U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
202-366-4000
The following traces the evolution of retroreflectivity and illumination requirements in the various editions of the MUTCD. These descriptions do not include language introduced in a revision to an MUTCD edition. [GH23]
All Stop, slow-type, and railroad advance warning signs shall be illuminated at night so as to be visible from all distances up to 350 feet.
The outlines [ed. note: border] and word "STOP" of Stop signs, the outlines of slow-type signs, and the outlines and letters "RR" of railroad advance warning signs shall be illuminated.
Less brilliant illumination of the message on slow-type signs is desirable but optional. If a sign is necessary in daylight, it has equal or greater value at night, inasmuch as night driving at high speeds is continually increasing. Illumination of the outline of STOP and slow-type signs will make them visible at ample distance to enable motorists to slow down in time to read and obey the specific message. Illumination of word message legible a sufficient distance would hardly be feasible. Illumination of the outlines, besides providing advance warning, should have much educational value in impressing upon motorists the significance of shapes.
In any program of replacement of existing equipment the first replacements with standard signs shall be at the most hazardous locations and at places where the signs are not otherwise adequately illuminated.
While illumination of parking signs is not ordinarily warranted, it is desirable to illuminate certain signal and other movement signs which should be readable at night and sometimes at considerable speed. It is recommended that route markers and destination signs also be illuminated. At least one state has already provided reflecting elements for all signs designating United States highways.
Illumination of signs shall be white, except the lettering on STOP signs, which shall be red.
The following methods of illumination are listed in the order of preference:
Flashing or steady lights within the sign, flashing or constantly luminous tubes, or flood lights provide the maximum visibility under all conditions. Because of their higher cost and the difficulty of service connections, however, they will probably not be used very frequently in rural areas.
Reflecting buttons outlining the shape of a sign should be placed on the yellow background just inside of the black border.
The importance of the STOP sign is such that it merits a distinctive color of illumination, and red is so generally recognized as a mark of danger requiring a vehicle to stop that it is the obvious choice for the purpose, especially as its visibility is adequate for permitted speeds.
Street lighting is not generally considered adequate illumination for signs because of inflexible location, reduced light due to foliage, and the fact that street lamps are sometimes not operated during all the hours of darkness.
All traffic signs shall be kept in good order and clearly legible at all times. Signs damaged or destroyed shall be replaced within 24 hours.
Signs shall be cleaned and inspected for repainting at least once a year.
All stop, slow-type, and railroad advance warning signs shall be illuminated or reflectorized so as to be visible at night from all distances up to 350 feet. All illuminated signs shall be extinguishable on short notice or be otherwise conformable to War Department blackout or dimout specifications.
The symbols and/or main messages of stop, slow-type and speed limit signs, and both the cross and letters "RR" of railroad advance warning signs, shall be illuminated or reflectorized.
Adequate illumination is generally to be preferred to reflectorization, where cost is not a controlling element. No method of illumination or reflectorization should be employed that will materially reduced daylight legibility.
Illumination may be by means of:
Illumination of signs shall be white, except that flashing red self-illumination may be used in the lettering on STOP signs.
Either the message or background of a sign may be reflectorized. Reflectorization of symbols or messages shall be white. If the background is reflectorized it shall be white or yellow in accordance with the color scheme outlined in section 126.
To improve legibility through greater contrast, a panel of a color to contrast with that of the message may be used.
All traffic signs shall be kept in good order and clearly legible at all times. Signs damaged or destroyed shall be replaced within 24 hours.
Signs shall be cleaned and inspected for repainting at least once a year.
Signs that carry messages of warning, important regulations, or essential directional information are at least as necessary by night as by day. All warning signs, including railroad-crossing signs, all Stop signs, all other regulatory signs with the exception of pedestrian signs and urban parking signs, and, on State highways and important local roads, all guide signs, shall be illuminated or reflectorized. Parking signs are ordinarily read at slow speeds and often receive some illumination from street lighting. Other exceptions to the general rule are the School sign and similar signs that have significance only during school or daylight hours, and the Men Working sign and other temporary signs, if used only during daylight.
Illumination may be by means of:
Ordinary street or highway lighting is not regarded as meeting the requirements for sign illumination. However, such lighting can aid visibility to an appreciable degree and, if present, should be taken into consideration in selecting the exact points at which unreflectorized signs are placed.
All illumination shall be by white light, except that a flashing light incorporated in a sign installation shall be yellow when displayed with a warning sign or red when displayed with a Stop sign, in accordance with the signal code set forth in section 167 of this manual.
