18.4 Sign Design to Improve Legibility

  • While road designers are not responsible for designing signs, they do review signing plans. As such, understanding methods for improving sign legibility is helpful for verifying that all signs installed along a route are readable and thus promote safe user behaviors. Table 18.2 lists methods to improve sign legibility.
Table 18.2 Guidelines for Improving Sign Legibility
Sign Design Characteristic Guideline
Sheeting Type
  • Signs made of microprismatic retroreflective sheeting are legible at longer distances than signs made of encapsulated retroreflective sheeting.
Legend Color
  • Light letters on a dark background are more legible than dark letters on a light background.
  • Users can detect black-on-orange and white-on-green signs at a greater distance than black-on-white signs.
Font Size
  • Legibility distance increases as letter height increases up to letter heights of 8 in.
  • Use a maximum legibility index of 40 ft./in of letter height.
Font Style
  • Microprismatic sheeting with Clearview alphabet over Series E (modified) improves the legibility of overhead guide signs and shoulder-mounted guide signs.
  • Using mixed-case text (i.e., a combination of upper-case and lower-case letters) increases legibility distance under daytime and nighttime conditions.
Symbol Contrast
  • Optimize sign legibility with a legend-to-background contrast value of 12:1.
  • Legibility distances for positive-contrast signs are greater than for negative-contrast signs.
Improvements for Older Drivers
  • Minimize symbol complexity by using few details.
  • Maximize the distance between symbol sign elements.
  • Use representational — not abstract — symbols.
  • Use solid figures instead of outline figures for design.
  • Standardize designs for arrowheads, human figures, and vehicles.
  • Maximize contrast between symbols and sign backgrounds.
  • Use a larger font when possible.
  • Several factors compromise motorists’ abilities to detect and read signs at night, including glare from oncoming headlights, fatigue or lack of alertness, lighting that does not adequately illuminate signs and text, poor visibility resulting from dim headlights, and glare on windshields or mirrors.

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