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Federal Highway Administration Research and Technology
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Publication Number: FHWA-HRT-07-042
Date: April 2007
Maintaining Traffic Sign Retroreflectivity: Impacts on State and Local Agencies
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1.1. System of Traffic Signs
Traffic signs are the principle medium by which highway agencies communicate regulatory, warning, guidance, or other information to road users. This means that traffic signs must be detectable, legible, and comprehensible to users at a distance commensurate with their purpose. Traffic signs are designed to satisfy these requirements by selection of sign size and color, the size and style of letters and numerals and application of symbols, and the retroreflective materials used for the background and legend. It is through the appropriate selection of the design parameters that State and local agencies develop signs intended to meet drivers' needs under both day and night conditions.
The FHWA has promoted efforts to design and implement improved traffic control devices (TCDs) that meet the needs of drivers under both day and night conditions. There has been considerable research to 1) understand driver needs, 2) develop improved TCDs (e.g., designs, materials, and technology) to meet those driver needs, and 3) establish sound practices for TCD application and management. The underlying motivation for these efforts has been the interest in promoting safety and efficient flow of traffic during all time periods. Crash data indicate, however, that about 50 percent of the traffic fatalities occur at night despite significantly lower volumes of traffic. This over-representation of night fatalities has persisted for more than 20 years. The nighttime crash rate has been estimated to be three times that during the day.2 The FHWA is, therefore, focusing more attention on the nighttime crash problem and reviewing the influences of highway design and control.
1.2. Night Visibility of Traffic Signs
The night visibility of traffic control devices (e.g., signs, pavement markings, and signals) is critical to the safe and efficient operation of roadways at night. TCDs also represent one area where immediate night visibility improvements are considered possible to enhance the delineation of the roadway, warn drivers aware of unexpected conditions, and facilitate their abilities to navigate the road system.
Every version of the Manual on Uniform Traffic Control Devices (MUTCD)3 has included requirements for nighttime sign visibility, since the first edition in 1935. Over that time, a variety of sign materials have evolved to provide options in meeting detectability and legibility objectives, but there have been no specific design or maintenance thresholds. The available materials vary in cost and performance, particularly relative to night visibility, complicating decisions for traffic sign design and budgets for sign programs.
1.3. Purpose of Retroreflectivity
Retroreflectivity, one of the factors associated with night visibility, is the property of a material to redirect light back towards its source. In the case of a traffic sign, light is redirected back from the sign face toward the vehicle's headlights, making the sign visibile to the driver. The American Society for Testing Materials (ASTM) has established testing procedures and manufacturing requirements for retroreflective materials and measurement equipment. These measures can be used in varying ways to support the different methods and procedures by which agencies assess and manage the retroreflectivity of traffic signs.
1.4. Traffic Sign Degradation
The retroreflectivity of signs gradually deteriorates over time, thus making signs progressively less visibile at night. While deterioration can occur in a number of ways, the primary mechanisms are the loss of retroreflectivity and the fading of the color portions. As the retroreflective properties deteriorate, the sign becomes less detectable and legible at night. When the colors fade, the sign loses a distinguishing feature and the contrast between legend and background is reduced. For critical signs, such as STOP signs, fading of the red background may make the sign less detectable and legible even during the daytime. Deterioration can occur for a variety of reasons, ranging from the environment in which the sign exists to poor workmanship during fabrication or improper installation of the sign. Highway agencies are faced with the challenge of determining when the deterioration has reached levels that warrant replacement of the sign while at the same time avoiding replacing a sign before its true useful life is reached. The useful life of a traffic sign is a critical factor in assessing sign maintenance costs for a highway agency.
1.5. Minimum Maintained Retroreflectivity
In 1992, the Congress directed the Secretary of Transportation to revise the MUTCD to include a standard for minimum levels of retroreflectivity that must be maintained for traffic signs.4 The FHWA already had an active research program investigating the nighttime visibility of TCDs to meet driver needs. In 1993, the FHWA responded to the congressional mandate by publishing a set of research recommendations for minimum maintained sign retroreflectivity levels.5 A series of tables was presented in the research report to establish minimum maintained retroreflectivity levels for regulartory, warning, and guide signs. Based on comments received in workshops with practitioners, revised recommended minimum levels were published in 1998 in a report entitled "An Implementation Guide for Minimum Retroreflectivity Requirements for Traffic Signs."6 Because of changes to U.S. headlight standards, the FHWA conducted additional research to develop minimum maintained retroreflectivity levels for overhead guide signs and street name signs, which were not included in the minimum levels published in 1998. The research was published in 2003,7 and culminated in the proposed minimum maintained retroreflectivity levels for traffic signs in July 2004.
1.6. Impacts of Proposed Rule
About 75 percent of the public roads in the United States are maintained by local agencies, 21 percent are maintained by State agencies, and the remainder are maintained by Federal agencies.8 Therefore, it is imperative that the impacts on State and local agencies resulting from the proposed addition to the MUTCD of minimum maintained traffic sign retroreflectivity levels be carefully assessed.
