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HFG CHANGEABLE MESSAGE SIGNS Version 1.0 PRESENTATION TO MAXIMIZE VISIBILITY AND LEGIBILITY Introduction Presentation to maximize visibility and legibility refers to how the photometric and physical characteristics of a CMS can be employed to positively affect readability. Because CMS characters or symbols are typically constructed using a relatively coarse matrix of pixels, the requirements for their visibility and legibility are more demanding than for standard, fixed signs. Also, the fixed matrix introduces limitations to character size, height-to-width ratio, spacing, and other geometric characteristics available for presenting messages. The MUTCD provides specific guidance about letter height, minimum legibility distance, and other characteristics. Additional recommendations for designing messages within the limitations imposed by CMS technologies, including guidelines for contrast ratio, luminance, character spacing, and resolution are provided below. Design Guidelines CMS Guideline Value Characteristic Contrast Ratio Optimal contrast ratio range = 8-12 (light-emitting where: CMS) Luminancemax Contrast ratio = Luminancemin Luminancemax = luminance emitted by the area or element of greatest intensity (text) Luminancemin = luminance emitted by the area or element of least intensity (background) Luminance Sun Overhead Overcast/Rain Nighttime (light-emitting CMS in cd/m2) Young (16-40) 850 350 30 Old (65+) 1000 600 30 Character Spacing (matrix CMS) Word spacing: 75-100% of the letter height Line spacing: 50-75% of the letter height Character spacing: 25-40% of the letter height Character Size should be consistent within a display Resolution 5 7 matrix: static or non-critical text 7 9 matrix: dynamic or critical text Based Primarily on Based Equally on Expert Judgment Based Primarily on Expert Judgment and Empirical Data Empirical Data 19-4

OCR for page 39
HFG CHANGEABLE MESSAGE SIGNS Version 1.0 Discussion Contrast ratio: The photometric and physical properties of signs directly affect the legibility of the sign elements. For example, contrast ratios are affected by photometric properties such as luminance, but can be reduced by physical properties such as dirty or scratched protective plexiglass sheeting (1). The guidance on acceptable ranges depends on the conditions present in the ambient environment and whether the CMS is light reflecting or light emitting. Light-emitting CMSs have minimum contrast ratios on sunny days when the sun increases the background sign luminance, whereas light-reflecting CMSs have minimum contrast ratios when the light falling on the sign is at a minimum (2). Weather conditions such as rain and fog can affect contrast ratios for both types of signs by reducing the illumination coming from the sign or light reflected by the sign. The optimal contrast ratio range is between 8 and 12, although Dudek (1) presents other acceptable ranges based upon European research. Luminance: Driver age and sun position affect the required CMS luminance significantly (3). Generally, greater luminances are required for older drivers than for younger drivers at a given distance. Garvey and Mace (3) found that during extreme backlit (sun behind the sign) and washout (sun directly on the sign) conditions, 1000 cd/m2 is a minimum value. However, at 650 feet, some drivers cannot be accommodated under these visibility conditions, at any luminance level. If luminance values are too high at night, the characters may appear to irradiate or bleed onto the background and blur due to the extreme contrast (4). Character spacing: Character spacing is limited by physical properties of the sign such as the matrix pattern of the LEDs. The spacing used should allow drivers to recognize (1) words as items rather than series of individual letters and (2) lines as separate entities. The included guidance is based upon the MUTCD, though Dudek (4) presents different values based upon the United Kingdom's draft CMS standards. Character resolution: Character resolution can affect the readability of text. Campbell, Carney, and Kantowitz (5) reported that for characters smaller than approximately 22 arcminutes, a 7 9 matrix led to faster reading times and fewer reading errors than a 5 7 matrix. A 7 9 matrix should be used to display dynamic or critical text, while a 5 7 matrix can display static or non- critical text. There are obvious trade-offs between the resolution used and the amount of text that can be fit on the sign. Design Issues Appropriate resolution is also affected by the case of the characters presented. All uppercase letters are often displayed on CMSs and are more difficult for people to read than mixed or lowercase letters (6). People are more accustomed to reading mixed or lowercase letters and can identify word shapes using the ascenders and descenders. However, lowercase letters require a higher resolution matrix (5 9) to accommodate these descenders (7). The readability of lowercase letters also depends on the display of curved lines, which is a challenge on matrix displays. Thus, there are trade-offs between readability and practicality for displaying letters in mixed cases. There are many types of CMSs available that utilize different technologies. Upchurch, Armstrong, Baaj, and Thomas (8) evaluated shuttered fiber-optic, LED, and flip disk signs to analyze the legibility distance of each. For backlit (sun directly behind sign) and nighttime conditions, LED and fiber-optic signs had better legibility distances than flip disk signs. For washout (direct sunlight on sign) and midday conditions, fiber-optic signs performed best for legibility distance. LED signs may interact negatively with sunglass filters. Sunglass lenses that have a notch filter, which attenuates light emissions in the same range that amber LEDs emit light (9), reduce the brightness of the LED, thereby decreasing the contrast and making CMS messages difficult to read. Cross References Key Components of Sight Distance, 5-2 Sign Design to Improve Legibility, 18-4 Composing a Message to Maximize Comprehension, 19-8 Key References 1. Dudek, C.L. (1997). NCHRP Synthesis of Highway Practice 237: Changeable Message Signs . Washington, DC: Transportation Research Board. 2. Dudek, C.L. (2004). Changeable Message Sign Operation and Messaging Handbook. (FHWA-OP-03-070). College Station: Texas Transportation Institute. 3. Garvey, P.M. and Mace, D.J. (1996). Changeable Message Sign Visibility. (FHWA-RD-94-077). Washington, DC: FHWA. 4. Dudek, C.L. (1992). Guidelines on the Use and Operation of Changeable Message Signs . (FHWA-TX-92-1232-9). College Station: Texas Transportation Institute. 5. Campbell, J.L., Carney, C., and Kantowitz, B.H. (1998). Human Factors Design Guidelines for Advanced Traveler Information Systems (ATIS) And Commercial Vehicle Operations (CVO), (FHWA-RD-98-057). Washington, DC: FHWA. 6. Proffitt, D.R., and Wade, M.M. (1998). Creating Effective Variable Message Signs: Human Factors Issues. (VTRC 98-CR31). Charlottesville: Virginia Transportation Research Council. 7. Hitchins, D. (2001). Lowercase font set development for variable message signs (VMS). Proceedings of the 8th World Congress on Intelligent Transport Systems [CD-ROM]. 8. Upchurch, J., Armstrong, J.D., Baaj, M.H., and Thomas, G.B. (1992). Evaluation of variable message signs: target value, legibility, and viewing comfort. Transportation Research Record, 1376, 35-44. 9. Halloin, D.M. (1996). Impediments to the effective use of portable variable message signs at freeway work zones. In C. Dudek (Ed.). Compendium of Graduate Student Papers on Advanced Surface Transportation Systems (pp. Ci-C34). College Station: Texas A&M University. 19-5