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Earlier versions of the MUTCD (in Section 2A.22) have stated, “All traffic signs should be kept properly positioned, clean, and legible, and should have adequate retroreflectivity.” However, the MUTCD did not define what was meant by “adequate retroreflectivity” until recently. While the ASTM Types have been used to specify the types of materials that agencies use, there were no criteria in the United States for “maintained retroreflectivity.” This need for common benchmarks to serve as the criteria for maintaining nighttime traffic sign visibility performance and to link it to sign maintenance processes was recognized in the late 1980s. Since then, considerable efforts have been devoted to developing maintenance criteria that can be used to assess the nighttime performance of traffic signs. These efforts have been based upon retroreflectivity, a convenient surrogate measure of nighttime visibility. This chapter describes the work that has been completed to establish minimum maintained traffic sign retroreflectivity levels. In addition, information relevant to the application of the minimum maintained retroreflectivity levels is provided.
The establishment of minimum maintained traffic sign retroreflectivity levels (also referred to as end-of-service-life values) is one of the latest steps in the evolution of providing a safe and efficient road transportation system. The progression of this concept in the United States was accelerated in 1984 when the Center for Auto Safety (CAS) petitioned the FHWA to establish retroreflectivity standards for signs and markings.(32) Congress then directed the Secretary of Transportation in 1992 to revise the Manual on Uniform Traffic Control Devices to include “a standard for a minimum level of retroreflectivity that must be maintained for pavement markings and signs which apply to all roads open to public travel.” (33)
Even before the CAS petition for retroreflectivity standards, the FHWA had an active research program addressing the nighttime visibility requirements of traffic control devices. This research program continued through the 1980s and into the 1990s, resulting in several research reports (see references 34, 35, 36, 37, and 38.), and culminating in 1993 with research recommendations for minimum in-service sign retroreflectivity levels.(39, 40) The minimum retroreflectivity levels were distributed for review and comment among highway agencies , and several national workshops were held. Concerns about the levels and the associated impacts o f implementing them became apparent. In 1995, a research effort was funded to revise the initial 1993 levels and to assess the impacts of the requirements on State and local agencies. The research efforts resulted in a pair of reports that were published by the FHWA.(41, 42) The revised 1998 set of minimum levels was presented in thes e reports.
The most evident change of the 1998 revisions was the removal of minimum levels for overhead signs because of significant variability associated with headlamp luminous intensity directed toward overhead signs. As the 1998 revised levels were being finalized, the Federal Motor Vehicle Safety Standard Number 108, “Lamps, Reflective Devices, and Associated Equipment” (FMVSS 108) was revised so that vehicle owners could easily aim their headlamps and therefore reduce the variability associated with headlamp aim. (FMVSS 108 is the document that sets the minimum and maximum luminous intensities for headlamps, headlamp mounting heights, and standardization of headlamps on new vehicles sold in the United States.) Because of these changes, the FHWA sponsored additional research to develop minimum in-service retroreflectivity levels for overhead and street name signs, which were not accounted for by the initial 1993 or revised 1998 levels. The research for overhead and street name signs was completed in early 2001.(43) Although the overall approach was different than earlier research related to minimum retroreflectivity levels, several of the same assumptions were maintained. One of the significant contributions of the overhead and street name sign research was a demonstrated need to update some of the fundamental assumptions associated with the earlier development of minimum in-service retroreflectivity levels. From early 2001 until 2003, research continued on the development of updated minimum maintained traffic sign retroreflectivity levels.
It should also be noted that in late 1998, the FHWA was close to issuing a proposed rule on minimum levels of sign retroreflectivity when the Board of Directors of the American Association of State Highway and Transportation Officials (AASHTO) requested that the FHWA delay any future action of minimum retroreflectivity levels until an AASHTO task force could review the proposed minimum requirements and their impacts on highway agencies. The AASHTO Standing Committee on Highways created the Special Task Force on Retroreflectivity in 1999, and it included representatives from Federal, State, city, and county transportation agencies, plus industry, research, and private sector entities. The efforts of the task force led to a resolution that was adopted by the AASHTO Board at its December 2000 meeting. The key points of the resolution were
The initial set of minimum retroreflectivity levels published in 1993 was derived from analyses based upon a theoretical computer model called Computer Analysis of Retroreflectance of Traffic Signs (CARTS).(39) The CARTS model is comprised of several sub models that work in series to determine retroreflectivity needs based on user selected inputs. The first sub model determines the minimum distance at which a sign must be legible in order for a motorist to respond appropriately and safely. This distance is termed the Minimum Required Visibility Distance (MRVD) and is the sum of distances associated with the following factors: detecting the sign, recognizing or reading the sign, deciding on the appropriate action, initiating the response, and completing the required maneuver (depending on the sign message, the latter factors may not be needed).
