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## Section 2: General Retroreflectivity Concepts

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### Introduction

The visibility of a pavement marking is determined by the amount of light reflected off the marking’s surface to a driver’s eye. During daylight hours, marking visibility is achieved through light from the sun striking the marking surface and scattering in all directions, some of which reaches the driver’s eyes. However, in dark environments at night (without roadway lighting), vehicle headlamps produce most of the light striking a pavement surface, and therefore the retroreflective properties of the pavement marking govern the amount of light that reaches the driver’s eyes.

Retroreflectivity in pavement markings is a measure of the amount of light from the vehicle’s headlamps that is reflected back to the driver’s eyes.

Retroreflectivity is a measure of how efficiently the pavement marking returns light from the headlamps back to the driver. In mathematical terms it is a ratio of the reflected luminance to the headlamp illuminance at a certain viewing geometry. Figure A-1 shows a simple illustration of pavement marking retroreflectivity.

Figure A-1. Pavement Marking Retroreflectivity at Different Geometries.

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### Factors Influencing Retroreflectivity

While glass beads are responsible for most of the retroreflectivity in pavement markings, retroreflectivity is influenced by numerous characteristics of the marking, including properties of the glass beads themselves. Table A-1 shows some of the major factors that influence the amount of retroreflectivity that a marking produces.

 Glass Beads Amount and Dispersion Embedment Depth Refractive Index Size Clarity Roundness Binder Material Color Type Thickness Other Pavement Surface Roughness Dirt or Other “Blinding” Material Type of Retroreflectometer Used for Measurements

Clearly, pavement marking retroreflectivity is a complex phenomenon that is influenced by many factors.