National Academies Press: OpenBook

Human Factors Guidelines for Road Systems: Second Edition (2012)

Chapter: Chapter 18 - Signing

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Suggested Citation:"Chapter 18 - Signing." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 18 - Signing." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 18 - Signing." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 18 - Signing." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 18 - Signing." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 18 - Signing." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 18 - Signing." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 18 - Signing." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 18 - Signing." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 18 - Signing." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 18 - Signing." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

General Principles for Sign Legends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18-2 Sign Design to Improve Legibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18-4 Conspicuity of Diamond Warning Signs under Nighttime Conditions . . . . . . . . . . . . . . . . .18-6 Driver Comprehension of Signs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18-8 Complexity of Sign Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18-10 18-1 C H A P T E R 18 Signing

GENERAL PRINCIPLES FOR SIGN LEGENDS Introduction Sign legends refer to the text and/or symbols composing the sign message. Legends that are too long or too complicated can lead to problems in comprehension. In general, the legend on a sign must be kept to a minimum, regardless of letter size, to maximize driver comprehension. Design Guidelines Type of Sign Example (all from MUTCD (1)) Guidelines Advance Guide Limit route and destination information to a total of three lines Do not use more than two destination/street names. Place intersecting streets on top line and distance to intersecting streets on bottom. Conventional Guide Limit route and destination information to a total of three lines. Exit Direction Limit route and destination information to a total of three lines. Do not include more than two destination/street names. Tourist Place symbols to the left of the word legend. Limit information to a total of two lines. Service Limit general road user services to six. Distance Limit traffic generators to three accompanied by the related distance. Keep the highest priority distance (nearest distance) at the top or left. Lane Control Place the legend at the top of the sign. Based Primarily on Expert Judgment Based Equally on Expert Judgment and Empirical Data Based Primarily on Empirical Data HFG SIGNING Version 2.0 18-2

Discussion The relatively sm all am ount of available space on roadway signs suggests the need to make the best use of this space when designing legends. The guidelines on the previous page have been adapted from the MUTCD ( 1 ) because of their comm on focus on legends and because they are provided across various sections/pages within the MUTCD and can be hard to find. In general, they reflect acceptable, best practices for sign legends. The legend for a sign should be selected to maxim ize information transm ission and comprehension, given both the nature of the sign’s me ssage and general roadway environment. Text-based signs are clearly mo re appropri ate than sym bolic signs for highly com plex me ssages, such as destination me ssages or hazards that are mo re quickly and easily presented via text rather than potentially am biguous or unfam iliar sym bols. There is a trade-off between the am ount of inform ation provided in a sign, the co mp lexity of the sign information, and its overall comprehensibility. Either through the use of more words or through the use of complex graphics, the density of in formation presented on a sign can be in creased, but often at the cost of legibility and/or comprehensibility. New sign designs (or even existing signs being used in a new location or a new way) should always be tested, using a representative group of drivers, to see if they support adequate levels of driver com prehension. Design Issues Sign placem ent and appropriate letter height are deter mi ned by a num ber of factors. A process for determining these values is presented in the Traffic Control Devices Handbook ( 2 ) and discussed in more detail in Tutorial 5. Appropriate sign placement is determined by the overall information presentation distance, which is the total distance at which the driver needs information about the choice point (e.g., intersection). This distance is the sum of the reading distance, the decision distance, and the ma neuve r distance. The reading distance is determ ined by the am ount of tim e that the driver needs to read the sign’s me ssage, depending on the nu mb er of words, num bers, and sym bols contained in the me ssage. The decision distance is deter min ed by the am ount of ti me needed to make a choice decision and initiate a maneuver. The decision time necessary depends on the complexity of the maneuver. The ma neuver distance is determ ined by the ti me necessary to com plete any ma neuver required by the choice. For a lane change ma neuver, this distance is the su m of the ga p search, lane change, and deceleration distances. These values are all influenced by the vehicle operating speed. Once the reading, decision, and maneuver distances are summ ed to find the inform ation presentation distance, the advance placement distance between the sign and the choice point can be subtracted to find the legibility distance, which is the distance at which the sign mu st be legible. The required letter height can be calculated by referencing the legibility index provided in the MUTCD (30 ft/in.). When the legibility distance is divided by the legibility index, the letter height is obtained. Information Presentation Distance Maneuver Distance Reading Distance Decision Distance Gap Search Lane Change Deceleration Legibility Distance Advance Placement Cross References Presentation to Maximize Visibility and Legibility, 19-4 Sight Distance Guidelines, 5-1 Key References 1. FHWA (2009). Manual on Uniform Traffic Control Devices for Streets and Highways . Wash ington, DC. 2. Pline, J.L. (Ed.). (2001). Traffic Control Devices Handbook . Washington, DC: ITE. 18-3 HFG SIGNING Version 2.0

