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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 8. FOLLOWUP RESEARCH
The completion of the research activities and findings described in the previous seven chapters convinced the researchers that a significant amount of work was still needed before the MR levels were ready for implementation. During the American Traffic Safety Services Association (ATSSA) Traffic Expo in Fort Lauderdale, FL, in 2001, the researchers presented their concerns to FHWA. The concerns were focused on the investigation and sensitivity of updated factors such as the driver's age, headlamps, vehicle type, and an inventory of available retroreflective sheeting materials and their performance levels. By summer 2001, the researchers were under contract to address the identified concerns. Since then, the researchers have addressed the concerns and developed an updated set of MR levels for traffic signs.(66-67) This chapter summarizes the research that was conducted from summer 2001 to the end of 2002. However, it is specifically focused on the updated MR levels for overhead guide signs and street-name signs.
The work that was done to update the MR levels was based on the concerns mentioned above. A summary of the decisions that were made is included in this chapter. The complete details are discussed elsewhere.(66-67)
One of the first issues that was addressed during the research to update the MR levels was the assumed visual capabilities of the driver. After studying the effect of changing the assumed nighttime needs of the driver, the researchers and FHWA decided that the thresholds assumed in the earlier chapters of this report are reasonable. More specifically, it was decided that the assumed demand minimum legibility luminance thresholds derived from the median accommodation levels of drivers age 55 and older were reasonable in terms of defining a visibility metric. The additional research also led to the discovery of additional data, which allowed the researchers to generate a better estimate of the actual accommodation level of nighttime drivers that is inferred by the previously stated assumptions. The result of the additional data revealed that the actual nighttime accommodation level corresponds to levels well above 90 percent (89 percent of the nighttime driving population plus 50 percent of the nighttime drivers older than the age 55 threshold). Generally, this process results in visual capabilities approximating a 62- to 65-year-old nighttime driver.
Additional analysis on headlamp profiles and their effect on nighttime sign luminance led the researchers to recommend an updated headlamp for modeling MR levels. The updated headlamp profile takes into account many of the changes that have occurred in the headlamp and automotive industries over the past decades. The researchers also studied additional headlamp sources, including the latest headlamp trend-HID headlamps. Based on the inconclusive results of some of the analyses and the slow implementation of HID headlamps on new vehicles sold in the United States, UMTRI's U.S. low-beam 50th percentile profile for the 2000 model year vehicles was selected for establishing updated MR levels for traffic signs.(68) However, it is important to note that as the technologies, specifications, and the composition of the vehicle fleet evolve, there will be a need to revisit the headlamp issues associated with the development of MR.
Prior to the work described herein to update the MR levels for traffic signs, all MR research used vehicle dimensions representing a large passenger sedan. However, U.S. model year 1999 vehicle sales statistics show that for the first time since records have been maintained, trucks (defined as pickups, SUVs, and minivans) outsold cars. (For that year, trucks had about 50.1 percent of the new vehicle market versus 49.9 percent for cars.) This trend has continued through 2001 (the last year of available data). The most recent data from J.D. Power and Associates representing vehicle sales in the United States for the first quarter of 2003 show that the trend for larger vehicles still continues in the United States.(69) The top three best-selling vehicles were full-size pickup trucks and less than 50 percent of the combined vehicle sales were passenger cars. Over the last decade, the number of registered passenger cars decreased by 0.1 percent, while the percentage of trucks has increased more than 60 percent.(70) Based on these data, the researchers decided to use an updated vehicle that better represents the trends in the U.S. vehicle fleet. This decision was also made because it provides no compromises in terms of reducing nighttime visibility. Vehicles such as passenger cars generally have headlamp and seating arrangements that result in smaller observation angles compared to larger vehicles such as trucks, and the performance of retroreflective sheeting increases as the observation angle decreases.
In November 2001, researchers measured the pertinent dimensions of the top 10 best-selling light trucks, minivans, and SUVs for model year 2000. The results were averaged to develop a set of dimensions representing a typical light truck/minivan/SUV that could be used to develop MR levels. The overall impact of this change is a larger observation angle associated with the vehicle dimensions (see table 36).
Table 36. Updated Vehicle Dimensions
Additional work was completed to determine the assumptions related to retroreflective sheeting performance as it weathers. Preliminary analyses indicated that the assumption of constant degradation rates in observation profiles within specific sheeting types was only valid for materials using glass beads as their retroreflective element. The findings for materials using microprisms as their retroreflective elements were inconclusive, but did show the need for additional research. Ultimately, it was decided that the retroreflectivity data from the computer program ERGO would be the most reasonable and complete set of data to use in the calculations of the MR levels. However, it is clear that additional retroreflectivity data sets need to be available in the public domain.