Reflectorization may be by means of:
All reflecting elements shall reflect white light, except that if a reflecting coating is used as a background of a yellow sign it shall reflect yellow light. If a black panel is used on the Stop sign the letters shall be reflectorized by clear reflector buttons or by white or yellow reflecting coating.
Reflector buttons in a highway sign are individual reflecting units arranged in rows or patterns to form the letters or symbols. In suitable sizes and spacings they give the visual effect of continuous lines or areas of light. Reflector buttons are made of glass or transparent plastic with lenses or prisms designed to reflect an incident beam of light directly back toward its source. It is this retrodirective reflection that distinguishes reflector buttons from any sort of specular or diffuse reflectors and gives them their great brilliance. An effective reflector button concentrates most of the reflected light into a narrow cone having only enough divergence to reach the driver's eyes in his normal position above the headlamps. This divergence angle should be on the order of one-half to one degree.
If made with multiple small prisms or lenses, reflector buttons need not be circular, though they remain essentially separate units.
A reflecting coating is a surfacing applied either to the background or to the symbol or lettering of a sign to give a uniformly brilliant reflection over the entire area so coated. The types of coating most commonly used are of retrodirective reflecting character, having minute glass spheres (or "beads") closely distributed and embedded in a flexible weather-resistant or painted surface. Each bead acts as an independent reflector button, but in mass effect the beads give the appearance of a uniformly brilliant area when viewed in the headlamp beam. A suitable incorporation of pigment in a reflecting coating causes it to reflect colored light, as for the yellow backgrounds of warning signs.
Sheets of polished metal so embossed as to produce a uniform pattern of small bright spots over the entire surface have also been used as reflecting coatings. These are diffuse rather than retrodirective reflectors, and consequently they reflect to the driver's eyes a part of the light from all sources within a fairly wide angular range. Their effective brilliance is not confined to a narrow cone around the axis of any incident light beam.
Reflecting coatings are particularly advantageous in signs having small lettering into which reflecting buttons cannot be fitted in a legible pattern. With a reflecting coating there is usually a minimum of difference between the daytime and nighttime aspects of a sign.
Individual reflectors or clusters of reflecting units, white or colored, used to mark obstructions or to delineate a highway, are treated as markings, in part II of this manual.
All traffic signs shall be kept in proper position, clean, and legible at all times. Damaged signs shall be replaced immediately.
Signs poorly maintained lose their authority as traffic control devices. Damaged, defaced, or dirty signs are ineffective and discredit the agency responsible for them.
To insure adequate maintenance a suitable schedule for inspection, cleaning, and replacement of signs should be established. All signs should be inspected at least twice a year and any that are defective should be cleaned, touched up, or replaced and taken in for repair and refinishing or scrapping. Employees of the highway department, traffic police, and other governmental employees whose duties require that they travel on the highways should be instructed to report any damaged or obscured sign at once.
Special care should be taken to see that weeds, shrubbery, construction materials, and snow are not allowed to obscure the face of any sign.
In the case of illuminated signs a regular schedule for replacement of lighting elements should be maintained, so that such elements will be renewed before they would normally be expected to burn out.
All signs that are to convey their messages during hours of darkness, other than urban parking signs, shall be reflectorized or illuminated. Much traffic moves after dark, and reflectorization or illumination may be necessary even on signs concerned primarily with daytime activities.
Street parking signs are ordinarily read at slow speed, are relatively closely spaced, and often receive some illumination from street lighting, hence there reflectorization is optional.
Overhead signs, extending upward from a minimum of 15 feet above the roadway, may not receive enough illumination from motor vehicle headlamps for adequate brightness. Overhead signs, accordingly, shall be illuminated where an engineering study shows that reflectorization will not give effective performance.
Illumination may be by means of:
Ordinary street or highway lighting is not regarded as meeting the requirements for sign illumination.
All illumination shall be such as to show the same sign colors be night and by day. A flashing light incorporated in a sign installation shall be red when displayed with a Stop sign or yellow when displayed with a warning sign or other regulatory sign.
In a reflectorized sign either the legend and border, or the background, or both, may be reflectorized, depending on the sign design and on local conditions. On any particular highway a consistent policy should be followed for signs of the same type, recognizing, however, differences in rural and urban requirements. As a general rule, both the legend and background of urban signs, if other than black, should be reflectorized.
Reflectorization may be by means of:
Sign reflecting materials shall reflect white light, or if a reflecting coating is used as the background of a colored sign, the color of the background.