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 (Sheeting Type) Microprismatic retroreflective sheeting provides longer legibility 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 Clearview™ 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 Older Drivers (all from FHWA (6 )) Minimize symbol complexity by using very few details. Maximize the distance between symbol sign elements. 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 Expert Judgment Based Equally on Expert Judgment and Empirical Data Based Primarily on Empirical Data HFG SIGNING Version 2.0 18-4

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 HFG SIGNING Version 2.0

C ONSPICUITY OF D IAMOND W ARNING S IGNS UNDER N IGHTTIME C ONDITIONS Introduction Conspicuity refers to how easy it is to see and locate a visual target. In the context of road signs, it represents how easy it is to distinguish a sign from the surrounding visual environm ent. Visual conspicuity is particularly im portant when providing important information because drivers are typically reluctant to spend more than 2 s with their eyes off of the roadway. Consequently, the easier drivers can find a sign, the more tim e they have to comprehend the sign information. Also, at a more basic level, increasing the co nspicuity of a si gn will reduce the chance that drivers will miss or be unable to read the sign information altogether. Nighttime visibility is a special problem for sign design, as reduced illu mi nance (relative to daytim e conditions) is associ ated with reduced target contrast and generally reduced visibility for drivers. Related to traffic control devices in general, the MUTCD (1) provides design considerations that specify that “devices should be designed so that features such as size, shape, color, composition, lighting or retroreflection, and contrast are co mb ined to draw attention to the devices.” As discussed in more detail below, a critical factor in facilitating the driver’s ability to find and comprehend warning signs at night is to maximize the sign’s visual conspicuity relative to surrounding background elements. The figure below illustrates the relationship between si gn recognition by dr iv ers, sign brightness, and the complexity of the sign’s immediate environment. Design Guidelines Sign Characteristics Environment Characteristics Increase sign brightness relative to its surround. Increase brightness contrast between different parts/elem ents of the sign. Increase the sign’s size relative to other objects in the visual field/environm ent. Use a sign hue that contrasts with other noise/background ite ms . Reduce the number and density of background noise items, especially those immediately adjacent to the sign. Increase the distance between the sign and noise items. Based Primarily on Expert Judgment Based Equally on Expert Judgment and Empirical Da ta Based Primarily on Empirical Dat a R ECOGNITION P ERFORMANCE BY V ISUAL C OMPL EX ITY AND S IGN B RIGHTNESS 500 600 700 800 900 1000 1100 1200 1300 1400 Low Medium High M ea n R ec o g n it io n P er fo rm an ce ( F ee t) Visual Complexity High Brightness Medium Brightness Low Brightness Source: adapted from Mace, King, and Dauber ( 2 ) HFG SIGNING Version 2.0 18-6