Using the computed MRVD value, the next sub model estimates the threshold legibility luminance needed for the sign. The major componenet of this sub model is a visibility model called PCDETECT,(44) which is based on data from the classical Blackwell experiments of the 1940s where subjects were tasked with the identification of circular targets against uniform backgrounds.(45) The last sub model takes the MRVD and estimates threshold legibility luminance and back-calculates the retroreflectivity needed at the standard measurement geometry of 0.2 and -4.0 degrees for the observation and entrance angles, respectively.
Because of the infinite number of possible scenarios in terms of the combination of sign types, sign locations, driver needs, headlamp performance variations, and the like, several scenarios were selected to represent typical or design conditions. For instance, the driver was assumed to be 47 years old, and the dimensions of the vehicle approximated a large passenger sedan. The assumed headlamp was a composite headlamp representing the median performance of 26 headlamps from passenger cars with model years ranging from 1985 to 1990.
The results of this initial work were summarized in four tables of minimum retroreflectivity levels, distinguished by the color of sign. There were separate tables for white signs, yellow and orange signs, green signs, and red signs. Depending on which of the four tables one was considering, the minimum retroreflectivity levels also depended on at least some of the following factors: roadway speed, sign size, type of retroreflective sheeting, sign location for green signs, and type of legend (symbol versus text). There was also a minimum contrast ratio of 4:1 required for white on red and white on green signs. Because this research was conducted in the early 1990s, the only types of microprismatic sheeting included in the recommendations were ASTM Types IV and VII.
After the 1993 proposed minimum retroreflectivity levels were published, the developers of CARTS received many comments questioning the modeling assumption of one headlamp with the driver directly above the headlamp (also called Cyclops modeling). In reality, this modeling represents a motorcycle rather than a four-wheeled vehicle. Because of retroreflective sheeting materials’ sensitivity to observation angle, a Cyclops modeling assumption can produce significantly different results than a model with the proper positioning of the headlamps with respect to the driver’s eye position. In July 1994, the developers of CARTS provided a refined version that accounted for the effect of two headlamps on the observation angle.(46)
Shortly thereafter, the FHWA sponsored two research projects to determine the adequacy of the 1993 minimum retroreflectivity levels.(47, 48) During the same period, the FHWA also sponsored three national workshops to solicit input regarding the 1993 minimum retroreflectivity levels. In 1998, a report by McGee and Paniati listed the following reasons for revising the minimum retroreflectivity levels:(41)
The revisions in 1998 resulted in several changes, but the most evident change was the removal of all minimum retroreflectivity levels for overhead signs because of many unresolved issues with vehicle headlamp performance specifications and the difficulty in measuring overhead sign retroreflectivity.(49) The minimum retroreflectivity levels for red, yellow, and orange signs were slightly reduced. Most of the minimum retroreflectivity levels for white signs were reduced, but a few were raised. The minimum retroreflectivity levels for ground mounted green signs, which did not include street name signs, stayed the same.
In March 1997, the NHTSA implemented a final rule that revised FMVSS 108 in order to address the issue of headlamp misaim, which was believed to be a significant factor related to the amount of glare and the variability of headlamp luminous intensity directed toward overhead signs. The final rule reflects the consensus of the negotiated rulemaking concerning the improvement of headlamp aim ability performance and visual/optical headlamp aiming.
The final rule established improved headlamp aiming features that provide more reliable and accurate aiming and help vehicle operators more easily determine the need for correcting aim. The rule introduced visually/optically aimed headlamps to the United States. The term “VOA” generically describes two types of visually/optically aimed headlamps: VOR and VOL headlamps. The VOL headlamp is a low beam with a horizontal cutoff to the left side of the beam. The VOR headlamp is a low beam and has a horizontal cutoff to the right side of the beam. VOL headlamps can reduce glare to oncoming drivers compared to conventional U.S. low beams. VOR headlamps have less ability to reduce oncoming glare but produce luminous intensity distributions more similar to conventional U.S. low beams.
Because of the NHTSA’s revision to FMVSS 108, the FHWA sponsored a research project to develop minimum retroreflectivity levels for overhead signs. In order to complete the initial set of minimum retroreflectivity recommendations, the FHWA also included minimum retroreflectivity levels for street name signs in the scope of the project. The research included the development of an analytical process to determine minimum retroreflectivity levels from a host of factors including demand luminance. To determine the adequate demand luminance values, the researchers performed a legibility study with full-scale guide signs and street name signs. Special emphasis was devoted to accommodating older drivers. The results of the study were published in 2003 and included research recommendations for a set of minimum retroreflectivity levels for overhead and street name signs.(51)
Besides providing recommendations for minimum retroreflectivity levels for overhead and street name signs, the researchers also performed sensitivity analyses to determine the relative impact of factors such as the assumed design driver capabilities, the headlamp type, and the vehicle type. This research identified a need to update some of the key assumptions of the initial 1993 and revised 1998 minimum retroreflectivity levels. In addition, there was a need to develop minimum retroreflectivity levels for the various types of retroreflective sheeting that had been introduced into the market since the earlier works’ completion.
