Listed in Highway Safety
Maintaining safe and visible traffic signs is one of the most important responsibilities of any highway department. Over time, sign sheeting materials degrade, reducing their visibility at night. The Manual on Uniform Traffic Control Devices (MUTCD) requires that agencies maintain sign retroreflectivity at or above minimum levels. To help agencies comply, the MUTCD outlines several assessment methods, with two primary options being most commonly referenced: Visual Nighttime Inspection and Measured Sign Retroreflectivity.
Here’s what every highway superintendent should know about these two methods.
Visual Nighttime Inspection
This is the most accessible method for most towns and small highway departments. It relies on trained personnel to inspect signs during nighttime conditions using vehicle headlights, simulating the typical driver's experience.
How It Works:
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Inspections are conducted from a moving vehicle at night.
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The inspector evaluates whether signs are still bright enough to be seen and read at a distance.
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The inspector should have normal (or corrected-to-normal) vision and be familiar with sign types and expected visibility standards.
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The sign is most often compared side-by-side with a new sign or sample which has been protected from light and UV rays.
Key Benefits:
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Low cost: Requires no special equipment beyond a vehicle.
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Practical: Evaluates signs in real-world conditions.
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Efficient for smaller towns with fewer signs.
Things to Keep in Mind:
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Training is critical. Personnel need to know what to look for and be consistent.
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Subjectivity can be a drawback—different inspectors may judge signs differently.
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Keep written records of inspection dates, observations, and signs replaced.
Measured Sign Retroreflectivity
This method involves using a retroreflectometer, a specialized tool that measures how much light a sign reflects back toward a light source, simulating the performance seen by a nighttime driver.
How It Works:
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The retroreflectometer is held against the sign face or aimed at it from a set distance.
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It provides a numerical value (typically in candelas/lux/m²).
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The measured value is compared to MUTCD minimum retroreflectivity levels.
Key Benefits:
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Objective and precise. Removes human judgment from the process.
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Documentation-friendly - easy to prove compliance during audits or legal claims.
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Good for high-volume sign inventories or critical sign locations (e.g., stop signs, school zones).
Things to Keep in Mind:
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High upfront cost. Equipment can run several thousand dollars.
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Training required to operate the device correctly.
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More time-consuming than visual inspection, especially if covering large areas.
Choosing the Right Method for Your Department
The MUTCD allows flexibility. Your department can use either method or a combination of both, depending on your resources, staff capacity, and inventory size.
Smaller towns often begin with visual nighttime inspection and adopt spot-checking with a retroreflectometer for high-risk areas. Larger municipalities with thousands of signs may lean more heavily on measured methods to efficiently track and schedule replacements.
Regardless of which method you choose, the MUTCD emphasizes the importance of having a documented maintenance policy, and following it.
Maintaining compliant sign retroreflectivity isn’t just about avoiding liability. It’s about keeping your roads safe for nighttime drivers. Whether you use your own eyes or a calibrated meter, staying on top of sign visibility helps prevent accidents and builds trust with your community.
If you're unsure where to start, consider developing a basic sign assessment plan and slowly build your approach from there. Remember: it’s not the method that matters most; it’s having a method in place and using it consistently.