Background and current status of the development of updated minimum in-service retroreflectivity levels for traffic signs in the United States are described. A summary of the earlier work, conducted from the late 1980s to the mid-1990s, is provided along with descriptions of the relevant validation work. Related events that occurred in the late 1990s are described, and it is shown how, when combined with the assumptions and limitations of the earlier minimum retroreflectivity research, they resulted in a need for updated minimum retroreflectivity levels. The research related to the updated minimum retroreflectivity levels is summarized. A brief description of a second round of national minimum retroreflectivity workshops is also provided. The most recent set of recommended minimum retroreflectivity levels for traffic signs is presented along with a list of suggested research topics based on the limitations associated with the recommendations.

Research and recommendations were produced on the basis of a study on the development of minimum retroreflectivity values for overhead guide signs and street name signs. The research reviewed the literature and available photometric models. A photometric model was created to develop minimum retroreflectivity values for overhead guide signs and street name signs. One model input was the minimum luminance required for legibility. To obtain minimum luminance requirements for overhead guide signs and street name signs, a field evaluation was performed. Accommodating older drivers was specifically emphasized. After the minimum luminance values were determined, the minimum retroreflectivity model was used to study the impacts of (and determine appropriate values for) factors that affect retroreflectivity, such as distance, headlights, and vehicle speed. After the appropriate values were determined, the minimum retroreflectivity model was executed for the final runs. The recommended minimum retroreflectivity values were grouped into one table that includes overhead guide signs, postmounted street name signs, and overhead (mast-arm– or span-wire–mounted) street name signs. The values depend on type of sheeting, speed, and sign position.

Research was conducted to review and develop minimum levels for pavement marking retroreflectivity to meet nighttime driving needs. A previous study performed in the 1990s using a computer model called the Computer-Aided Road-Marking Visibility Evaluator resulted in a table of minimum levels of pavement marking retroreflectivity values that FHWA used to develop its initial set of minimum pavement marking retroreflectivity levels. Since then, additional research has been completed as well as development of a newer, more feature-intensive computer model called the Target Visibility Predictor (TarVIP). The research presented used TarVIP to study pavement marking retroreflectivity needs while using the most recently available information pertaining to driver, vehicle, and headlamp trends in the United States. In this research, previous pavement marking research efforts that included findings or recommendations related to minimum retroreflectivity are summarized. Next, a comprehensive survey on the factors that affect pavement marking visibility and minimum RL levels was performed, with key factors identified. They included pavement marking configuration, pavement surface type, vehicle speed, vehicle type, and presence of raised reflective pavement markers. From findings of the key factor reviews, the TarVIP model was used to generate preliminary results that could then be analyzed by sensitivity analysis. The research resulted in a set of recommended minimum pavement marking retroreflectivity levels for typical conditions on U.S. roadways. Limitations of the research were listed as well as concepts for future work.

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