Factors Affecting Sign Visibility, Conspicuity, and Legibility: Review and Annotated Bibliography

Main Article Content

John Bullough

Abstract

This paper summarizes published research studies, technical reports and codes and standards related to the visibility (i.e., conspicuity and legibility) of signage. In the summary that follows, publications are grouped and discussed according to several different topics. First, the typographic and symbolic characteristics of signs and the information they carry are described (e.g., letter size, font selection, etc.); second, photometric, colorimetric and temporal properties of signs as they affect visibility; finally, environmental considerations (e.g., daytime versus nighttime viewing, whether a sign is located in a rural or urban area, etc.) as they influence sign design are reviewed. Annotated summaries of each publication in the literature review are included at the end of this paper.

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References

Akagi Y, Seo T, Motoda Y. 1996. Influence of visual environments on visibility of traffic signs. Transportation Research Record 1553: 53-58.
• The average detection distances for signs decreased from 110 ft with minimum visual noise to 60 ft with high levels of visual noise.

Allen TM. 1958. Night legibility distances of highway signs. Highway Research Bulletin 191: 3-40.
• Optimal sign luminances for nighttime legibility were found to be around 35 cd/m².

Allen TW, Dyer FN, Smith GM, Janson MH. 1967. Luminance requirements for illuminated signs. Highway Research Record 167: 16-37.
• Minimum nighttime sign luminances of 35 cd/m² are appropriate in rural locations, with a maximum of 100 cd/m².
• On illuminated highways or in the presence of substantial glare from opposing vehicle headlights, sign luminances between 70 and 340 cd/m² are recommended.
• In very brightly lighted urban locations, a minimum luminance of 700 cd/m² with a maximum of 1700 cd/m² might be appropriate.

American Association of State Highway and Transportation Officials. 2005. Roadway Lighting Design Guide. Washington, DC: American Association of State Highway and Transportation Officials.
• Nighttime sign luminances in areas of low, medium and high ambient luminance should be 20-40 cd/m², 45-90 cd/m² and 90-180 cd/m², respectively.
• A maximum-to-minimum sign luminance ratio of 6:1 is recommended.
• External lighting, if used, should not direct light into drivers' eyes.

Arditi A, Cho J. 2005. Serifs and font legibility. Vision Research 45: 2926-2933.
• Reading speed for normal-sighted and low vision observers did not differ whether fonts has serifs or not.
• Acuity was slightly improved when a font with serifs was used in place of one without serifs.

Arditi A, Liu L, Lynn W. 1997. Legibility of outline and solid fonts with wide and narrow spacing. Trends in Optics and Photonics, 5 p.
• Acuity for outline fonts was worse for outline fonts than for solid fonts.
• Outline characters needed to be 1.8 times larger than solid characters for equivalent legibility.

Beijer D, Smiley A, Eizenman M. 2004. Observed driver glance behavior at roadside advertising signs. Transportation Research Record 1899: 96-103.
• Signs with dynamic content made up half of the signs observed in one study, but received 70% of glances by drivers.
• Active signs received twice as many glances as non-active ones.

Bernard M, Liao CH, Mills M. 2001. The effects of font type and size on the legibility and reading time of online text by older adults. Proceedings of the Conference on Human Factors in Computing Systems, pp. 175-176.
• On average, legibility by older people of 14-point type was greater than for 12-point type.
• A 12-point serif font was less legible than a 12-point non-serif font, but the reverse effect of serifs occurred at 14 points.

Bertucci A. 2003. On-Premise Signs: Guideline Standards. Bristol, PA: United States Sign Council Foundation.
• A methodology for calculating the necessary size of a sign for various conditions (e.g., vehicle speed, type of reaction needed, letter type) is presented.

Bertucci A. 2006. Sign Legibility: Rules of Thumb. Bristol, PA: United States Sign Council Foundation.
• A legibility index of 30 ft/in is recommended for signage.

Bertucci A, Crawford R. 2015. Best Practice Standards for On-Premise Signs. Bristol, PA: United States Sign Council Foundation.
• Letter height needs to increase by 15% when all-uppercase letters are used, compared to mixed case.
• A legibility index of 30 ft/in. is recommended for adequate sign legibility.
• In conditions of moderate visual complexity, the recommended legibility index should be multiplied by 0.83; under high complexity, the legibility index should be multiplied by 0.67.

