient illumination (only about 50 percent of the vehicles provided adequate illumination to meet the legibility criteria).

Other criteria established for headlamp adequacy include a viewing distance of 500 ft, straight and flat roadways, a minimum luminance of 3.2 cd/m², and new type III sheeting.

FINDINGS

The review of the literature yielded the following findings related to MR levels for overhead guide signs and street-names signs:

• Overhead Signs
  • Measuring the retroreflectivity of overhead signs is not as practical as measuring the retroreflectivity of roadside signs.
  • The majority of the recommendations for minimum luminance or retroreflectivity levels were developed through theoretical or laboratory research efforts.
  • The minimum luminance needed for legibility is about 2.5 to 3.5 cd/m² for the legend, although substantial variability exists in the research. These findings are for young drivers and low background complexity. Older drivers and more complex backgrounds may increase the minimum luminance needed for the legibility of overhead signs.
  • Type III sheeting viewed under low-beam conditions provides marginal luminance for the legibility of overhead signs.
  • Types I and II sheeting do not provide adequate luminance for legibility on overhead signs.
  • There appears to be some support for the need for a minimum internal contrast ratio. A minimum ratio of 3:1 or 4:1 has been recommended most frequently. For background areas with high visual complexity, the minimum internal contrast is critical; however, for backgrounds with low visual complexity, the legend luminance is more important.
• Street-Name Signs
  • The literature review has shown that minimum photometric requirements for street-name signs have not been researched or recommended. In fact, street-name signs are usually an afterthought or at least not a primary concern.
  • The legibility of street-name signs depends on many factors; however, the location is most important. Left-shoulder-mounted signs will require a significantly greater amount of retroreflectivity because of headlamp beam patterns.
• Headlamps
  • A substantial difference in headlamp beam patterns exists between U.S. and European standards.

Based on a random sample of 1500 vehicles passing under an underpass on an Interstate highway in Kansas, headlamps in use on today's roadways provide marginal illumination for overhead signs. Research shows that only about 50 percent of the 1500 randomly sampled vehicles provided enough illumination to satisfy the assumed criteria of a viewing distance of 500 ft, type III sheeting, and a minimum luminance of 3.2 cd/m².

[1] Reported findings in foot-lamberts (ft-L) were converted to cd/m2 (1 ft-L = 3.426 cd/m²).

[2] SIA is expressed as candelas of reflected light per footcandle of incidental light per square foot of target (cd/fc/ft²). It is equivalent to cd/lx/m².

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