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Synthesis on the Legibility of Variable Message Signing (VMS) for Readers with Vision Loss

Print Email The ability to electronically change a sign’s message has existed for many years, however it was not until the 1970’s that the matrix design so common today enabled signs to display “free text programmability” (Stainforth and Kniveton, 1996), or the ability to display any textual message desired.(A review of the history of VMS usage on North American and European highways can be found in Dudek, 1991).

Willis (1995) reported that the VMS technologies used in transit are reflective disk (RD), liquid crystal (LCD), light emitting diode (LED), and television monitors. While there are many different types of VMS, they can be placed into three major functional categories based on how the sign messages are illuminated: light reflecting; light emitting; and hybrid (Dudek, 1997). Each group has benefits and drawbacks with regard not only to visibility but also price, maintenance, message flexibility, color rendition, and many other variables. A thorough description and evaluation of VMS transit applications was published in 1996 as “TCRP Report 12: Guidelines for Transit Facility Signing and Graphics” (Earnhart). Much of the information in that document holds true today and will form the foundation of this section. Given the rapid growth of VMS design technologies, however, there have been some dramatic advances and performance improvement since that document was written, particularly with the light emitting and hybrid VMS. Therefore, the information from Earnhart (1996) will be supplemented with as much recent data as was available at the writing of this report.

Jenkins (1991), Upchurch, et al. (1992), and Garvey and Mace (1996) evaluated the relative legibility of various VMS technologies and found that, in general, VMS that used either light emitting or a combination of light emitting and light reflecting technologies (hybrids) outperformed light reflecting technologies, especially at night. However, while it is tempting to make these type of inter-technology comparisons, as Jenkins (1991) cautioned, “The results of these experiments pertain to the particular samples tested, the formats of the symbols and characters used, the mounting assemblies they were in, and the lighting systems that were used for illumination…. The conclusions found should not be generalized to other configurations.” With the technological advancements that have occurred in the past decade, this caveat is even more applicable today.

Light Reflecting Static-like This category includes any VMS that is only capable of displaying a set of prefabricated “hard copy” sign messages. These technologies (discussed in detail in Dudek, 1991) represent the oldest VMS technology, but can still be found in use today. They include fold out,rotating scroll, and rotating drum signs. Of these, rotating drum is the only technology that has any current adherents. The rotating drum (or prism) sign has the appearance of a standard or staticsign, however lines of text are mounted on a rotating drum with different messages on each side. When the drum rotates, a new message appears on the sign.

Advantages: Can useany desired font Low cost Good resolution Unlimited color capability Can display upper and lowercase letters Disadvantages: Limited message selection Mechanical parts subject to maintenance “Old fashioned” or dated appearance Utility in the Transit Environment This type of VMS has been used extensively in the transit environment in the past, but has fallen out of favor because of the disadvantages listed above, and the availability and increased capabilities of LED and video display unit/cathode ray tube (VDU/CRT) technologies.

Reflective-Disk (RD) Matrix Also called flip-dot or flip-disk, these signs consist of a matrix of circular,square, or rectangular elements that are reflective on one side and matte black on the other. Electromagnetic current rotates the elements to display either the black or reflective side, with the reflective elements creating the message. The reflective coating can be paint or vinyl, fluorescent or retro reflective. The most common color is yellow, but white and orange can also be found.

During the daytime, sun or ambient lighting illuminates these signs, while under low ambient light conditions they are illuminated by an internal light source. The RD VMS that use retroreflective disks (exclusively for highway use) receive nighttime illumination from vehicle headlamps as do static retroreflective signs. Some also have an internal bank of fluorescent lamps located behind the elements to supplement vehicle headlighting.

The design of RD VMS can be described as module character, continuous line, or full matrix. In a modular character matrix sign, each character is displayed as a discrete unit typically consisting of a matrix of five horizontal and seven vertical elements (i.e., 5x7 charactermatrix). In a continuous line matrix sign, there are no vertical breaks between elements, but there are horizontal breaks between lines of text. An example of this is the standard single-line VMS often found in the transit environment; in highway usage they typically have three lines. In a full matrix sign there are no breaks at all and the sign is one continuous matrix. The latter is the most flexible as it allows for various letter heights, widths, stroke-widths, inter-character and inter-word spacing, and the use of symbols. The former is the most restrictive as it only allows for alphanumerics and significantly restricts letter characteristics (e.g., stroke width, letter width, and letter height).

