National Academies Press: OpenBook

Aesthetic Concrete Barrier Design (2006)

Chapter: Chapter 3 - Considerations for Developing an Aesthetic Barrier

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Suggested Citation:"Chapter 3 - Considerations for Developing an Aesthetic Barrier." National Academies of Sciences, Engineering, and Medicine. 2006. Aesthetic Concrete Barrier Design. Washington, DC: The National Academies Press. doi: 10.17226/13888.
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Suggested Citation:"Chapter 3 - Considerations for Developing an Aesthetic Barrier." National Academies of Sciences, Engineering, and Medicine. 2006. Aesthetic Concrete Barrier Design. Washington, DC: The National Academies Press. doi: 10.17226/13888.
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Suggested Citation:"Chapter 3 - Considerations for Developing an Aesthetic Barrier." National Academies of Sciences, Engineering, and Medicine. 2006. Aesthetic Concrete Barrier Design. Washington, DC: The National Academies Press. doi: 10.17226/13888.
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Suggested Citation:"Chapter 3 - Considerations for Developing an Aesthetic Barrier." National Academies of Sciences, Engineering, and Medicine. 2006. Aesthetic Concrete Barrier Design. Washington, DC: The National Academies Press. doi: 10.17226/13888.
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Suggested Citation:"Chapter 3 - Considerations for Developing an Aesthetic Barrier." National Academies of Sciences, Engineering, and Medicine. 2006. Aesthetic Concrete Barrier Design. Washington, DC: The National Academies Press. doi: 10.17226/13888.
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Suggested Citation:"Chapter 3 - Considerations for Developing an Aesthetic Barrier." National Academies of Sciences, Engineering, and Medicine. 2006. Aesthetic Concrete Barrier Design. Washington, DC: The National Academies Press. doi: 10.17226/13888.
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Suggested Citation:"Chapter 3 - Considerations for Developing an Aesthetic Barrier." National Academies of Sciences, Engineering, and Medicine. 2006. Aesthetic Concrete Barrier Design. Washington, DC: The National Academies Press. doi: 10.17226/13888.
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Suggested Citation:"Chapter 3 - Considerations for Developing an Aesthetic Barrier." National Academies of Sciences, Engineering, and Medicine. 2006. Aesthetic Concrete Barrier Design. Washington, DC: The National Academies Press. doi: 10.17226/13888.
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Suggested Citation:"Chapter 3 - Considerations for Developing an Aesthetic Barrier." National Academies of Sciences, Engineering, and Medicine. 2006. Aesthetic Concrete Barrier Design. Washington, DC: The National Academies Press. doi: 10.17226/13888.
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Suggested Citation:"Chapter 3 - Considerations for Developing an Aesthetic Barrier." National Academies of Sciences, Engineering, and Medicine. 2006. Aesthetic Concrete Barrier Design. Washington, DC: The National Academies Press. doi: 10.17226/13888.
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Suggested Citation:"Chapter 3 - Considerations for Developing an Aesthetic Barrier." National Academies of Sciences, Engineering, and Medicine. 2006. Aesthetic Concrete Barrier Design. Washington, DC: The National Academies Press. doi: 10.17226/13888.
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Suggested Citation:"Chapter 3 - Considerations for Developing an Aesthetic Barrier." National Academies of Sciences, Engineering, and Medicine. 2006. Aesthetic Concrete Barrier Design. Washington, DC: The National Academies Press. doi: 10.17226/13888.
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Suggested Citation:"Chapter 3 - Considerations for Developing an Aesthetic Barrier." National Academies of Sciences, Engineering, and Medicine. 2006. Aesthetic Concrete Barrier Design. Washington, DC: The National Academies Press. doi: 10.17226/13888.
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Suggested Citation:"Chapter 3 - Considerations for Developing an Aesthetic Barrier." National Academies of Sciences, Engineering, and Medicine. 2006. Aesthetic Concrete Barrier Design. Washington, DC: The National Academies Press. doi: 10.17226/13888.
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16 CHAPTER 3 CONSIDERATIONS FOR DEVELOPING AN AESTHETIC BARRIER The evaluation plan for developing a guide for designers for aesthetic treatments of concrete barriers originally ad- dressed both safety performance and the application of context-sensitive principles for selecting the appropriate bar- rier for the drivers and their environment. Research into the principles for selecting the appropriate barrier for drivers and their environment was performed early in the study. A re- view of the literature and an investigation into context- sensitive principles was performed. However, following the submittal of the project interim report and the project panel members’ meeting with the researchers, the scope of the project and research objectives were modified to only (1) assemble a collection of examples of longitudinal traffic barriers exhibiting aesthetic characteristics and (2) develop engineering design guidelines for aesthetic surface treat- ments of concrete safety shape barriers (e.g., New Jersey and F-shape profiles). The focus of the design guidelines would be to determine the crashworthy geometric configura- tion of surface asperities that could placed into the traffic face of New Jersey and F-shape concrete barriers. The inves- tigation of geometric configurations of crashworthy surface asperities would be performed using finite element simu- lation and full-scale crash testing. Therefore, additional research relating to driver behavior and context-sensitive design principles was stopped. The information presented hereafter was gathered prior to the change of project scope and is presented as documentation of project work per- formed. This information may be considered incomplete and is only presented for the reader’s edification. Aesthetic barrier design has been poorly described as a tech- nique dealing strictly with aesthetics. Previous studies dealing with barrier rail design have focused on structural performance and testing rather than the evaluation of aesthetic characteris- tics of a barrier and its effect on the driver, the roadway, and environment. The project work plan originally addressed de- veloping guidelines for designers of aesthetic treatments of concrete barriers that addressed both safety performance and context-sensitive principles. The researchers approached this plan from both the viewer preference level and the behavioral level. The viewer preference level focused on applying char- acteristics that cause a rail or barrier to elicit a favorable response from a viewer, either consciously or subconsciously. The behavioral characteristics study identified for the engi- neer/designer those characteristics of rails that promote im- proved visibility, principally as a combination of color, line, and contrast with other roadway elements or activities. DEVELOPING A DEFINITION FOR AESTHETICS The issue of when something is considered “aesthetic” is important because many people consider aesthetics to be a heavily subjective assessment. For the sake of discussion regarding bridge rails and barriers, a more objective defi- nition is needed. This is because, when a community asks for a more “aesthetically pleasing” design, designers must be able to know what that means. The 2002 AASHTO Roadside Design Guide(18) describes an aesthetic barrier to be a barrier that harmonizes with the nat- ural environment. This definition is clearly appropriate where the natural environment provides a strong visual presence, but the definition offers little when dealing with urban contexts. A general definition of aesthetic barrier that has emerged and has been suggested in the literature is “anything different from what is now used.” This is a reaction of course to the common safety shape barrier or other smooth concrete barrier designs. This definition, however, offers no guidance on what makes a barrier “aesthetic.” For that, more objective criteria are required. A starting place is with the characterization of the common traffic barrier. The common concrete barrier is probably considered non- aesthetic based simply on its unadorned, utilitarian character. This is characterized by: • Uniformity of line. Line typically infers linear direc- tionality in the context of a barrier. Line is found in the edges of surfaces, shapes, or patterns, but most promi- nently as the edge of the structure that is silhouetted against the background. Unchanging lines over long distances can become static and boring. • Uniformity of profile. Profile is the shape of the barrier. Typically this is a form constantly repeated throughout the length of the barrier. A consistent profile can also become boring, particularly if it is a very simple form. • Uniformity of surface. A uniform surface can also be- come static and boring over long distance simply due to its plainness. • Lack of color. White in the context of most highway structures is not a color but rather may be considered an absence of color. This includes the lighter shades of gray.