This section describes the analyses that were conducted to develop a preliminary set of updated MR levels for guide signs and street-name signs. It includes the demand luminance criteria and other related conditions that were used to establish the MR levels.
Overhead Guide Signs
Using the updated factors as described above, overhead guide 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 right edgeline of the travel lane (i.e., centered above the left adjacent lane).
Using the demand luminance data determined from the human factors task of this research, the MR levels for overhead 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. The corresponding MR levels needed to satisfy these demand luminance values are shown in table 37. It is important to note that the demand luminance values and, therefore, the MR levels shown in table 37 represent the white legend for white-on-green signs.
The MR levels for the green background were determined by first calculating a white-to-green color ratio using the retroreflectivity standards shown in ASTM D4956.(5) Then, the white-to-green color ratio was multiplied by the MR levels shown in table 37, which are for the white part of the guide signs. This same process was used for all positive-contrast signs.
Table 37. Initial MR Levels for Overhead Guide Signs (cd/lx/m2)
Because street-name signs are installed in somewhat unique positions compared to other white-on-green signs, the researchers felt that 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 was varied depending on the speed of the roadway under consideration. In general, FHWA's proposed recommendations for the second revision of the Millennium MUTCD were used to select letter height as a function of speed. Table 38 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 38. Assumed Characteristics and Criteria for Street-Name Signs
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 right edgeline of the travel lane. 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 were used to determine the initial MR levels for street-name 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 39 shows the preliminary set of updated MR levels associated with these criteria.
Table 39. Initial MR Levels for Street-Name Signs (cd/lx/m2)
During summer 2002, FHWA conducted a second round of national workshops dedicated to the MR concept (the first set of workshops was held in 1995). A total of four workshops were held in Lakewood, CO; Hudson, WI; College Station, TX; and Hanover, MD.(71) The goal of the workshops was to solicit comments from public agencies regarding the implementation of minimum inservice retroreflectivity guidelines for traffic signs. During the workshops, draft MUTCD language for section 2A.09 was presented and revisions were suggested by the workshop participants. The workshops also included a nighttime demonstration of a variety of signs at various levels of retroreflectivity. The most current research recommendations regarding the MR levels were presented and discussed (as the workshops were conducted, the research regarding the updated MR levels progressed). One of the most consistently and frequently heard comments during the four national MR workshops held during summer 2002 was that the MR levels need to be easy to manage and implement. The results of the workshops will be used by FHWA to draft the MUTCD language used in the rulemaking process for the MR levels.
The recommended MR levels resulting from the work summarized in this chapter are shown in table 40. Table 40 also includes consideration of other white-on-green signs such as destination and distance signs and shoulder-mounted guide signs. The MR levels shown in table 40 represent the most current research recommendations, but are limited to the current knowledge of the nighttime requirements for traffic signs. It should be noted that, because of the limitations that are described below, there will be conditions where the levels shown in table 40 will not provide adequate retroreflectivity levels. It should also be noted that if the worst-case scenario were 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. Furthermore, environmental conditions such as dirt accumulation, dew, and/or frost were not necessarily considered in the development of the MR levels.
Table 40. Research Recommendations for Updated MR Levels
The development of the updated MR levels consisted of many different scenarios, including a variety of practical and typical speeds, roadway cross sections, vehicle types, sign positions, sign sizes, headlamp types, etc. Ultimately, the MR levels were derived from the equilibrium point of the 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. Luminance is the perceived brightness of a sign and retroreflectivity is a property of the sign that describes its ability 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 MR level associated with that situation. However, from a practical point of view, the MR levels need to be easy to manage and implement. This requires that the infinite number of MR 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 MR levels of 1993 and 1998 was a particularly significant issue according to the AASHTO Retroreflectivity Task Force. As the research effort 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 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 ultimately resulted in some degree of compromise between the precision and the brevity of the MR levels. The final research report provides a detailed description of how the MR levels were consolidated into an implementable format.(66)
The recommended updated MR levels presented in this chapter represent the most recent results of a series of dedicated research studies that have been undertaken over the past two decades. They also represent the latest efforts in a long series of safety considerations related to providing safe and efficient roadways. The key assumptions that are associated with the updated MR levels are described below:
FUTURE RESEARCH NEEDS
While significant progress has been made in the last couple of decades regarding the nighttime visibility requirements of traffic signs, there is a need for additional research. The following research topics, which are based on the assumptions and limitations associated with the proposed MR levels, are recommended by the research team: