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Measuring and Monitoring Sign Visibility

October 24, 2019

Sign visibility is extremely important because it can make a difference in preventing accidents. A stop sign, for example, spray-painted a dark color will not be visible at night and can lead to collisions. It’s up to local leaders to periodically inspect street signs to ensure they are up to code. Here are important facts to know for measuring and monitoring sign visibility.

Visual Perception

Sign visibility encompasses visual perceptions of the individuals engaging with the sign. The science of vision describes how light passes through the human eye, then is focused by the cornea and lens. An image is generated at the back of the eye on the retina, which is a group of cells sensitive to light, that connect with a network of nerve cells and light receptors.

Light images picked up by these receptors are converted to electrical signals that are transmitted to the brain. Cones, which determine perception of color and detail, are receptors that operate best under high light intensities. Rods, on the other hand, are more sensitive in low light without contributing to color or detail perceptions.

Sign Visibility Factors

The age of a sign often determines its quality of visibility and legibility. The older a sign gets, the more it will likely fade, especially if it’s always exposed to sunlight. A sign can also lose visibility from the wear and tear of severe weather, in which debris obscures the message. Another factor is the driver’s vision, which can be impaired by alcohol or medication.

Time of day often plays into visibility, even though road signs use reflective coating on aluminum that makes them shine at night. Studies show that most accidents happen at night, which is why extra care should be put into making signs more visible at night.

Regular inspections will help determine sign vulnerabilities so that problems can be fixed in their early stages. It’s best to correct a sign as quickly as possible if it’s been vandalized, whereas a slowly fading sign doesn’t demand as much immediate attention. Inspecting signs after heavy storms will help detect damage early.

Measuring Legibility

An important subset of sign visibility is sign legibility, which is the ease at which sign content can be read and perceived. Sign legibility refers to reading unfamiliar signs, whereas sign recognizability refers to reading familiar signs. The distance at which a person can read a sign is known as legibility distance. These measurements play into testing and sign placement.

The legibility index (LI) is used by government officials to determine if a sign needs to be repaired or replaced due to low visibility. LI is a product of dividing the sign’s legibility distance by the letter height. Using these values, local officials can place signs where they are most conspicuous. Retroreflectivity, size and color are crucial factors to check for evaluating sign legibility.

Other variables to visibility include the amount of lighting and its system design and contrast from other lighting. The key to ensuring sign visibility is to not just rely on a formula, but to have an expert inspect the sign and make a determination based on several variables.


Several factors affect sign visibility, from age to climate to vandalism. Inspecting signs periodically is an essential part of extending their longevity. Contact Zumar at our Arizona, California or Washington location to learn more about making signs more visible day or night.

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