Bullough JD, Skinner NP. 2011. Luminance criteria and measurement considerations for light-emitting billboards. Transportation Research Board Annual Meeting, 7 p.
• A maximum allowable daytime billboard luminance of 23,000 cd/m² is proposed.
• A maximum allowable nighttime billboard luminance of 280 cd/m² is proposed.

Bullough JD, Skinner NP. 2016 [in press]. High visibility reflective sign sheeting materials: Field and computational evaluations of visual performance. Transport, 9 p.
• The relative visual performance model shows that large changes in luminance have small impacts on visibility for highway signs.
• Font size is a primary reason signs are not legible from large distances.

Bullough JD, Skinner NP, O’Rourke CP. 2010. Legibility of urban highway traffic signs using new retroreflective materials. Transport 25: 229-236.
• Retroreflective materials can compensate for a lack of external sign illumination in overhead guide signs.

Bullough JD, Sweater Hickcox K. 2012. Interactions among light source luminance, illuminance and size on discomfort glare. Society of Automotive Engineers International Journal of Passenger Cars - Mechanical Systems 5(1): 199-202.
• Ratings of discomfort glare from large-area sources are influenced by the illuminance produced by the source at observers' eyes and by the maximum luminance of the source of glare.

Cai H, Green PA. 2009. Legibility index for examining common viewing situations: A new definition using solid angle. Leukos 5(4): 279-295.
• A legibility index based on the subtended solid angle of a sign character rather than its height is proposed,
• The revised legibility index performed well at predicting critical legibility levels for many different viewing angles in which the characters' subtended angle would differ.

Carter R, Day B, Meggs P. 1985. Typographic Design: Form and Communication. New York, NY: Van Nostrand Reinhold.
• Text in all-uppercase letters is more difficult to read than mixed-case text.
• Serif and non-serif fonts can provide equal legibility.
• Research is described that finds the optimal font size at normal reading distances to be 9-12 points.

Center for Inclusive Design and Environmental Access. 2010. Design Resources: Text Legibility and Readability of Large Format Signs in Buildings and Sites, DR-11. Buffalo, NY: University at Buffalo.
• Research is cited stating that setting letter width to be the same as letter height results in greater legibility distances.
• A legibility index of 35 ft/in. is recommended.
• Positive contrast text is recommended.

Charness N, Dijkstra K., 1999. Age, luminance, and print legibility in homes, offices, and public places. Human Factors 41(2): 173-193.
• Reading task background luminances of 100 cd/m² are recommended for proficient reading.

City of Bellflower. 2016. Signage Design Guidelines. Bellflower, CA: City of Bellflower.
• Intricate typefaces for signs are prohibited.
• Lettering on a sign should not occupy more than 75% of the sign face area.
• The number of colors used on a sign should not exceed three.
• Excessively bright and fluorescent colors should be avoided.
• Internally-illuminated or back-lighted signs are preferred over external illumination.

City of Davis. 2010. Davis Citywide Sign Design Guidelines. Davis, CA: City of Davis.
• Messages on signs should be brief.
• Letters should occupy no more than 75% of the sign face area.
• High contrast between letters/symbols and their backgrounds should be used.
• External lighting should be shielded from view.
• Neon light signs are discouraged.
• Animation, blinking or other changes in intensity and color are prohibited.

City of Hutto. 2014. Site Design Standards. Hutto, TX: City of Hutto.
• Blinking or flashing on signs is prohibited.
• Electronic signs should not exceed a luminance of 7000 cd/m² during the daytime and 500 cd/m² during the nighttime.

City of Melbourne. 2009. An Ordinance of the City of Melbourne, Brevard County, Florida, Relating to Signs and Advertising. Melbourne, FL: City of Melbourne.
• Rotating or animated signs (except for changeable copy) are prohibited.

City of Mesa. [Undated.] Sign Regulations. Mesa, AZ: City of Mesa.
• Signs with flashing illumination or other animation or movement are prohibited.
• A sign with an LED display cannot exceed a luminance of 3150, 6300, 4690 or 7000 cd/m² for red, green, amber or full color signs, respectively, during daytime; or 1125, 2250, 1675 or 2500 cd/m² for red, green, amber or full color signs, respectively, during nighttime.
• Light sources for any external illumination should not be directly visible.