Reflecting material. An effective reflecting material reflects a large proportion of an incident beam of light directly back toward its source, in a narrow cone having only enough "divergence" to reach the driver's eyes in his normal position above the headlamps. It is this "retrodirective" reflection that distinguishes reflector buttons and reflecting coatings from any sort of specular or diffuse reflectors and gives them their great brilliance.
Reflector buttons. Reflector buttons in a highway sign are individual reflecting units arranged in rows or patterns to form letters, symbols, or borders. In suitable sizes and spacings they give the visual effect of continuous lines or areas or light. They are made of glass or transparent plastic with lenses or prisms designed for retrodirective reflection.
Reflecting coating. A reflecting coating is a coating or sheeting applied either to the background or to the legend of a sign, or to both, to give a bright reflection over the entire area so covered. The coating or sheeting is of a retrodirective reflecting character, usually having minute glass spheres ("beads") closely distributed and embedded in a painted surface or in a flexible plastic sheeting, or minute lenses molded in the surface of a plastic sheeting. Each bead or lens acts as an individual reflecting unit, but the effect is that of a uniformly brilliant area when viewed in the headlamp beam. A suitable incorporated of pigment in a reflecting coating causes it to reflect colored light.
All traffic signs shall be kept in proper position, clean, and legible at all times. Damaged signs shall be replaced immediately.
Signs poorly maintained lose their authority as traffic control devices. Damaged, defaced, or dirty signs are ineffective and discredit the agency responsible for them.
To insure adequate maintenance a suitable schedule for inspection, cleaning, and replacement of signs should be established. All signs should be inspected at least twice a year and any that are defective should be cleaned, touched up, or replaced and taken in for repair and refinishing or scrapping. Employees of the highway department, traffic police, and other governmental employees whose duties require that they travel on the highways should be instructed to report any damaged or obscured sign at once.
Special care should be taken to see that weeds, shrubbery, construction materials, and snow are not allowed to obscure the face of any sign.
In the case of illuminated signs a regular schedule for replacement of lighting elements should be maintained, so that such elements will be renewed before they would normally be expected to burn out.
As a useful guide to sign maintenance the American Association of State Highway Officials' Policy on Maintenance of Safety and Traffic Control Devices is recommended.[1]
Regulatory and warning signs, unless excepted in the standards covering a particular sign or group of signs, shall be reflectorized or illuminated to show the same shape and color both by day and night. All overhead sign installations should be illuminated where an engineering study shows that reflectorization will not perform effectively. Reflectorization, non-reflectorization, or illumination of guide signs shall be as provided in subsequent sections.
Illumination may be by means of:
The requirements for sign illumination are not considered to be satisfied by street or highway lighting, or by strobe lighting.
Reflectorization may be by means of:
Regulatory and warning signs, unless excepted in the standards covering a particular sign or group of signs, shall be reflectorized or illuminated to show the same shape and color both by day and night. All overhead sign installations should be illuminated where an engineering study shows that reflectorization will not perform effectively. Reflectorization, non-reflectorization, or illumination of guide signs shall be as provided in subsequent sections.
Illumination may be by means of:
The requirements for sign illumination are not considered to be satisfied by street or highway lighting, or by strobe lighting.
Reflectorization may be by means of:
Regulatory and warning signs, unless excepted in the standards covering a particular sign or group of signs, shall be reflectorized or illuminated to show the same shape and color both by day and night. All overhead sign installations should be illuminated where an engineering study shows that reflectorization will not perform effectively. Reflectorization, non-reflectorization, or illumination of guide signs shall be as provided in subsequent sections.
Illumination may be by means of:
The requirements for sign illumination are not considered to be satisfied by street or highway lighting, or by strobe lighting.
Reflectorization may be by means of:
Support:
There are many materials currently available for retroreflection and various methods currently available for the illumination of signs. New materials and methods continue to emerge. New materials and methods can be used as long as the signs meet the standard requirements for color, both by day and by night.
Standard:
Regulatory, warning, and guide signs shall be retroreflective or illuminated to show the same shape and similar color by both day and night, unless specifically stated otherwise in the text discussion in this Manual of a particular sign or group of signs.
The requirements for sign illumination shall not be considered to be satisfied by street, highway, or strobe lighting.
Guidance:
All overhead sign installations should be illuminated unless an engineering study shows that retroreflection will perform effectively without illumination.
Option:
Sign elements may be illuminated by the means shown in Table 2A-1.
Retroreflection of sign elements may be accomplished by the means shown in Table 2A-2.
Support:
(This section is reserved for future text based on FHWA rulemaking.)
