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Publication Number: FHWA-RD-03-082
Date: December 2003
Minimum Retroreflectivity Levels for Overhead Guide Signs and Street-Name Signs
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CHAPTER 1. INTRODUCTION
The development of minimum inservice levels of retroreflectivity (end-of-service-life values) for signs is a critical step in the evolution of providing a safe and efficient road transportation system. Recent activity in this arena began in 1984, when the Center for Auto Safety petitioned the Federal Highway Administration (FHWA) to establish retroreflectivity standards for signs and markings. In 1993, Congress required the Secretary of Transportation to revise the Manual on Uniform Traffic Control Devices (MUTCD) 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."(1) Because of the work in progress, FHWA was able to develop suggested minimum retroreflectivity (MR) levels for signs in a relatively short time. Initial recommendations included overhead signs, but were later removed because of many unresolved issues with vehicle headlamp performance specifications and the difficulty of measuring overhead sign retroreflectivity.(2-3) Since the initial recommendations were made, vehicle headlamp performance specifications have been revised.(4) This research project was conducted to determine MR levels for overhead guide signs and street-name signs.
As a direct result of the congressional mandate for minimum levels of retroreflectivity and the recently revised vehicle headlamp performance specifications, FHWA identified the need to conduct research to determine MR levels for overhead guide signs and street-name signs. The research project was awarded to the Texas Transportation Institute (TTI) in late 1999 and was started in mid-February 2000.
The research project was a 15-month effort. The research activities are described below:
Chapter 7 provides the initial recommendations made as a result of the research described in the report. However, additional research was conducted that resulted in revised recommendations. This additional research was not part of this project; however, it directly affects the results. Therefore, chapter 8 was included to describe the revisions and the subsequent recommendations for overhead guide signs and street-name signs. Chapter 8 also provides a list of future research topics.
The number of factors related to highway sign visibility can be overwhelming. The factors identified through the literature review can be categorized into four main headings as shown in table 1. Under each category are the corresponding design elements.
Table 1. Legibility Factors
While each of the design elements listed above affect visibility on some level, not every element has the same effect and not all factors act independently. Given the limited time and resources associated with this project, it was not reasonable to explore each of the elements listed above. Furthermore, all of these elements can be reduced to three main components that impact visibility: the amount of light reaching the sign (illuminance), the efficiency of the retroreflective material (retroreflectivity), and the returned light that makes the sign appear bright (luminance). These three main components can be combined with a variety of other issues, such as the visual ability of the driver and the vehicle type, to determine the required luminance for the traffic signs. The luminance and contrast determine the legibility and recognition of highway signs. Therefore, these issues were explored using past research findings to help define and quantify those factors that are most influential in overhead and street-name sign visibility.
Traffic signs use retroreflective sheeting to help ensure that the signs communicate the same message day and night. Retroreflectivity redirects vehicle headlamp illuminance back toward the driver. There have been substantial improvements in retroreflective technology since it was first introduced using large glass beads called "cat's eyes." The currently available retroreflective technology is defined and described in American Society for Testing and Materials (ASTM) D4956.(5) As of 2001, ASTM has defined seven types of retroreflective sheeting approved for traffic signs. These types of sheeting can be broadly classified into two groups: one that uses microsized glass beads to retroreflect headlamp illuminance and another that uses microsized prisms to retroreflect the light. Table 2 includes a list of the currently defined retroreflective sheeting available for permanent traffic signs (according to ASTM D4956). This report uses the ASTM-type designation when referring to specific sheeting types.
As mentioned, in the mid-1990s, one of FHWA's greatest concerns regarding the initially proposed MR levels for overhead signs was the global harmonization efforts related to headlamp specifications. In 1997, the Federal Motor Vehicle Safety Standards (FMVSS) related to headlamp specifications for vehicles sold in the United States were revised to include harmonized headlamp specifications. The research effort used currently available headlamp profiles as identified in the literature review and recently obtained illuminance data from the roadway to identify the headlamp profile that best replicates those currently found on the roadway. The advantage of this approach is that real-world factors, such as headlamp misalignment, headlamp cleanliness, and variations in available voltage, are considered, rather than using an exclusively theoretically based headlamp profile.