City of Saratoga Springs. 2012. Signage: Historic District Design Guidelines. Saratoga Springs, NY: City of Saratoga Springs.
• Sign messages should be short (no more than 8 words) and use three or fewer colors.
• Light sources for external illumination should be inconspicuous.

City of West Hollywood. 2002. Sign Design Guidelines. West Hollywood, CA: City of West Hollywood.
• Contrasting colors between letters and the sign background should be used to maximize legibility.
• An excessive number of sign colors can reduce legibility.
• A sign designed to be viewed from 60 ft requires 3.5 in. letters; to be viewed from 100 ft requires 5.5-6 in. letters.
• Symbols and pictograms are stated to be more effective than text.
• Letters should not take up more than 75% of the space on a sign panel.
• External sign lighting should be shielded to avoid glare; back lighting is encouraged.

Cornog DY, Rose FC. 1967. Legibility of Alphanumeric Characters and Other Symbols, II: A Reference Handbook. Washington, DC: National Bureau of Standards.
• Excessive brightness of a display can lead to irradiation that reduces legibility of characters and symbols.

Crawford A. 1962. The perception of light signals: The effect of the number of irrelevant lights. Ergonomics 5: 417-428.
• Flashing lights increase their conspicuity relative to steady lights.

Duncanson JP. 1994. Visual and Auditory Symbols: A Literature Review. Atlantic City, NJ: Federal Aviation Administration.
• It is proposed that an effective sign symbol is simple rather than complex, large rather than small, and solid rather than hollow or outlined.

Eastman AA. 1968. Color contrast versus luminance contrast. Illuminating Engineering 63: 67.
• Color contrast has little to no influence on visibility of objects unless the luminance contrast approaches zero.

Elstad JO, Fitzpatrick JT, Woltman HL. 1962. Requisite luminance characteristics for reflective signs. Highway Research Bulletin 336: 51-60.
• Optimal nighttime sign luminances were found in rural and suburban locations to be between 35 and 70 cd/m².
• In bright urban locations, nighttime sign luminances between 250 and 400 cd/m² were judged as prominently visible.

Federal Highway Administration. 2004. Standard Highway Signs. Washington, DC: Federal Highway Administration.
• Guide signs on conventional roads in rural locations should have letters at least 6 in. high; in urban locations with low speed limits (25 mph) letter height should be at least 4 in.
• Street name signs should have a letter height of 6 in.
• For signs other than on interstate highways, a legibility index of 40 ft/in. should be used.
• Nearly all signs should have borders of the same color as the sign letters.

Fletcher K, Sutherland S, Nugent K. 2009. Identification of Text and Symbols on a Liquid Crystal Display, Part II: Contrast and Luminance Settings to Optimise Legibility. Edinburgh, Australia: Defence Science and Technology Organisation.
• For a positive contrast display, character luminance is recommended to be 20 cd/m² under dark lighting conditions, and 60 cd/m² under bright conditions.
• The background screen luminance is recommended to be 1 cd/m².

Forbes TW. 1972. Visibility and legibility of highway signs. In Human Factors in Traffic Safety Research. New York, NY: Wiley.
• A formula relating the conspicuity detection distance for a sign to its luminance, the contrast between the sign letters and their background, and the letter height is provided.

Forbes TW, Pain RF, Fry JP, Joyce RP. 1967. Effect of sign position and brightness on seeing simulated highway signs. Highway Research Record 164: 29-37.
• At night, higher sign luminance tended to be more likely to be detected.
• Under daytime conditions, darker signs were often most likely to be detected, but so were brighter signs, for many observers. The contrast between letters and the sign background might sometimes overcome the contrast between the sign and its own background.

Forbes TW, Snyder TE, Pain RF. 1965. Traffic sign requirements: I. Review of factors involved, previous studies and needed research. Highway Research Record 70: 48-56.
• Research is cited finding about 85% legibility to signs with a legibility index (ft of legibility distance per in of letter height) of 50 ft/in.
• Only 3-4 short, familiar words can be read in a single glance at a sign.
• Letter-height to stroke-width ratios of 4-6 appear to be optimal for legibility.
• Color combinations providing the highest luminance contrast tend to provide the highest legibility.
• Use of fluorescent colors appears to have some advantages for sign detection.
• Brightness changes and motion are salient cues for peripheral vision.

