Roadside Measurement requirements
The standard used for signs is the Coefficient of Retroreflection (RA). A description of this can be found in ASTM Standard E808. The metric unit for RA is candelas per lux per square meter (cd/lx/m²). The English units for RA (cd/fc/ft²) are sometimes used, but have the same numerical value as the metric equivalents. It is also often referred to as Specific Intensity per unit Area (SIA) in older documents and specifications; use of this term should be discouraged.
RA is a ratio of returned intensity to incident illumination divided by the area of the retroreflector. Signs make the measurement of these quantities simpler because they have a fixed area. In addition, the measured geometry is arranged so that the plane of the sign is perpendicular to car headlights, thus the area does not change as fast as angles near horizontal. This makes the measurement much simpler and more accurate.
For field measurements of signs, handheld devices are most commonly deployed. These devices must meet the requirements of at least one standard. The most common standards are ASTM E1709, ASTM E2540, and EN 12899-1. Other countries may have their own standards, but most are very similar to the ASTM and EN standards.
Sign Retroreflectivity Requirements
The RoadVista Model 922 is an integral component of any program to maintain minimum retroreflectivity levels. Further information about the current version of the MUTCD can be found on the FHWA website.
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