Advantages Low cost Performs well in bright daylight (if protective plastic screen is kept clean and scratch-free and the elements are not faded) Low power consumption Disadvantages Mechanical failure of some or the entire message is common (disk failure) Nighttime illumination is often not uniform across the sign Legibility is not optimal Limited to the character matrix, they create artificial fonts with poor “resolution” Can effectively display only numerals and uppercase letters Reflective disks fade over time resulting in significant reduction in luminance contrast and, therefore, legibility Protective plastic often reflects light (especially when dirty or scratched)seriously degrading visibility Utility in the Transit Environment Earnhart (1996) wrote, “…flip-dot displays are most suitable for on vehicle displays wherespace limitations and cost limitations exist.” Reflective disk VMS have been, and continue to be, used extensively on and within buses. In 1992, Cobern, Martin, Thompson, and Norstrom stated “Flip dot signs have been used on the exterior of buses because of the problems that natural light poses for the other technologies. LED and LCD have proven to be less visible in natural light.”

Light Emitting Blank Out These are signs that display a discrete number of “hard wired” messages. They employ a wide variety of technologies including fiber optic, neon, and incandescent or fluorescent bulbs. A typical application of this technology is the “open” or “closed” signs for truck weigh-stations, HOV lanes, and toll plazas.

Advantages Low cost Good resolution Unlimited color capability Disadvantages Limited message selection Lamp replacement Utility in the Transit Environment This technology has only limited utility in the transit environment for the same reason it has limited utility in the highway environment: the lack of flexibility in message display.

Lamp (Bulb) Matrix These signs are created out of an array of light bulbs that are turned on or off to display the intended message. Like RD VMS, these signs can be modular character, continuous line, or full matrix.

Advantages Solid-state design (no moving parts to maintain) Very high brightness High visibility Disadvantages Lamp replacement Limited to the matrix characters creates artificial font with poor “resolution” High power consumption Can effectively display only numerals and uppercase letters Phantom effects in direct sunlight Utility in the Transit Environment Although popular with the advertising community, very few highway agencies have continued the use of this technology since their peak deployment in the 1970’s. In a survey of 27 states and one Canadian Provence,Dudek (1997) reported that only California had purchased a bulb matrix VMS since 1976. It is unlikely that, given the advances in other light emitting technologies (LED in particular), the advantages found with lamp matrix signs are sufficient to outweigh the disadvantages.

VDU or CRT Known commonly as television monitors, transportation application of this VMS technology is exclusive to transit, and because of size and glare considerations is restricted to indoor use.

Advantages Low cost Good resolution Graphics and animation capabilities Unlimited color capability Can display upper and lower case characters using any font Disadvantages Small screen size Typically use very small letter height (less than one inch) to maximize sign content Subject to glare Deep cabinet Utility in the Transit Environment “…VDU displays are more suitable for indoor displays where better control of lighting and more space exist, than at vehicle stops, or onboard vehicles.” (Earnhart, 1996) Because of its display flexibility, this technology has the capability of providing information legible to travelers with vision impairments. Unfortunately the alphanumerics used are typically far too small which, when combined with mounting height, makes their messages difficult even for fully sighted individuals to read.

Liquid Crystal Display (LCD) The liquid crystals work as a “solid state light shutter” creating an image on the screen by blocking some portions of light and allowing other portions to pass through (Stainforth and Kniveton, 1996). There are three basic types of LCD VMS: reflective; transmissive; and transflective. Reflective LCD VMS have reflective sheeting behind the crystals and use ambient illumination reflected through the crystals to illuminate the sign. Transmissive LCD VMS are illuminated with an internal light source located behind the crystals.Transflective LCD VMS combine the reflective and transmissible properties of the other two (these are addressed in the hybrid VMS section below).