Freyssinier JP, Narendran N, Bullough JD. 2006. Luminance requirements for lighted signage. Proceedings of the SPIE, Vol. 6337, 63371M.
• Illuminated sign luminances between 40 and 190 cd/m² are optimal when no nearby signs are present.
• Illuminated sign luminances between 65 and 230 cd/m² are optimal when nearby signs are present.

Freyssinier JP, Zhou Y, Ramamurthy V, Bierman A, Bullough JD, Narendran N. 2003. Evaluation of light-emitting diodes for signage applications. Proceedings of the SPIE, Vol. 5187, pp. 309-317.
• The contrast of luminance variations within a sign character should be no greater than 0.2-0.4 to achieve 80% acceptability.
• The size or spatial frequency of the luminance variations are relatively unimportant to judgments of acceptability.

Funkhouser D, Chrysler S, Nelson A, Park ES. 2008. Traffic sign legibility for different sign background colors: Results of an open road study at freeway speeds. Proceedings of the Human Factors and Ergonomics Society 52nd Annual Meeting, pp. 1855-1859.
• Green and purple highway signs performed equivalently in a driving test in terms of legibility distances during daytime and nighttime.

Garvey PM. 2006. On-Premise Signs: Determination of Parallel Sign Legibility and Letter Heights. Bristol, PA: United States Sign Council Foundation.
• Reading performance begins to decline as the viewing angle changes from perpendicular with the sign surface to between 20o and 40o from perpendicular.

Garvey PM. 2007. Urban wayfinding signs: Evaluating exceptions to FHWA’s standard alphabet. Transportation Research Board Annual Meeting, 17 p.
• A study of the use of the Futura font in wayfinding signs in Miami Beach found that it resulted in equivalent legibility as standard highway sign fonts.

Garvey PM, Chirwa KN, Meeker DT, Pietrucha MT, Zineddin AZ, Ghebrial RS, Montalbano J. 2004. New font and arrow for National Park Service guide signs. Transportation Research Record 1862: 1-9.
• A new highway sign font resulted in smaller word "footprints" but increased legibility distances by 10%.

Garvey PM, Klena MJ, Eie W-Y, Meeker DT, Pietrucha MT. 2016. Legibility of the Clearview typeface and FHWA standard alphabets on negative- and positive-contrast signs. Transportation Research Record 2555: 28-37.
• Signs using the Clearview font outperformed identical signs using standard highway alphabets in terms of legibility distance.
• Predictions of relative visual performance were correlated with legibility distances for individual fonts, but legibility distances were lower than predicted by the visual performance model when the font aspect ratio was narrow.

Garvey PM, Kuhn BT. 2011. Highway sign visibility. In Handbook of Transportation Engineering (Kutz M, editor). New York, NY: McGraw-Hill.
• Internally-illuminated signs and neon signs resulted in 40%-60% improvements in nighttime legibility over externally-illuminated signs.

Garvey PM, Pietrucha MT, Cruzado I. 2009. The Effects of Internally Illuminated On-Premise Sign Brightness on Nighttime Sign Visibility and Traffic Safety. Bristol, PA: United States Sign Council Foundation.
• Recognition distances at night to signs tend to increase as sign luminance increases, but decrease at the very highest luminances.
• Daytime signs were 43% more legible than poor nighttime signs, but only 13% more legible than well designed nighttime signs.

Garvey PM, Pietrucha MT, Meeker D. 1997. Effects of font and capitalization on legibility of guide signs. Transportation Research Record 1605: 73-79.
• Nighttime legibility distances to highway signs increased by 16% when Clearview font was used in place of the standard highway font.

Garvey PM, Zineddin AZ, Pietrucha MT. 2001. Letter legibility for signs and other large format applications. Proceedings of the Human Factors and Ergonomics Society 45th Annual Meeting, pp. 1443-1447.
• A study of visual acuity using various fonts found Bank Gothic Light, Dutch Regular and Dutch Bold to be most legible, with Commercial Script Regular least legible.
• Letter width serves as a better predictor of legibility than stroke width.

Gates TJ, Carlson PJ, Hawkins HG. 2004. Field evaluations of warning and regulatory signs with enhanced conspicuity properties. Transportation Research Record 1862: 64-76.
• Use of fluorescent colors in highways signs increased desired driving maneuvers.
• A red border around speed limit signs reduced daytime driving speeds and reduced the number of speed violators during daytime and nighttime.

Goodspeed C, Rea MS. 1999. The significance of surround conditions for roadway signs. Journal of the Illuminating Engineering Society 28(1): 164.
• The speed with which observers could identify Landolt ring target orientation was correlated with the predicted relative visual performance model quantity.

