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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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Appendix A - Proposed Minimum Maintained Levels of Traffic Sign Retroreflectivity
In 1985, the USDOT was petitioned to require minimum levels of retroreflectivity for signs and pavement markings. In 1992, Congress directed the US DOT to "revise the MUTCD to include a standard for a minimum level of retroreflectivity that must be maintained for pavement markings and signs, which shall apply to all roads open to public travel." Since that time significant strides have been made in developing sound practical minimum requirements, as well as in means to facilitate their implementation. The information provided in this report represents one of the latest products of the FHWA's efforts.
The FHWA has been involved in research investigating driver night visibility needs since the early 1980s. This research led to the publication of a report entitled "Minimum Retroreflectivity Requirements for Traffic Signs" (FHWA-RD-93-152, October 1993) which translated driver's needs for sign luminance for various types of signs and applications into minimum levels of retroreflectivity. Retroreflectivity was selected as the evaluation criterion since it could be conveniently measured in the field. The minimum retroreflectivity values recommended in the 1993 report were modified somewhat after workshops with practitioners in 1995. These requirements were published in 1998 in a report entitled "An Implementation Guide for Minimum Retroreflectivity Requirements for Traffic Signs" (FHWA-RD-97-052). In 1999, the FHWA initiated further research to define minimum requirements for sign types not covered in the 1993 report with a project entitled "Minimum Retroreflectivity Values for Overhead Guide Signs and Street Name Signs" (publication pending). This project developed a new analysis tool, incorporated newly acquired field luminance requirements data gathered from older driver subjects, and calculated minimum retroreflectivity requirements for overhead and street name signs.
In efforts to combine the results from the two research efforts, it became apparent that there was a need to revisit both research efforts to incorporate data that reflected current conditions. In 2000, the FHWA funded a project entitled "Updated Minimum Retroreflectivity Levels for Traffic Signs," (draft report December 2002). In this project, the basic inputs for the analytical derivation of driver luminance needs (translated to retroreflectivity measures) were updated. This included changes to reflect the characteristics of newer headlights, the capabilities of older drivers, the influences of larger-sized vehicles in the current fleet, the properties of sign materials that did not exist when the earlier research was undertaken, and other factors. A more powerful computer analysis tool was used to determine minimum driver retroreflectivity requirements. The project generated numerous detailed tables that reflected various sign positions, traffic speeds, and other factors. These tables were collapsed and consolidated to provide an easier-to-use benchmark. Table 1 provides the most recent version of the minimum requirements for traffic sign retroreflectivity. This single table combines the requirements for all color and sign applications, except for signs with blue or brown backgrounds.
Why have these minimums at all? Hasn't the FHWA indicated that they are really interested in better night visibility for drivers? It is believed that the minimum requirements that have evolved from the recent research provide useful benchmarks that are needed to support efforts by agencies to assess the night visibility of their in-place signs, determine those needing replacement, and apply more rigorous sign management programs.