Because of a demonstrated need to update the minimum retroreflectivity levels for traffic signs, FHWA sponsored additional research focused on the investigation and sensitivity of updated factors such as the driver age, headlamps, vehicle types, and retroreflective sheeting. More specifically, this work included the use of an older design driver, newer style headlamps, larger vehicle types such as sport-utility vehicles (SUVs), and much more robust retroreflectivity prediction tools. The specifics of the update are reported in an FHWA report, and a summary paper is provided in the Transportation Research Record 1824.(52, 53)
The key updates included using an older driver in the analyses, a larger vehicle that is more representative of large SUVs, a more modern headlamp profile, and additional sign sheeting materials that were not included in the initial analyses.
The results of all this research led to the minimum maintained traffic sign retroreflectivity levels shown in table A1.
| Sign Color | Sheeting Type (ASTM D4956-04) | Additional Criteria | ||||||
|---|---|---|---|---|---|---|---|---|
| Beaded Sheeting | Prismatic Sheeting | |||||||
| I | II | III | III, IV, VI, VII, VIII, IX, X | |||||
| White on Green | W*; G ≥ 7 | W*; G ≥ 15 | W*; G ≥ 25 | W ≥ 250; G ≥ 25 | Overhead | |||
| W*; G ≥ 7 | W ≥ 120; G ≥ 15 | Ground-mounted | ||||||
| Black on Yellow or Black on Orange | Y*; O* | Y ≥ 50; O ≥ 50 | (2) | |||||
| Y*; O* | Y ≥ 75; O ≥ 75 | (3) | ||||||
| White on Red | W ≥ 35; R ≥ 7 | (4) | ||||||
| Black on White | W ≥ 50 | — | ||||||
| (1) The minimum maintained retroreflectivity levels shown in this table are in units of cd/lx/m2 measured at an observation angle of 0.2 ° and an entrance angle of -4.0 °. (2) For text and fine symbol signs measuring at least 1200 mm (48 inches) and for all sizes of bold symbol signs (3) For text and fine symbol signs measuring less than 1200 mm (48 inches) (4) Minimum Sign Contrast Ratio ≥ 3:1 (white retroreflectivity ÷ red retroreflectivity) * This sheeting type should not be used for this color for this application. |
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| Bold Symbol Signs | ||||||||
|
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| Fine Symbol Signs – Symbol signs not listed as Bold Symbol Signs. | ||||||||
| Special Cases | ||||||||
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It was noted that there may be conditions where the minimum retroreflectivity levels shown in table A1 would not provide adequate nighttime visibility. It was also noted that if the worst case scenario was chosen for the analyses, there would be no type of retroreflective sheeting that could provide adequate luminance levels to achieve detection and legibility for all drivers.
The development of the updated minimum retroreflectivity levels consisted of many different scenarios comprised of a variety of practical and typical speeds, roadway cross sections, vehicle types, sign positions, sign sizes, headlamp types, etc. Ultimately, the minimum retroreflectivity levels were derived from the equilibrium point of demand and supply luminance levels, which also vary as a function of the aforementioned factors. (It is important to note that luminance and retroreflectivity are not synonymous terms. Briefly, luminance is the perceived brightness of a sign, and retroreflectivity is a property of the sign that describes its efficiency to return headlamp illuminance back toward the driver.) Technically, each specific scenario for each specific driver has a unique minimum luminance and therefore a unique minimum retroreflectivity level associated with that situation. However, from a practical point of view, the minimum retroreflectivity levels need to be easy to measure and manage. This requires that the infinite number of minimum retroreflectivity levels associated with the infinite number of driving scenarios be consolidated to a practical and manageable number.
The level of complexity of the framework of the minimum retroreflectivity levels of 1993 and 1998 was a particularly significant issue as seen by the AASHTO Special Retroreflectivity Task Force. As the research effort to update the minimum retroreflectivity levels was nearing completion, the researchers focused on consolidating the recommendations into an easy-to-use format. In consolidating the minimum retroreflectivity levels, certain decisions were made regarding the resolution of the levels. For example, factors such as sign size and roadway speed were collapsed into one level representing the majority of typical driving scenarios for a given sign type.
The consolidation efforts were progressing as a second round of national minimum retroreflectivity workshops was underway.(54) The most current research recommendations regarding the consolidation of the minimum retroreflectivity levels were presented to the participants of each of the four workshops. The input received from the workshop participants helped shape the outcome of the recommended minimum retroreflectivity levels presented in this paper.
The consolidation efforts ultimately resulted in some degree of compromise between the precision and the brevity of the minimum retroreflectivity levels. The final research report provides a detailed description of how the minimum retroreflectivity levels were consolidated to an implementable format.(52)
The minimum retroreflectivity levels shown in table A1 represent the most recent results of a series of research studies that have been undertaken over the past two decades. They also represent the latest efforts in a long series of considerations related to providing safe and efficient roadways. The key assumptions associated with table A1 are
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