Graham JR, Fazal A, King LE. 1997. Minimum luminance of highway signs required by older drivers. Transportation Research Record 1573: 91-98.
• Young drivers require sign luminances of 30 cd/m² for correct identification from 90 m, and 2 cd/m² from 60 m.
• Older drivers require sign luminances in excess of 40 cd/m² for correct identification from 90 m, and 7 cd/m² from 60 m.

Hawkins HG, Picha DL, Wooldridge MD, Greene FK, Brinkmeyer G. 1999. Performance comparison of three freeway guide sign alphabets. Transportation Research Record 1692: 9-16.
• A comparison of different highway sign fonts showed increased legibility with Clearview over the standard highway font; the advantage was between 2% and 8%.

Hawkins HG, Rose ER. 2005. A human factors study of the effects of adding dual logo panels to specific service signs. Transportation Research Board Annual Meeting, 18 p.
• Using two logos in the space normally allocated to a single logo on service signs resulted in lower recognition, but not so much that using dual logos should be prohibited in the authors' opinion.

Holick AJ, Carlson PJ. 2002. Model of overhead-sign luminance needed for legibility. Transportation Research Record 1801: 80-86.
• An equation for the sign luminance needed to achieve legibility as a function of driver age, visual acuity, stroke width and viewing distance is provided.

Institution of Lighting Engineers. 2001. Brightness of Illuminated Advertisements. Warwickshire, UK: Institution of Lighting Engineers.
• Large illuminated sign luminance at night should be limited to 300 cd/m² in low district brightness areas and 600 cd/m² in medium and high district brightness areas; large illuminated signs should not be used in intrinsically dark areas.
• Small illuminated sign luminance at night should be limited to 100 cd/m² in intrinsically dark areas, 600 cd/m² in low district brightness areas, 800 cd/m² in medium district brightness areas and 1000 cd/m² in high district brightness areas.

International Sign Association. 2007. Conspicuity and readability. Signline 51: 1-8.
• At a speed of 55 mph, a sign should be legible from a distance of 440 ft; at a speed of 30 mph, it should be legible from 240 ft.
• On-premise signs should use letter heights of 7 in. for traffic at 25 mph, and 15 in. for traffic at 55 mph.

Kinney GC, Showman DJ. 1967. Studies in Display Symbol Legibility: Part XVIII. The Relative Legibility of Uppercase and Lowercase Typewritten Words. Bedford, MA: The Mitre Corporation.
• Word forms produced by combinations of uppercase and lowercase letters were equivalent in legibility to those by all-uppercase letters.
• Uppercase letters are recommended for displays and applications other than "normal reading" of text.

Kuhn BT, Garvey PM, Pietrucha MT. 1997. Model guidelines for visibility of on-premise advertising signs. Transportation Research Record 1605: 80-87.
• The contrast between a sign and its immediate background is the primary determinant of one's ability to detect the sign in visually simple environments, perhaps more than size.
• Increased sign luminance results in increased conspicuity and can help overcome visual complexity of the sign's background in most cases.
• Sign color can increase the sign's conspicuity.
• Contrast between sign letters and the sign background is important for legibility with a luminance ratio of 12:1 being close to optimal.
• Increasing sign luminance generally improves nighttime legibility up to an optimal value of 75 cd/m². Sign luminance at night should not be below 2.4 cd/m².
• Legibility distances for graphical symbols were nearly always longer than for alphanumeric characters.
• Mixed-case characters result in greater legibility distances than uppercase-only.
• The optimal stroke-width to height ratio for positive contrast is 1:5, and 1:7 for negative contrast text.
• Positive contrast results in greater legibility than negative contrast.
• To read a sign, process the information, and execute a driving maneuver in response to it requires 5.5 seconds with signs containing five or fewer critical elements.

Kuhn BT, Garvey PM, Pietrucha MT. 1998. Sign Legibility: The Impact of Color and Illumination on Typical On-Premise Sign Font Legibility. Bristol, PA: United States Sign Council Foundation.
• Internal illumination and neon signs outperformed externally-lighted signs in terms of sign legibility.
• Positive contrast signs outperformed negative contrast signs in terms of legibility.
• No legibility differences between serif and non-serif fonts were identified.

