In 2001 the Public Rights-of-Way Access Advisory Committee made the following statement:
“Research is needed to determine specifications for changeable message signs that will result in equivalent legibility for readers having visual acuities from 20/20 to 20/200, in outdoor situations. Technical specifications are needed for changeable message signs of different types and colors, in situations in which messages are either static or dynamic, and in situations in which either the viewer or the sign are in motion.”
Whether or not the goal of “equivalent” legibility for people with vision impairments can ever be attained is subject to debate, however the present research has identified the existence of certain design criteria that, if met, will be capable of significantly improving the legibility of VMS for a large percentage of individuals with vision impairments. The question is, are the current findings sufficiently robust to be used to make solid recommendations.
In a very recent synthesis on LED/LCD VMS for use on transit vehicles conducted for the FTA, Wourms,et. al. (2001) concluded that there was sufficient available guidance on several indices, including: the position of on-vehicle VMS; character height; width-to-height ratio; stroke width-to-height ratio; and inter-character spacing. Conversely, these researchers stated that there were insufficient data regarding: visibility under varying lighting conditions; streaming and paging style and rate; sign color; relative motion between the sign and the observer; and glare effects.
The present research supports Wourms and his colleagues’ conclusions regarding the need for further research to fill the gaps they identified, particularly with regard to paging and streaming rates and time allotted for individuals with vision impairments to read VMS. The present study however demonstrates that other issues (e.g., letter height, color, and luminance), alone and in combination, may still benefit from additional study.
In particular, appropriate letter heights should be established using new, brighter light emitting VMS, in combination with streaming text for subjects who have a variety of functional visual impairments (e.g., scotoma, CFL, and PFL). The use of an analytical model to determine letter heights (as suggested by Wourms, et al. 2001 and others) simply does not address the complexity of VMS reading by individuals with vision impairments in real-world environments. A summary of recommendations regarding the application of current knowledge to future VMS design and recommendations for future research to fill the gaps in that current knowledge can be found in Table 2.
Table 2.Recommended values for VMS characteristics, issues, and future research needs.
| VMS Characteristic | Recommended Values | Issues and Future Research |
| Letter Height for VMS on Vehicles | Not less than 10 inch(will allow for 30 ft legibility distance at an LI of 3 ft/in)* | – Using 8-inch letters, Bentzen, et. al. (1994) found mean VMS legibility distance to be less than 20 feet for subjectswith 20/80 to 20/160 acuity.– Research should be conducted to determine what letter height will optimize reading speed and legibility distance for individuals with vision impairments reading dynamic messages. |
| Letter Height for VMS in Facilities | Not less than 6 inch(will allow for 18 ft legibility distance at an LI of 3 ft/in) | |
| Width to Height Ratio | 0.7 to 1.0 | – Existing research on this variable is fairly strong and, at a minimum, supports the use of 5x7 versus a 4x7 character matrix. |
| Stroke Width to Height Ratio | 0.2 | – Existing research on this variable is fairly strong and supports the useof a 1:5 ratio. |
| Text Color | Green or Yellow | – Existing research indicates that these two colors provide the best legibility for readers with vision impairments.– Additional research should be conducted to determine if other colors now available in high brightness LEDs provide any benefit to the visually impaired. |
| Font | 5x7 for Uppercase7x9 for Lowercase | – Existing research on this variable is fairly strong. |
| Luminance | Night: 30cd/m2Day: >1,000cd/m2 | – Existing research on this variable is fairly strong for individuals without vision impairments.– Additional research should be conducted to determine if these levels are sufficient for individuals with vision impairments.– New European standards currently under development should be tracked to determine measurement methods and recommended levels. |
| Luminance Contrast | (Lt-Lb)/Lb = 8 to 12 | – Existing research on this variable is strong for individuals without vision impairments.– Research should be conducted to determine if these levels are sufficient for individuals with vision impairments. |
| Inter character Spacing | 25 to 40%letter height | – Existing research on this variable is strong. |
| Inter word spacing | 75-100%letter height | – Existing research on this variable is strong. |
| Inter line spacing | 50 to 75%letter height | – Existing research on this variable is strong. |
| Case | Uppercase or mixed case for single words | – Existing research on this variable is strong; however attaining high-quality lowercase letterforms using a matrix format is difficult. If this cannot be attained, then all uppercase letters is preferable. |
| Lowercase for longer textual messages | ||
| Contrast Orientation | Positive | – Existing research on this variable is strong for individuals without vision impairments.– Research should be conducted to determine if the findings are the same for individuals with vision impairments. |
| Sign Width | Dynamic text should be capable of displaying 6-7 characters | – Additional research on this variable should be conducted to determine if this basic research finding holds up in real-world VMS reading by individuals with vision impairments. |
| Paging or Streaming | Streaming | – Additional research should be conducted. |
| Static Display Time | 10 Seconds | – This is a very weak recommendation and very much in dispute.– Additional research must be conducted to determine the appropriate reading time needed for sign comprehension by individuals with vision impairments. |
| Streaming Rate | 2.74 seconds per frame dwell time | – This is a very weak recommendation as it is based on a single research study (Bentzen and Easton, 1996).– Additional research must be conducted to determine the appropriate streaming rate individuals with vision impairments. |
*LI – Legibility Index (legibility distance in feet per inch of letter height).
