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HFG SIGNING Version 1.0 SIGN DESIGN TO IMPROVE LEGIBILITY Introduction Sign design refers to the design parameters of signs that impact the legibility of text placed on the sign. Sign legibility is greatly affected by specific design characteristics of signs that contribute to drivers' ability to perceive and understand a sign's message in order to promote safe driving behaviors. Key design parameters determining the legibility of signs include retroreflectivity (sheeting type) and legend color, font size, and font style. Design Guidelines The following guidelines can be used to improve sign legibility. Sign Design Characteristics Guidelines Retroreflective Microprismatic retroreflective sheeting provides longer legibility (Sheeting Type) distances than encapsulated retroreflective sheeting by 9.5% (1). Legend Color Light letters on a dark background are superior to dark letters on a light background (2). Black-on-orange and white-on-green signs are detected at greater distances than black-on-white signs (3). Font Size A maximum legibility index of 40 ft/in. of letter height should be used (4). Research indicates that legibility distance increases as letter height increases, although the benefits are not proportional above letter heights of about 8 in. (3). Font Style Legibility of overhead guide signs and shoulder-mounted guide signs is increased with microprismatic sheeting with ClearviewTM alphabet over Series E (modified) (5). Increased legibility distance is found with mixed-case text under daytime and nighttime conditions (3). Symbol Contrast Optimal legend to background contrast value for sign legibility is 12:1 (3). Positive-contrast signs provide greater legibility distances than negative-contrast signs (3). General Improvements for Minimize symbol complexity by using very few details. Older Drivers (all from Maximize the distance between symbol sign elements. FHWA (6 )) Use representational rather than abstract symbols (see also Campbell, Richman, Carney, & Lee (7 )). Use solid rather than outline figures for design. Standardize the design of arrowheads, human figures, and vehicles. Retain maximum contrast between the symbol and the sign background. Use a larger font when possible. Based Primarily on Based Equally on Expert Judgment Based Primarily on Expert Judgment and Empirical Data Empirical Data 18-4
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HFG SIGNING Version 1.0 Discussion The table on the previous page summarizes key design guidelines that can help improve sign legibility and safety. A great number of studies have examined specific properties of roadway signs that affect legibility, and many of the results from these studies are reflected in the MUTCD. Garvey, Thompson-Kuhn, and Pietrucha (3) contributed a number of the guidelines on the previous page; this data source was a comprehensive review and synthesis of existing research associated with the use and design of roadway signs. Design Issues Drivers cannot see as well under nighttime conditions as they can under normal daytime conditions. Additional factors that compromise vision at night, consequently affecting legibility distances, are summarized in the following table. Factors that Compromise Vision at Night Glare Glare from headlights, overhead signs, and construction lights can cause problems for approaching drivers. Drivers traveling in the same direction may experience glare issues when lights shine in their rearview mirrors. Fatigue/Lack of Alertness Fatigue and lack of alertness problems increase at night. The degree of these problems may be more apparent as drive time increases. Poor Lighting When driving during the daytime there is usually enough light to see well. This is not true at night. Even with the presence of lights, the road scene may still be confusing as signs may be hard to see amongst other signs, shop windows, and other lights. Headlights Headlights provide the main source of light for drivers to see and be seen under nighttime conditions. Drivers cannot see as far or see as much detail with headlights as compared to daytime driving conditions. Also, drivers tend to overdrive their headlights under certain conditions at night. Typically, the maximum distance for which modern headlamps provide reasonable illumination is between 150 and 250 ft, depending on headlamp characteristics and the reflectivity of the object being seen (8). In urban/suburban areas, drivers normally dim their headlights, which reduces visibility distance. Prismatic grade sign sheeting helps improve driver visibility in these areas. Windshield and Mirrors Bright lights at night can cause dirt on windshields or mirrors to create glare. Cross References Driver Comprehension of Signs, 18-8 Key References 1. Carlson, P.J., and Hawkins, G. (2003). Legibility of overhead guide signs with encapsulated versus microprismatic retroreflective sheeting. Transportation Research Record, 1844, 59-66. 2. Sivak, M., and Olson, P.L. (1983). Optimal and Replacement Luminances of Traffic Signs: A Review of Applied Legibility Research. (UMTRI-83- 43). Ann Arbor: University of Michigan Transportation Research Institute. 3. Garvey, P.M., Thompson-Kuhn, B., and Pietrucha, M.T. (1996). Sign Visibility: Research and Traffic Safety Overview. Bristol, PA: United States Sign Council. 4. Chrysler, S.T., Carlson, P.J., and Hawkins, H.G. (2002). Nighttime Legibility of Ground-Mounted Traffic Signs as a Function of Font, Color, and Retroreflective Sheeting Type. (FHWA/TX-03/1796-2, TTI: 0-1796). College Station: Texas Transportation Institute. 5. Hawkins, H.G., Jr., Picha, D.L., Wooldridge, M.D., Greene, F.K., and Brinkmeyer, G. (1999). Performance comparison of three freeway guide sign alphabets. Transportation Research Record, 1692, 9-16. 6. FHWA (1995). Improvements in Symbol Sign Design to Aid Older Drivers. Summary Report. (FHWA-RD-95-129). McLean, VA. 7. Campbell, J.L., Richman, J.B., Carney, C., and Lee, J.D. (2004). In-Vehicle Display Icons and Other Information Elements. Volume I: Guidelines (FHWA-RD-03-065). McLean, VA: FHWA. (http://www.tfhrc.gov/safety/pubs/03065/index.htm). 8. Schiller, C., Holger, S., Groh, A., Böll, M., and Khanh, T.Q. (2009). HID vs. Tungsten Halogen Headlamps: Driver Preferences and Visibility Distance. (2009-01-0550). Warrendale, PA: Society of Automotive Engineering. 18-5