Appropriate letter heights are attainable Solid-state design Flat cabinet design Graphics and animation capabilities Can display upper and lower case characters Disadvantages Expensive Light loss when viewed from an angle Utility in the Transit Environment There is some disagreement regarding LCD applicability in the transit environment.Earnhart (1996) stated that they provide, “Good performance in displaying schedule information in transit facilities,” and that, “Mosaic tile TNLCDs present a very readable character, even for those with visual impairments.” He went on to write; “TNLCD displays are most suitable for on vehicle or vehicle stop displays where space limitations, vibration, and the desire for advertising revenue exist.” However, Stainforth and Kniveton (1996) reported poor visibility in general, and legibility in particular, and poor color capability.

Fiber optic (FO) Matrix FO VMS(also called Light Conductor Technology - Bendekovics and Zelisko, 1996) work by running fiber optic cable from a light source within a sign cabinet to small holes in the sign face. The holes are then alternately blocked or exposed by electromechanical shutter devices. The holes form a matrix in the sign face which, when the light is let through,create the message. As with RD and bulb matrix, the sign can be modular character, continuous line, or full matrix; however in practice they are mainly modular character matrix.

Advantages High luminance Uniform day and night illumination Control over sign luminance (night dimming capabilities) Disadvantages Possible mechanical failure of some orall of the message (shutter failure) Limited to the character matrix creates artificial font with poor “resolution” Can effectively display only numerals and uppercase letters Lamp replacement Difficult to read in direct sunlight (glare) Some loss of sign brightness at large viewing angles (addressed with some recent models) Utility in the Transit Environment Bendekovics and Zelisko (1996) reported that Austria’s rail network (OeBB) has been using fiber optic VMS for over 20 years, although it is not clear if these were matrix or blank-out signs. Regardless of the specific technology application, however this usage seems to be the exception, for while FO technology has made some market headway in highway VMS, there has not been much interest shown by the transit community.

From a visibility perspective, they are as legible as LED VMS and suffer the same problems with glare in bright daylight. FO VMS, however, have the added potential maintenance problems associated with reflective disk signs (i.e., electromechanical shutters) and bulb matrix VMS (i.e., lamp replacement).

LED Matrix Modern LED VMS are constructed of a matrix of LEDs, with clusters of LEDs acting as a single element. The number of LEDs per cluster is dependent on sign size. Most transit VMS useeither a single LED per element on signs with three-to-four inch letter heights and four LEDs per element for the larger signs. The illumination of select LEDs creates the message. As with RD, bulb, and fiber optic signs, the LED VMS can be modular character, continuous line, or full matrix.

In the mid to late 1980’s, highway agencies began experimenting with the use of LED VMS with mixed results (Garvey, 1996). However, there have been a number of new advances in LED technology. As recently as the early to mid 1990’s, outdoor LED VMS were limited to the use of the color red, because no other color was bright enough to satisfy daytime transportation needs. Indeed,Wourms, et al. (2001) stated that red is still the only color LED that is capable of producing sufficient light for outdoor daytimeuse, however this is not the case. With the advent of new semiconductor materials such as InGaN in 1993 and even more recently AlInGaP, it is possible to make VMS using essentially any color desired while maintaining high luminance and wide viewing angles (Thomas, 1996; Yoshida, 2000).

The combination of high brightness, multiple colors, long life span (10 years estimated - Swinea, 1999), and lower power consumption that LEDs provide, is why they are used so frequently as replacement technology for red, yellow, and green incandescent traffic signal heads (i.e., traffic lights) and for construction work zone arrow panels, which require a visibility distance of one mile (Lewis, 2000). Research has not been conducted, however, to determine how best to implement this emerging technology to provide optimal VMS readability for individuals with vision impairments.

