Science of Retroreflectivity

Retroreflective signs help address nighttime driving safety issues such as older drivers, glare, visual clues and driver expectations.
Upcoming MUTCD standards are structured around basics of sheeting retroreflectivity. The fundamental question was how to relate "standard retroreflectivity" to visibility of traffic signs:

Lighting parameters ...
Headlights emit light measured in candelas (intensity [cd])
When the beam from a headlight strikes the sign surface, the amount of light meeting the surface is measured in lux (illuminance)
When this light is reflected toward an observer, the observer sees brightness, or luminance, measured in candelas per meter squared (cd/m2)
Coefficient of retroreflection is the ratio of this luminance or brightness to the illuminance (RA = cd/m2/lux)

Retroreflection: A special type of reflection where incoming light is returned toward its source.
Entrance Angle: This is a measure of how the light is entering the sign. If the light is entering the surface perpendicularly, the entrance angle is zero.
Observation Angle: This is the angle between the illumination path (from a headlight to the sign) and the observation path (from driver eye to the sign). In a sense, it is the measure of separation of the driverís viewpoint from a headlight.

Coefficient of retroreflection (RA) is a measure of retroreflective efficiency of a retroreflector at a specific set of entrance angle and observation angle, which specifies one certain viewing geometry.
Minimum retroreflectivity requirements refer strictly to 0.2 degree observation angle and -4 degree entrance angle.
Retroreflective efficiency: The percent of light returned back toward its source by a retroreflector. Typically, beaded sheetings have 7% to 14%, truncated cube microprismatic sheetings have about 32%, and a full-cube sheeting has 58% retroreflective efficiency.
There is no direct correlation between the retroreflective efficiency and ASTM sheeting types.

15 Traffic sign retroreflectivity "sign retro 101", FHWA Visibility Team PowerPoint

Owners or parties responsible for such private roads have two years from the date of the ruling (January 17, 2007) to bring traffic control devices into compliance with the MUTCD and other applicable State Manuals.