Discussion Mace et al. ( 2 ) describe a study conducted to establish luminance levels for conspicuity of yellow diamond warning signs at night. A key finding of the study was that, while many factors influence the visibility of a road sign, the visual co mp lexity of a scene is most important in determining nighttime sign luminance requirements. Specifically, the complexity of the area immediately surrounding a sign (e.g., other signs, lights, structures, trees, etc.) greatly influences a driver’s ability to perceive and extract information from a sign. When sites are assessed or classified on their visual com plexity, the following factors are rated: The am ount of detail visible in the visual scene, quantified as the num ber of objects or percentage of the scene with visible detail The number of bright light sources—streetlights, si gns, cars, billboards, store windows, reflection, etc.—located in the scene The am ount of visible detail contained in the cone (that portion on the right-hand side of the roadway where a driver would typically look for road signs) of the scene The visual demands associated with the portion of the roadway associated with the sign (i.e., the percentage of the driver’s time that would be spent looking for driving-relevant inform ation while at that location) A broader sum mary of relevant research provided in Mace et al. ( 2 ) concluded that the attention-getting value of a target increases as (1) the target’s brightness increases, (2) the brightness contrast between the target and its surround increases, (3) the brightness contrast between different parts of the target increases, (4) the target’s size increases relative to other stimuli in the visual field, (5) the shape of the target contrasts with noise items, (6) the target’s hue contrasts with noise, (7) the number of noise elements in the visual field decreases, (8) the overall density of noise ite ms in the visual field decreases, (9) the density of noise items immediately adjacent to the target decreases, (10) the distance between the target and noise increases, (11) the num ber of irrelevant classes of stim uli in the visual field decreases, and (12) the variability within each irrelevant cla ss of stimuli decreases. Although sign conspicuity is clearly important, compliance with the specifications set by the MUTCD for sign shape and other characteristics is essential. Design Issues A key factor to consider in improving the conspicuity and visibility of highway signs is the importance of individual differences across the driver population. In particular, older drivers have poorer rates of detection and recall of signs than do younger drivers ( 3 ), and slower response ti mes ( 4 ). Thus, conspicuity and visibility for older drivers should be a key concern in the design and placem ent of signs. Another factor in driver reaction to signs is their relevance to the drivers at a particular time and place. A series of studies have dem onstrated that the greater the relevance to a particular trip and the greater their need for the information provided by the sign, the more likely that drivers will pay attention to the sign ( 5 ). Cross References Presentation to Maximize Visibility and Legibility, 19-4 Key References 1. FHWA (2009). Manual on Uniform Traffic Control Devices for Streets and Highways . Wash ington, DC. 2. Mace, D.J., King, R.B., and Dauber, G.W. (1985). Sign Luminance Requirements for Various Background Complexities (FHWA-RD-85- 056). McLean, VA: FHWA. 3. Al-Gadhi, S.A., Naqvi, S.A., and Ab dul-Jabbar, A.S. (1 994). Driver factors affecting traffic sign detection and recall. Transportation Research Record, 1464 , 36-41. 4. Garvey, P.M., and Kuhn, B.T. (2004). Highway sign visibility. In Handbook of Transportation Engineering . (Chapter 11). New York: McGraw-Hill. 5. MacDonald, W.A., and Hoff ma nn, E.R. (1984). Drivers' Awareness of Traffic Sign Information. (AIR 382-1). Vermont South, Victoria: Australian Road Research Board. 18-7 HFG SIGNING Version 2.0

DRIVER COMPREHENSION OF SIGNS Introduction Sign comprehension refers to a driver’s or road user’s ability to interpret the meaning of a sign. Signs should be designed and presented so that their message is comprehended and understood by users. As discussed in Campbell, Richman, Carney, and Lee (1), in the context of icons and symbols, there are three stages associated with the comprehension and use of signs: legibility, recognition, and interpretation. Legibility reflects the relationships among the driver, the sign, and the environment; it is essential for the initial perception of the sign and includes parameters such as luminance uniformity, contrast, and size. Recognition reflects whether or not the driver can readily distinguish the sign, especially in the context of other signs and stimuli. Interpretation reflects the relationships among the driver, the sign, and the referent or message associated with the sign; it includes parameters such as whether the driver comprehends the meaning, intent, or purpose of the sign. This guideline identifies message format recommendations for improving drivers’ comprehension of road signs. As shown below, information can be presented in a text-only, graphic/icon-only, or mixed text–graphic format. Design Guidelines The following guidelines provide parameters for the use of text-only, graphic/icon-only, or mixed text–graphic formats. Format Example Guidelines Text Only Use for highly complex messages. Use when indicating hazards. Use for destination information. Use in areas requiring unexpected or unique driver actions, e.g., frequent lane shifts. Graphic / Icon Only Use for safety and warning information. Use for prohibited actions. Use in visually degraded conditions. Use in areas with higher posted speeds. Use diagrammatic graphics when road geometry violates driver expectancies. Minimize symbol complexity by using few details. Mixed Add text when symbols alone are unintuitive. Keep text to no more than two to three words. Use a clear and simple font for the text. Based Primarily on Expert Judgment Based Equally on Expert Judgment and Empirical Data Based Primarily on Empirical Data HFG SIGNING Version 2.0 18-8

