MUTCD Visibility of Traffic Signs - Retroreflectivity
MUTCD minimums establish a means to improve the nighttime visibility of traffic signs to promote safety, enhance traffic operations and facilitate comfort for drivers of all ages and abilities.
MUTCD standards for sign retro-reflectivity or illumination have remained essentially unchanged for 45 years.(2) In the 1980's, the FHWA began nighttime sign visibility research and in 1993 Congress directed the FHWA to revise the MUTCD to include minimum levels.(12) That ruling has produced minimums for critical traffic signs with a table of specific requirements and implementation (referenced below). FHWA Standard
One or more of the following assessment or management methods shall be used to maintain sign retroreflectivity above the minimum levels:
Visual nighttime inspections Measured retroreflectivity Expected life Blanket replacement Control signs
Minimum retroreflectivity standards also apply to Private Roads. Effective January 17, 2007, MUTCD regulations apply to "all roads open to public travel." Includes toll roads and roads within shopping centers, parking lots, airports, sports arenas, and other similar business and recreation facilities that are privately owned but where the public is allowed to travel without access restrictions.
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.
Source(13):
The FHWA-published table of minimum retroreflectivity values establishes standards of compliance. These values are considered minimums. The agency may decide to use higher values based on their own replacement criteria.
Determines the ratio of retroreflective contrast and sign size ("Conditions"). For example, white-on-red retroreflective signs such as Stop or Do Not Enter signs require a 3:1 (white:red) contrast ratio. When the contrast ratio is lower, the sign should be replaced. Indicates with an "*" where ASTM I (engineer grade) should not be used such as black on orange/yellow signs. Sheeting material for white-on-red signs should not fall below an RA (coefficient of retroreflection) of 35 measured in candelas per lux per meter squared. Provides some exclusions for signs such as Parking, Walking, Adopt-A-Highway, Bikeway, etc.)
FWHA Standards
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:
Parameters
Retroreflection
Retroreflective Efficiency
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)
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 driver’s 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.
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.
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.
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