Advantages Appropriate letter heights are attainable Relatively low cost Solid-state design Flat cabinet design Graphics and animation capabilities Low power consumption Control over sign luminance Long life expectancy of the LED elements Disadvantages Subject to glare from direct light on the sign face Large viewing angles reduce illumination (although newer LEDs and the use of lenses to distribute the light have addressed this issue) Mainly display uppercase (but are capable of displaying upper and lower) Utility in the Transit Environment Iannuzziello’s (2001) reported of a 63 transit agency survey across 29 states and 4 Canadian provinces found LED destination signs to be among communication modes most frequently identified as “very effective.” Earnhart, 1996 wrote, “LED displays are most suitable for on vehicle or vehicle stop displays where space limitations, vibration, and the desire for advertising revenue exist.” Cobern, Martin, Thompson, and Norstrom (1992) wrote “LED and LCD…have more applicability inside the vehicle where they are not subject to direct sunlight.” However, Hunter-Zaworski (1994) reported that France, Sweden, and Canada were using monochromatic and colored LED readerboards for those applications as well as within terminals. The bus stop signs displayed “route, destination, schedule and delay information” while the in-vehicle signs informed passengers of the next stop and any required transfers. Hunter-Zaworski also reported that transit authorities in the Netherlands and England were using LED signs in bus stations to convey real-time bus arrival information. Willis (1995), in reporting the results of a transit agency survey, stated that LED displays were “the most commonly used technology.” This was consistent with her survey of vendors that found LEDs to be the most commonly used technology in bus interiors.

In 1996, Dobies reported that “LED electronic wayside signs mounted above a bus shelter are more visible than most static displays, and messages can be changed much more easily.” He also reported that Denver, CO replaced their RD signs with LED VMS due, at least in part, to maintenance considerations. Those LED signs provided route and gate numbers and scheduled departure time. Denver’s future plans were to link the LED signs with ITS technology to provide real-time departure and arrival times.

Hybrid The hybrid VMS technologies are basically an attempt to counteract the shortcomings associated with light emitting and light reflecting technologies used alone. Specifically, light emitting technologies suffer under bright ambient conditions where they lose contrast. This results in an effect known as veiling luminance (or glare) in which the message is “washed out.” A severe reduction in contrast creates signs that are unreadable at any letter height. While some light emitting technologies, such as bulb matrix and newer LED signs, can at least partially overcome this problem, the addition of reflective elements can greatly enhance bright daylight VMS visibility. Clean, new, light reflecting technologies work well under bright ambient conditions, but suffer at night. Nighttime lighting of reflective VMS, however, requires lighting the entire sign (both “on” and “off” elements), which reduces contrast. Furthermore, most of the nighttime lighting is either inadequate or has a non-uniform distribution resulting in “hot spots” and “dark spots.” Garvey and Mace (1996) in evaluating the nighttime legibility of VMS found significant reduction in nighttime legibility distance with reflective signs.

Liquid Crystal Transflective This is a type of LCD that combines the illumination techniques of the reflective and transmissive LCDs discussed earlier. Transflective LCD’s are illuminated from the front either by lighting internal to the sign cabinet or by ambient lighting and are also backlit by lights situated in the sign cabinet behind the liquid crystals. These VMS have some of the same disadvantages as the other LCD types, however because they reflect and emit light, they can be used under both indoor and outdoor conditions. As with most of the other technologies, there are no human factors studies evaluating the legibility of transflective LCD for subjects with vision impairments.

FO/RD Matrix This is a hybrid of reflective disk matrix signs with fiber optics added for nighttime lighting to avoid the problems associated with the external “flood lighting” used with standard RD VMS. By the early 1990’s, highway agencies had become disenchanted with the visibility they were able to achieve with traditional RD VMS. At the same time, fiber optics and LEDs were being introduced as stand alone technologies for VMS. A number of agencies had their old RD signs retrofitted with FO/RD components. The principal advantage is uniform lighting across all the characters at night and very good performance in bright daylight as long as the protective “glare screen” is kept clean and scratch free and the elements are not faded. The disadvantages are the same as those reported for the FO and the RD signs (e.g., bulb replacement and moving parts). Applications to the transit environment are the same as with RD VMS.

LED/RD Matrix This is a hybrid of reflective disk and LED technologies. The rational for this technology is the same as with the FO/RD VMS. The advantages are the same, except that the life of an LED is substantially longer than that of an incandescent lamp, so lamp replacement would not be as much of an issue. A disadvantage is that power consumption is higher in the LED/RD matrix VMS than with either the standard RD or LED VMS because the LEDs are constantly illuminated. Applications to the transit environment are the same as with RD VMS.