Discussion The figure below shows the three stages that appear to be associated with comprehension and use of signs: legibility, recognition, and interpretation. As shown below, this sequence of icon comprehension refers to the perceptual and cognitive process by which users interpret the meaning of a sign. Legibility Recognition Interpretation • Can the driver see the sign? • Is it legible at various distances? • Can it be seen under both nighttime and daytime lighting conditions? • How well do the parts of this sign relate to one another? • Does the construction of the sign support accurate recognition? • Is it easily confused with other signs? • How well does the sign convey the message? • Will it be understood when presented in the appropriate context? • Does it require special knowledge particular to a culture, language, or driver age? Source: adapted from Campbell et al. (1) The format of a sign—i.e., text only, graphic/icon only, or mixed—should be selected to maximize information transmission and comprehension, given the nature of both the sign’s message and the general roadway environment. Text-based signs are clearly more appropriate for highly complex messages, such as destination messages or hazard warnings that are more quickly and easily presented via text. It has long been recognized that well-designed icons are generally recognized more accurately and quickly than text-based signs meant to convey the same message (2) and that icons can be presented in a much more spatially condensed form (3, 4, 5) than can most text-based messages. Road signs also have a limited amount of space for presenting information and must take advantage of the ability of icons to present more information to the driver than can be presented textually. Research in this domain has shown that icons can be recognized more rapidly and are legible at greater distances than information presented in other formats (6, 7). The absolute numerical differences in mean reaction times are not relevant because of the differences between the task performed in the study and the actual driving task. Design Issues Comprehension tests are evaluation techniques that provide a means to determine whether a candidate sign for a roadway message is likely to be properly understood by typical roadway users. Overall, a rigorous and iterative evaluation process will increase the likelihood that the implementation of the sign on the roadway will improve overall traffic safety, and not detract from it. A number of procedures can be used to measure driver comprehension of signs, including the recently released J2830, Process for Comprehension Testing of In-Vehicle Icons, an SAE Information Report within the SAE Standards series. Also, road engineers may consider message format based on location and driver demographics. For example, non- native-English speakers can correctly interpret graphic messages without relying on their knowledge of the English language. An increased use of transportation graphic signs in the vicinity of non-native-English-speaking population areas may be appropriate. Cross References Presentation to Maximize Visibility and Legibility, 19-4 Key References 1. 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). 2. Edworthy, J., and Adams, A. (1996). Warning Design: A Research Prospective. Bristol, PA: Taylor & Francis. 3. Zwaga, H.J., and Boersema, T. (1983). Evaluation of a set of graphic symbols. Applied Ergonomics, 14, 43-54. 4. Rohr, G., and Keppel, E. (1984). Iconic Interfaces: Where to Use and How to Construct Human Factors in Organisational Design and Management. The Netherlands: North-Holland. 5. Hemenway, K. (1982). Psychological issues in the use of icons in command menus. Proceedings of the CHI 1982 Conference on Human Factors in Computer Science (pp. 20-23). New York: Association for Computing Machinery. 6. Ells, J.G., and Dewar, R.E. (1979). Rapid comprehension of verbal and symbolic traffic sign messages. Human Factors, 21, 161-168. 7. Jacobs, R.J., Johnston, A.W., and Cole, B.L. (1975). The visibility of alphabetic and symbolic traffic signs. Australian Road Research, 5(7), 68-86. 18-9 HFG SIGNING Version 2.0