Lerner ND, Collins BL. 1983. Symbol sign understandability when visibility is poor. Proceedings of the Human Factors Society 27th Annual Meeting, pp. 944-946.
• The polarity of symbols and backgrounds made little difference on the recognition of symbolic signs.
• Filled symbols outperformed outline symbols in terms of recognition.

Lewin I. 2008. Digital Billboard Recommendations and Comparisons to Conventional Billboards. Scottsdale, AZ: Lighting Sciences, Inc.
• It is recommended that the illuminance from a digital billboard at a distance between 150 ft (for small billboards) and 350 ft (for very large billboards) not exceed 3 lx.

Milburn NJ, Mertens HW. 1997. Evaluation of a Range of Target Blink Amplitudes for Attention-Getting Value in a Simulated Air Traffic Control Display, DOT/FAA/AM-97/10. Washington, DC: Federal Aviation Administration.
• Flashing or blinking text is more difficult to read than steady text.

Millar K. 2011. Designing for legibility. SignCraft (January/February): 42-44.
• A rule of thumb for letter height at various viewing distances is given: 4 in. per 100 ft of viewing distance.
• At 30 mph, 8 in. letters are needed to ensure 5 seconds of readability; 4 in. letters ensure 3 seconds of legibility.
• At 60 mph, 16 in. letters are needed to ensure 5 seconds of readability; 8 in. letters ensure 3 seconds of legibility.

Pankok C, Kaber D, Rasdorf W, Hummer J. 2015. Driver attention and performance effects of guide and logo signs under freeway driving. Transportation Research Board Annual Meeting, 11 p.
• A comparison of guide signs and logo signs on the highway showed that guide signs received fewer and shorter visual fixations.
• Guide signs had more consistent eye-scan patterns than logo signs, probably because of the left-to-right nature of reading text on guide signs.

Powers LD. 1965. Effectiveness of sign background reflectorization. Highway Research Record 70: 74-86.
• Study participants were instructed to drive along a highway at night and exit following the presence of test signs equipped with no, low or highly-reflective green background sheeting material (resulting in different background luminances), with white reflectorized letters.
• No differences among the background conditions were found in terms of accuracy in responding to the test signs.

Rea MS (editor). 2000. IESNA Lighting Handbook: Reference and Application, 9th ed. New York, NY: Illuminating Engineering Society.
• Equations are provided for the spacing of individual lamps in exposed-letter signs, and for lamp wattages in different ambient environments.
• Sign luminance recommendations include 70-350 cd/m² for lighted fascia signs, 250-500 cd/m² for bright fascia signs, 450-700 cd/m² for low brightness areas, 700-1000 cd/m² for average commercial areas, 1000-1400 for areas with high sign competition, and 1400-1700 cd/m² for emergency traffic control.
• Floodlighted signs in bright surrounds should be illuminated to 1000 lx if reflectance is low, and 500 lx if reflectance is high; in dark surrounds, half these illuminances are recommended.

Rea MS, Ouellette MJ. 1991. Relative visual performance: A basis for application. Lighting Research and Technology 23(3): 135-144.
• The speed and accuracy of visual processing such as identifying characters in printed text is systematically related to its contrast, size, and the luminance of the background.

Schieber F, Goodspeed CH. 1997. Nighttime conspicuity of highway signs as a function of sign brightness, background complexity and age of observer. Proceedings of the Human Factors and Ergonomics Society 41st Annual Meeting, pp. 1362-1366.
• Increasing sign luminance had no benefit in terms of response times or response accuracy to signs when backgrounds were simple, but did improve detection times and accuracy in visually complex environments.

Schnell T, Atkan F, Li C. 2004. Traffic sign luminance requirements of nighttime drivers for symbolic signs. Transportation Research Record 1862: 24-35.
• Sign luminance, letter contrast and the type of symbol displayed all influenced the legibility distance of sign symbols.

Schnell T, Yekhshatyan L, Daiker R. 2009. Effect of luminance and text size on information acquisition time from traffic signs. Transportation Research Record 2122: 52-62.
• The relative visual performance model resulted in close agreement with visual acquisition times in a study of sign character legibility under different luminances, sizes and contrasts.

Shurtleff D, Botha B, Young M. 1966. Studies in Display Symbol Legibility: Part IV. The Effects of Brightness, Letter Spacing, Symbol Background Relation and Surround Brightness on the Legibility of Capital Letters. Bedford, MA: The Mitre Corporation.
• Letters with high contrast against their backgrounds are recommended for highest acuity.
• Polarity of contrast is unimportant to legibility.
• Background luminances of 70 to 140 cd/m² are recommended.

