Manual of Traffic Signs - Sign Sheetings (original) (raw)
Introduction
Important Note: The state of the retroreflective sheeting market continuously changes, due to development of new sheetings, requested changes to ASTM D4956, and changes in product offerings by manufacturers.
For nearly all traffic signs on public roadways in the United States, the faces of these signs are covered with a thin plastic material known as retroreflective sheeting. This material is designed to show a bright and uniform color and to reflect light back to a viewer's eyes so as to be visible in both daylight and darkness. If properly applied and sealed to a good surface, nearly all retroreflective sheetings are water, ice, and salt resistant and durable under normal outdoor conditions. To provide color, the sheeting either has colored pigments built into or applied to the material or has a translucent colored overlay film.
In the early days of highway travel, traffic signs were painted using common paints, and were either nonreflective or had retroreflectivity added by mixing glass beads in the paint or by using glass spheres or reflectors in the sign legend. Painted signs have not been in common use for over half a century.
There are two national specifications that define types of retroreflective sheeings - AASHTO M268 or ASTM D4956. The types used in the ASTM D4956 specifications do not necessarily imply relative performance; i.e. a "higher"-type sheeting isn't necessarily better than a "lower"-type sheeting - it just meets different performance characteristics. Also note that some sheetings on the market can meet the criteria of several ASTM types. The chart below describes each type of sheeting, sorted by its categorization under ASTM D4956.
Retroreflective Sheeting Types
| AASHTO M268 Type | ASTM D4956 Type | Status | Description | Typical Uses |
|---|---|---|---|---|
| (none) | I | Limited Use | Trade name: Engineer Grade. Basic reflective sheeting, made up of either very small glass beads enclosed in a translucent pigmented substrate or a dual-layer prismatic sheeting with moderate retroreflectivity. Has no distinctive identifying pattern, other than, of course, it reflects. This material is one of the most durable (in its ability to withstand rough handling) of all retroreflective sheeting products. Generally regarded to have a seven year service life. | Vehicle graphics Advertising signs Conspicuity markings No longer in general use for traffic signs |
| II | Limited Use | Trade name: Super Engineer Grade. Similar to Type I, except it uses larger glass beads in a denser matrix, providing about twice the level of reflectivity of Type I sheeting. This sheeting can be identified by small trademarks which are screened into the sheeting (varying by manufacturer). Also very durable. Generally regarded to have a ten year or greater service life. | Vehicle graphics Advertising signs Conspicuity markings No longer in general use for traffic signs | |
| A | III | Limited Use | Trade name: High Intensity Grade. This is an "encapsulated lens" sheeting, made of 2 layers - an outer translucent colored or clear layer, and an inner reflective layer faced with glass beads. The two layers are connected by a lattice, hence its distinctive 'honeycomb' appearance, where the lattice pattern varies by manufacturer for easy identification. Generally regarded to have a ten year service life. | Superseded by newer alternatives such as Type IV |
| B | IV | Current | Trade names: High Intensity Prismatic or Microprismatic Grade. This is also a multi-layer sheeting, except that the reflective layer is made of microscopic cube-corner reflectors instead of glass beads - known as a "microprismatic" layer. This sheeting can be distinguished by a pattern of small squares or bars superimposed upon the lattice grid. Generally regarded to have a ten year service life. | All types of post-mounted signs Barricades and other rigid temporary traffic control devices |
| (none) | V | Limited Use | A sheeting made of a metallized microprismatic material. Intended for applications requiring flexibility and durability. Generally regarded to have a five year service life. | Cones, delineators, and other flexible channelizing devices Roll-up signs Vehicle graphics Conspicuity markings |
| VI | Limited Use | A vinyl backed beaded or microprismatic material. This sheeting differs from all other types by being composed of a flexible cloth. Generally regarded to have a two year service life, depending on handling and use. | Roll-up signs Retroreflective clothing Retroreflective flags, patches, and similar items | |
| (none) | VII | Discontinued | Trade name: Diamond Grade LDP. A microprismatic sheeting distinguished by a diamond-shaped lattice separating the sheeting layers, a "coarse" grain to the microprisms, and high retroreflectivity levels at shallow viewing angles. Generally regarded to have a ten year service life. | No longer manufactured (superseded by Type XI) |
| B | VIII | Current | Trade names: Crystal Grade, Maximum Visual Performance Grade. A microprismatic sheeting similar in design to Type VII and IX, but with distinguishing characteristics similar to Type IV. Generally regarded to have a ten year service life. | Post-mounted signs Overhead signs Internally-illuminated signs Barricades and other rigid or semi-rigid temporary traffic control devices |