In recent years, there has been a concentrated effort to accommodate the needs of older drivers. This is especially critical for the establishment of MR levels since a driver's vision generally degrades with age, thus requiring brighter signs. The research conducted as part of this study focused on accommodating the needs of older nighttime drivers.
The establishment of minimum levels of retroreflectivity for overhead and street-name signs is only one part of the process of ensuring that these signs have adequate nighttime visibility. Once minimum levels are developed, agencies need to be able to measure their signs and compare the measurements to the minimums. This is a challenge for both types of signs as discussed below.
Because of the position of overhead signs, the measurement of the retroreflectivity of these signs introduces a significant challenge. Except for the FHWA mobile retroreflectometer and the LaserTech Impulse® retroreflectometer, measurement of overhead sign retroreflectivity requires contact with the specific part of the sign being measured. Both of the noncontact instruments require data manipulation to provide retroreflectivity measurements representing the standard measurement geometry of 0.2° and -4.0°. As a result, current measurements of overhead sign retroreflectivity require lane closures and a worker on a sign bridge or in a bucket truck.
In addition to the difficulty of measuring overhead sign retroreflectivity, the large size of these signs requires a substantial number of measurements to provide a representative sample of the overall sign retroreflectivity. The current ASTM procedure for measuring sign retroreflectivity with a portable sign retroreflectometer (ASTM E1709) states that four measurements should be made. Assuming that this applies to a typical roadside sign, this results in a general average of about one reading for every 0.1 to 0.2 square meters (m2) of sign area. If a similar proportion were to be used on overhead signs (using an assumed sign size of 1x1 m), approximately 50 measurements of the sign background would be needed to get a reasonable representation of the overall sign retroreflectivity. Furthermore, with large guide signs, the legend also needs to be measured. There are no guidelines that indicate whether every letter in a sign needs to be measured, nor is there guidance on the number of measurements needed per sign. There are still a large number of signs in the field with button copy, and there are no field devices capable of accurately measuring the retroreflectivity of button copy. When the background and legend are both considered, the total number of retroreflectivity measurements could be 50 to 100 measurements for a typical sign.
These factors indicate that numerically based MR levels may not be an effective means of ensuring adequate retroreflectivity of overhead signs. Other procedures may also need to be developed for the minimum numbers to have any practical value. Alternative procedures should be based on the numerical minimums, but should not require actual retroreflectivity measurements. Examples of alternative procedures include minimum visibility distances or using a tracking schedule combined with sheeting-life curves and MR levels.
Just as with overhead signs, there are some practical limitations on the ability to measure the retroreflectivity of street-name signs. Because of the height of street-name signs (i.e., above arm's reach), a pole-mounted retroreflectometer will typically be needed. However, street-name signs typically have crowded legends, leaving little open space for measuring the retroreflectivity of the background, especially if the positioning of the retroreflectometer is accomplished using a 2-meter (m) pole. The letter height and stroke width of street-name signs combine to provide a letter stroke that is too narrow for most retroreflectometers to measure without also measuring some of the background (green) retroreflectivity. Even if the legend retroreflectivity is to be measured, once again, accurately positioning the retroreflectometer on the end of a pole is a challenge. Finally, many street-name sign blanks are ribbed, with a thick section at the top and bottom of the blank to add rigidity. If the retroreflectometer is using a faceplate to help provide a flush and perpendicular position, then the unit may not be able to make proper contact with the face of the sign. Not using the faceplate may reduce the accuracy of measurements because of lack of proper alignment with the sign face.
These factors indicate that measuring the retroreflectivity of both the legend and the background of street-name signs may not be a practical undertaking. Again, alternative procedures may be needed, such as a minimum visibility distance or a maximum sign age.
From this point hereafter, the units of this report are presented using the common terminology among practicing traffic engineers and visibility experts. The photometric terms are expressed in SI units, as that is the standard in the industry. Sign size, letter height, and other sign-related dimensions (including legibility index) are expressed in English units because that is still the preferred practice by the transportation profession. Table 3 can be used to supplement the conversion table shown on page ii.
Table 3. Sign Dimension Conversions