COMPLEXITY OF SIGN INFORMATION Introduction The complexity of sign information refers to the number of information units presented as part of a roadway sign message. In this context, an information unit can describe geography (e.g., city), type of roadway (e.g., highway), event causes (e.g., stalled vehicle), event consequences (e.g., traffic jam), time and distances, and proposed actions. Therefore, information units can be described as the relevant words in a message. Much of the guideline information presented below has been adapted from Campbell, Carney, and Kantowitz (1). Design Guidelines Messages that require an urgent action should be a single word or a short sentence with the fewest number of syllables possible. Drivers should be able to understand the message immediately. Messages that are not urgent or for which a response may be delayed can be a maximum of 7 units of information in the fewest number of words possible. If the information cannot be presented in a short sentence, the most important information should be presented at the beginning and/or the end of the message. Navigation instructions should be limited to 3 or 4 information units. Based Primarily on Expert Judgment Based Equally on Expert Judgment and Empirical Data Based Primarily on Empirical Data DETERMINING THE NUMBER OF INFORMATION UNITS 4 units Road Construction Ahead at Jaspertown 8 units Road Construction on Interstate 5 for next 10 miles Take Highway 99 11 units Interstate 80 closed for construction between Iowa City and Cedar Rapids Exit at West Liberty and drive north on Highway 16 16 units Accident Ahead Exit 215 closed to Dover Traffic detoured to Exit 216 Follow Highway 46 to Chester and turn east onto Inglenook Road EFFECTS OF INFORMATION COMPLEXITY Length of Message 3-4 units 6-8 units 10-12 units 14-18 units Duration of Each Glance 1.08 s 1.18 s 1.20 s 1.35 s Number of Glances 3.8 6.9 9.6 15.5 Memory Recall 100% 97.5% 75.4% 52.4% Source: Labiale (2) HFG SIGNING Version 2.0 18-10

Discussion The longer the message, the more processing time the driver requires. Therefore, messages that require the driver to make an immediate response should be as short as possible. One-word messages informing the driver of the appropriate action to take might work best in these situations. As the response required by the driver becomes less and less urgent, the messages can become more detailed; however, an effort should still be made to make the messages as concise as possible. Zwahlen, Adams, and DeBald (3) analyzed the number of lane deviations that occurred while drivers were operating a CRT touch screen. The results suggest that the number of glances away from the roadway should be limited to three and that glance durations that exceeded 2 s in duration are unacceptable. Zwahlen et al. (3) examined the amount/complexity of information necessary for evoking these unsafe glance frequencies and durations. The results of this on-road study suggest that although the duration of glances does not increase dramatically as the number of information units increase, the number of glances does. Therefore, the shortest information message (3 to 4 units) would be the most appropriate for keeping drivers’ attention on the forward roadway. The driver’s ability to recall information was also examined in Labiale (2): only 75% of a 10- to 12-unit message could be recalled, in comparison to 100% of a 3- to 4-unit message and 98% of a 6- to 8-unit message. This finding is consistent with Miller (4), which proposed that the maximum capacity of working memory is “seven, plus or minus two” chunks of information. Again, this finding suggests that keeping the message short, 3 to 8 information units, would increase the likelihood that it will be recalled by the driver. Design Issues Complexity is a function of how much information is being provided and how difficult it is to process. The phrase “information units” is used to describe the amount of information presented, in terms of key nouns and adjectives contained within a message. High-Complexity Examples Low-Complexity Examples > 9 information units 3-5 information units Processing time > 5 s Processing time < 5 s Examples: Topographical representations of a route, or full route maps, or schedules for alternative modes of transportation. Examples: Directions of turns, or estimates of travel costs. Cross References Driver Comprehension of Signs, 18-8 Key References 1. 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. 2. Labiale, G. (1996). Complexity of in-car visual messages and driver’s performance. In A.G. Gale et al. (Eds.). Vision in Vehicles, 5 (pp. 187-194). Bron Cedex, France: INRETS. 3. Zwhalen, H.T., Adams, C.C., Jr., and DeBald, D.P. (1988). Safety aspects of CRT touch panel controls in automobiles. In A.G. Gale et al. (Eds.). Vision in Vehicles, 2 (pp. 335-344). Amsterdam: Elsevier Science. 4. Miller, G.A. (1956). The magical number seven plus or minus two: Some limits on our capacity for processing information. Psychological Review, 63, 81-97. 18-11 HFG SIGNING Version 2.0

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 600: Human Factors Guidelines for Road Systems: Second Edition provides data and insights of the extent to which road users’ needs, capabilities, and limitations are influenced by the effects of age, visual demands, cognition, and influence of expectancies.

NCHRP Report 600 provides guidance for roadway location elements and traffic engineering elements. The report also provides tutorials on special design topics, an index, and a glossary of technical terms.

The second edition of NCHRP 600 completes and updates the first edition, which was published previously in three collections.

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