Impacts take many forms and can be considered positive or negative. For this analysis, the concerns identified by participants in four workshops on nighttime sign visibility conducted for the FHWA at locations across the country in 2002 were used as a starting point.9 About 100 State and local officials participated in these workshops, which were organized to present updated information on the FHWA's plans to implement new minimum maintained sign retroreflectivity levels through changes to the MUTCD. During these workshops, the participants cited numerous perceived impacts that the new levels would have on their agencies, which are outlined in Table 1. It needs to be stressed that many of these items were perceptual, as many of the agencies had not begun thinking about how they would determine their degree of compliance or how they would implement more rigorous sign management processes to address night visibility needs.
It should be noted that most of the participant discussion in the workshops focused upon the negative impacts of implementing minimum levels of traffic sign retroreflectivity. The extent of those negative impacts will vary from agency to agency, depending upon current sign replacement practices. Negative impacts are expected to be smaller for agencies that currently have proactive sign management practices. There is also the potential for positive impacts from improved signing, including lower overall sign costs due to more effective sign replacement strategies and improved safety and mobility for the driving public due to better sign visibility. Participants recognized their agency roles and noted that adoption of minimum levels could be useful in obtaining increased funding for sign improvements. The workshop participants suggested that the new minimum levels should not be imposed without Federal funding assistance. However, the means to offset the costs of sign inspection, replacement, and long-term maintenance of adequate night visibility is not a topic that is covered in this report.
The concerns presented in Table 1 cover a broad spectrum, some of which can be readily translated into dollar amounts. However, it is difficult to generate reliable financial estimates for many of the concerns, and for some concerns it is not considered necessary.
1.7. Benefits of Improved Visibility of Traffic Signs
The FHWA believes that although improving sign retroreflectivity will be especially beneficial to older drivers, all drivers, including younger drivers, will find that improved sign retroreflectivity will be beneficial for their nighttime driving experience. All drivers need legible signs in order to make important decisions at key locations, such as intersections and exit ramps on high-speed facilities. This is particularly true for regulatory and warning signs. This is fundamental to safe driving, and the lack of uniform retroreflectivity standards has led to wide variations in maintenance levels of these critical signs. As discussed in the Supplemental Notice of Proposed Amendments (SNPA),10 there have been some investigations that demonstrate potential safety benefits of upgrading sign materials. More importantly, maintaining sign retroreflectivity is consistent with one of the FHWA's primary goals, which is to improve safety on the nation's streets and highways. Improvements in sign visibility will also support the FHWA's efforts to be responsive to the needs of older drivers, which is important because the number of older drivers is expected to increase significantly in the next 30 years.
1.8. Organization of Report
This report is organized to address questions associated with the concerns at increasingly higher degrees of detail. Chapter 2 addresses questions about the cost impact at the individual sign level, such as how much will need to be spent on sign face materials. Chapter 3 addresses questions at the agency level, such as the number of in-place signs that do not meet the proposed minimum levels, and the impacts of procedures to implement and administer management practices to comply with the proposed new rule. Previous impact analyses are reviewed in Chapter 4, including one State report that addressed tort liability concerns. All of this information is then used to develop the National Impact Assessment that is presented in Chapter 5. Finally, the conclusions are provided in Chapter 6.
1.9. Background and Assumptions
This analysis of the impacts of the proposed minimum maintained traffic sign retroreflectivity levels updates FHWA-RD-97-053, "Impacts on State and Local Agencies for Maintaining Traffic Signs within Minimum Retroreflectivity Guidelines," April 1998.6 In addition to updating the 1998 report, this report addresses concerns expressed by the 2002 workshop participants. The primary sources of information for this effort are previous studies related to the benefits of improved signage and the impacts of implementing sign system upgrades. No new or better data were discovered, necessitating a reliance on previously gathered data. More detailed information on retroreflectivity and the research efforts that have led to the proposed minimum maintained retroreflectivity requirements for traffic signs can be found in the 1998 report.
It is important to emphasize that the proposed minimum retroreflectivity levels for traffic signs presented in Appendix A represent a minimum threshold needed to accommodate older drivers in dark rural conditions. There are uncertainties associated with the minimum levels due to assumptions about viewing position and surrounding conditions, modeling weathered retroreflective sheeting materials, and aggregation. Furthermore, there are conditions when agencies may choose to use higher minimum maintained retroreflective levels. The MUTCD does not restrict agencies from using higher levels of retroreflectivity if, based on engineering judgment or studies, the agencies determine that higher levels are warranted. The presence of the minimum retroreflectivity levels in the MUTCD does not imply that all pertinent signs must meet the minimum retroreflectivity levels at all times. The FHWA understands that there will be cases where issues such as weather, vandalism, or damage influence the visibility of a sign.
Because many factors influence the visibility of a particular traffic sign under actual nighttime driving conditions, efforts to schedule sign upgrading or replacement actions should be triggered as a sign approaches the threshold so that it never reaches a level that is inadequate to meet drivers' needs.
This impacts analysis was conducted with the following assumptions:
Under these assumptions, the analysis indicates that impacts are distributed over a long enough period of time to allow most sign replacements to occur under normal maintenance cycles.
Topics: research, safety, visibility and retroreflectivity
Keywords: research, safety, retroreflectivity, maintained retroreflectivity, minimum maintained retroreflectivity, impact, impact assessment, impacts analysis, national impact assessment
TRT Terms: reflective signs, retroreflectivity