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HFG SIGNING Version 1.0 CONSPICUITY OF DIAMOND WARNING SIGNS UNDER NIGHTTIME CONDITIONS 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 environment. Visual conspicuity is particularly important 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 time they have to comprehend the sign information. Also, at a more basic level, increasing the conspicuity of a sign 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 illuminance (relative to daytime conditions) is associated with reduced target contrast and generally reduced visibility for drivers. Related to warning signs 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 combined 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 sign recognition by drivers, sign brightness, and the complexity of the sign's immediate environment. Design Guidelines Sign Characteristics Environment Characteristics Increase sign brightness relative to its surround. Reduce the number and density of background Increase brightness contrast between different noise items, especially those immediately parts/elements of the sign. adjacent to the sign. Increase the sign's size relative to other objects in Increase the distance between the sign and noise the visual field/environment. items. Use a sign hue that contrasts with other noise/background items. Based Primarily on Based Equally on Expert Judgment Based Primarily on Expert Judgment and Empirical Data Empirical Data RECOGNITION PERFORMANCE BY VISUAL COMPLEXITY AND SIGN BRIGHTNESS 1400 1300 Mean Recognition Performance (Feet) 1200 1100 1000 High Brightness 900 Medium Brightness 800 Low Brightness 700 600 500 Low Medium High Visual Complexity Source: adapted from Mace, King, and Dauber (2) 18-6
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HFG SIGNING Version 1.0 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 complexity 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 complexity, the following factors are rated: The amount of detail visible in the visual scene, quantified as the number of objects or percentage of the scene with visible detail The number of bright light sources--streetlights, signs, cars, billboards, store windows, reflection, etc.--located in the scene The amount 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 information while at that location) A broader summary 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 items 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 number of irrelevant classes of stimuli in the visual field decreases, and (12) the variability within each irrelevant class 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 times (4). Thus, conspicuity and visibility for older drivers should be a key concern in the design and placement 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 demonstrated 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 (2007). Manual on Uniform Traffic Control Devices for Streets and Highways. Washington, 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 Abdul-Jabbar, A.S. (1994). 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 Hoffmann, E.R. (1984). Drivers' Awareness of Traffic Sign Information. (AIR 382-1). Vermont South, Victoria: Australian Road Research Board. 18-7