Smiley A, Persaud B, Bahar G, Mollett C, Lyon C, Smahel T, Kelman WL. 2005. Traffic safety evaluation of video advertising signs. Transportation Research Record 1937: 105-112.
• Video advertising is stated to have potential to distract drivers inappropriately, but overall impacts on safety are likely to be small.

Tinker MA. 1966. Experimental studies on the legibility of print: An annotated bibliography. Reading Research Quarterly 1(4): 67-118.
• Research is cited stating that letters with serifs are more legible than those without serifs.
• A study found that white numbers printed on a black background were 8% more legible than black numbers printed on a white background.
• The poorest color combinations for reading text were found in one study to be red type on black background, or vice versa. Luminance contrast is one of the most important factors in legibility.
• The optimal character stroke width was identified in research as being 18% of the character height or width.
• Research stating that increasing illumination could overcome a type size change from 12 to 6 points is cited.

Town of Bermuda Run. 2013. Sign Design Guidelines. Bermuda Run, NC: Town of Bermuda Run.
• A viewer reaction time of 8 seconds is recommended for signs along roads with a speak limit of 45 mph, when six or fewer words are on the sign.
• The ideal letter height for signs is stated to be between 8 and 13 in.
• For improved legibility, block (non-script) text and mixed case is preferred.
• Using no more than two colors is stated to increase legibility.
• Positive contrast signs are stated to increase legibility, but the degree of improvement depends upon illumination and contrast.

Town of Huntersville. 2009. Suggestions for Designing Effective Signs. Huntersville, NC: Town of Huntersville.
• High contrast between sign letters and their backgrounds is desirable for legibility.
• Light letters on dark backgrounds are preferable to the opposite for ease of reading.
• For 2-lane roads, 30 mph traffic requires 8-in. letters and 55 mph traffic requires 12-in. letters.
• For 4-lane roads, 30 mph traffic requires 10-in. letters and 55 mph traffic requires 15-in. letters.
• Sign letters should occupy no more than 40% of the sign area.

Ullman BR, Ullman GL, Dudek CL, Ramirez EA. 2005. Legibility distances of smaller letter light-emitting diode changeable message signs. Transportation Research Board Annual Meeting, 23 p.
• LED letters on a changeable message sign with a height of 9 in. were legible from 228 ft in the daytime and 114 ft at night.
• LED letters on a changeable message sign with a height of 10.6 in. were legible from 324 ft in the daytime and 203 ft at night.

U.S. Small Business Administration. 2003. The Signage Sourcebook. Washington, DC: U.S. Small Business Administration.
• It is recommended that a sign be legible from a distance (in ft) equal to a vehicle's speed limit (in mph) multiplied by 8.
• Signs mounted on the left side of the road require letters to be one-third larger than those on the right side of the road, for equal legibility.
• Recommended sign heights range from 12 ft for 25-mph traffic to 50 ft for 55-mph traffic.

Van Houten R, Healey K, Malenfant JEL, Retting R. 1998. Use of signs and symbols to increase the efficacy of pedestrian-activated flashing beacons at crosswalks. Transportation Research Record 1636: 92-95.
• Adding a pedestrian symbol sign near a flashing warning beacon increased the number of drivers who yielded to pedestrians.

Yager D, Aquilante K, Plass R. 1998. High and low luminance letters, acuity reserve, and font effects on reading speed. Vision Research 38: 2527-2531.
• At a high background luminance (150 cd/m²) there is no difference in reading rates between serif and non-serif fonts.
• At a low background luminance (0.15 cd/m²) a non-serif font resulted in improved reading rates over a serif font.

Young SL, Laughery KR, Bell M. 1992. Effects of two type density characteristics on the legibility of print. Proceedings of the Human Factors Society 36th Annual Meeting, pp. 504-508.
• Type width is stated to affect legibility more than inter-character spacing.
• Reducing the space between characters improved legibility for standard type widths, but decreased legibility for the narrowest fonts.

Zwahlen HT, Schnell T. 1998. Legibility of traffic sign text and symbols. Transportation Research Record 1692: 142-151.
• Sign legibility distances are 1.8 times longer in the daytime than they are at night.