| IX | Current | Trade names: Diamond Grade VIP, Omniview, Oralite. A microprismatic sheeting very similar to Type VII, distinguished by a "fine" grain of the microprisms. This sheeting provides relatively high retroreflectivity levels at a wide range of viewing angles. Generally regarded to have a ten year service life. | Post-mounted signs Overhead signs Internally-illuminated signs Signs with fluorescent backgrounds | |
| (none) | X | Discontinued | A microprismatic sheeting very similar to Types VII-IX, and has similar performance characteristics. Generally regarded to have a ten year service life. | Many Type X sheetings reassigned into Type VIII |
| D | XI | Current | Trade names: Omnicube, Diamond Grade DG3. A microprismatic sheeting very similar to Types VII-IX, but providing high retroreflectivity at both shallow and wide angles. Generally regarded to have a ten to twelve year service life. | Post-mounted signs Overhead signs Internally-illuminated signs Signs with fluorescent backgrounds Barricades and other rigid temporary traffic control devices |
Nearly all sheetings are available with pressure-sensitive backings for attachment to sign surfaces. In the past, sheetings were available with heat-activated backings, but with changes in sign manufacturing technology this type of adhesive is no longer widely used.
Sign Legends
Legends (letters, numeral, symbols, and borders) for traffic signs are produced by the following methods:
- **Direct-Applied:**Cut-out letters, symbols, and borders directly applied to the sign face. For legends on darker-background signs, retroreflective sheeting is used. For legends on light-background signs, non-reflective vinyl or acrylic films are used. Acrylic film generally has a longer service life than vinyl, but typically has a higher initial cost. This can be an economical process for fabricating very large signs, signs with specific legends such as destinations or distances, or small to medium quantities of identical signs.
- Demountable Copy: This legend is made of sheeting applied to thin metal, which is cut out into the letter and legend shapes and then riveted to the sign face. This permits the sign legend to be changed or removed without having to replace or overlay the sign panel. This can be an economical process for fabricating very large signs, or signs with a possibility of legend changes during their service lives.
- Positive Screen: Used for signs with legends darker than the background, such as most regulatory and warning signs. Special ink formulated for traffic sign use is applied directly onto the lighter-colored sign face using a screen printing process where the screen stencil prevents ink from being applied except in the area of the sign legend. Black ink is opaque, whereas colored inks are translucent. This can be an economical process for fabricating large numbers of identical signs.
- Negative Screen: Used for signs with legends lighter than the background, such as stop signs, colored route markers, or some guide signs. The process generally begins with a white retroreflective sign face, then a special translucent colored ink formulated for traffic sign use is applied onto the sign face using a screen printing process where the screen stencil prevents ink from being applied in legend areas or regions of other colors. This produces a white legend on a colored background. Note that some colored inks may be susceptible to fading unless a clear protective film is applied. This can be an economical process for fabricating large numbers of identical signs.
- Overlay Film: Generally used for signs with legends lighter than the background. The process begins with a white retroreflective sign face, then the overlay film in the appropriate color is cut and the sections where the white is to show through is removed. The overlay film is then applied onto the sign face. This can be an economical process for fabricating signs with specific legends such as destinations or distances, or small to medium quantities of identical signs.
- Electronic Printing: Colored legends are applied to white retroreflective sign sheeting in a proprietary electronic printer that "digitally prints" the sign legend in a manner similar to a computer printer, but using special sign inks or pigments. The printed sign face is then applied to the sign substrate material. A clear protective film is often applied on top of the sign face to reduce damage or fading. This can be a time-efficient process for fabricating signs with specific legends such as destinations or distances, signs with complex pictographs, or signs with more than two colors. As initial and operating costs for sign printing technology decreases, this may replace other methods.
Colored inks or pigments used in screen or electronic printing are translucent to allow reflectivity through the ink, whereas black ink used for signs is opaque.
Some sign sheetings can be sensitive to visual orientation; i.e. the retroreflective performance can vary with the angle at which the sheeting is placed on the sign panel. If a sheeting is susceptible to this effect, it should be used in the orientation recommended by the manufacturer, especially if used for cut-out or demountable legends.