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    Visual Nighttime Inspection Evaluates Retroreflectivity


    Assessment methods ensure compliance with the proposed standards by evaluating all signs in place on the roadways.(16)

    Assessments may be made by observation

    Nighttime visual inspection may be conducted by qualitative observation using one or any combination of three methods:
    1. Calibration signs The agency obtains sample signs that are at or near minimum levels. On the night of inspection, the observer views the sample signs from the vehicle to compare against in-place signs to indicate those that need to be replaced. Signs are needed for each color for which there is a minimum level.(28)
    2. Comparison panels The agency obtains sample panels at or near minimum levels. Upon observing a questionable sign, the inspector stops and clips the panel to the sign to make a direct comparison. If the panel appears brighter, the in-place sign needs to be replaced. (16)
    3. Consistent parameters This method uses consistency as a reasonable way to inspect for non-compliant signs. Observations are made at regular highway speeds from the travel lane (right lane). To produce consistency, this method:(16)
      • Uses human observers (trained inspectors, 60 years or older)
      • Evaluates sign retroreflectivity at a viewing distance that would provide adequate driver response
      • Observes using low-beam headlights on a full-size SUV or pickup truck model year 2000 or newer

    Assessments may be made by calculation

    Retroreflectivity is measured quantitatively using a retroreflectometer. In one recommended process to accomplish this, the agency:
    • Takes at least four retroreflectivity readings, averaging the results
    • Compares these with minimum values
    • Determines which signs are approaching minimums.
    Measurements may be taken by handheld devices or prototypical mobile units developed by the FHWA. The primary advantage of the measured retroreflectivity method is that each sign's actual retroreflectivity is known. This method can quantitatively prove compliance to FHWA standards.(16)


    16 Harris, E.A., et al, Analysis of traffic sign asset management scenarios, TRB 2007 Annual Meeting CD-ROM
    28 Maintaining traffic sign retroreflectivity, FHWA-SA-03-027
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