Retroreflectivity, or retroreflection, is an optical phenomenon in which reflected rays of light are preferentially returned in directions close to the opposite of the direction from which the rays came. This property is maintained over wide variations of the direction of the incident rays. Retroreflection is achieved through multiple reflections within a retroreflector. Common retroreflectors are cube corners and microspheres of glass or plastic.
Retroreflection is used for highway safety and measuring distance. The method used for measuring distance is called time-of-flight measurement, which can be used for great distances. In time-of-flight measurements, the distance from light source to target is determined by measuring the time it takes light to travel to the target and return to the sensor. Time-of-flight distance measurements can be made using pulse-type systems or modulated beam systems. Pulsed-type systems are used for measuring great distances, whereas modulated beam systems are typically used for intermediate range distance measurements. A cube corner array, for example, was left on the moon to allow accurate measurement of its distance from the earth.
The most common and practical use of retroreflection technology is its application to highway safety. Pavement markings and road signs are two of the most important means of ensuring the safety of motorists during their travels. Retroreflectivity, or nighttime visibility of signs and pavement markings, is essential for efficient traffic flow, driving comfort, and highway safety in general. Pavement marking and road signs are visible at night because the light from headlights is reflected back into drivers’ eyes by retroreflectors embedded into the signs or road stripes.
Determining the Visibility of a Sign or Pavement Marking
Legibility
This refers to the probability that the driver will understand the message that the roadway delineation or sign is meant to convey. Legibility relies upon many factors. In addition, the criteria by which legibility may be judged differ for different types of delineation. Legibility is an even less tangible quantity that conspicuity.
Luminance
The total amount of light a driver receives from a sign or marking. Luminance of the sign or marking is directly proportional to the amount of light energy that is directed back towards your eyes.
Visibility Distance
The range at which a marking or sign can be seen. Visibility distance only specifies the distance at which a given driver is capable of seeing a marking. It is not a guarantee that the driver will see it. It is important that you keep the visibility factor in mind as you move into the application of retroreflectivity to highway safety.
Contrast
The ratio of luminance from the marking to luminance from its surroundings, as measured from the driver’s position. Contrast is much more important for overall visibility than luminance because contrast defines how clearly a target stands out from its background. Contrast is a better measure of marking and sign visibility than is luminance.
Conspicuity
This refers to the likelihood that a driver will notice a selected object at a given distance. It is probably the best measure of visibility, but it is difficult to quantify. Unlike luminance and contrast, conspicuity is not an easily determined optical quantity. It is difficult to determine an estimate for the conspicuity factors, and very difficult to combine them in a way that yields a numerical measure of conspicuity. Conspicuity is a quantity that can only be determined empirically.
Color
The color of highway marking signs are specified for different purposes. For example, white with black lettering is used for regulatory information such as speed and traffic flow direction; red is used for stop and wrong way signs; yellow and new fluorescent yellow are used for warning signs. Color can increase the perceived contrast of the sign with its surroundings.
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