Typical concrete barriers are simple in shape, featureless, repetitive, and utilitarian in appearance. For most viewers, an aesthetically treated barrier will be different in some or all of these categories. The selection of one alternative aesthetic treatment over another will still be a subjective process. How- ever, an objective set of aesthetic criteria can consist of line, profile, surface, and color. CHANGING THE AESTHETICS OF A LONGITUDINAL CONCRETE TRAFFIC BARRIER Concrete traffic barriers are linear elements by design, and this linear character cannot be fundamentally altered. Improv- ing the aesthetic character means that methodologies must be found to add interest to the structure without compromising the functionality of the structure. Interest may be achieved by modulating the linear character of the barrier. Modula- tion may be defined as a change in rhythmical measure. In terms of a linear structure we may take this to mean the change in the amplitude, frequency, or intensity of the line, color, pattern, or form. The combination of these aspects will do one of two things: • Reinforce the linear character of the structure. Intro- duce linear lines that parallel the edge of the structure or use a short, consistent sequence or repeating pattern. • Lessen the linear character of the structure. Segment the rail by varying the height, introducing vertical lines, or using long, discontinuous patterns. Because barriers are experienced while the viewer is in motion, the structure is experienced as a thing with a begin- ning and end, perceived over a period of time. This temporal aspect means that the structure is experienced as a pattern of both rhythm and sequence. The common concrete rail has an aesthetic character composed of a singular, boring rhythm and a lack of any sense of sequence. Therefore, creating an aesthetic barrier rail means creating a pleasing rhythm and sequence to the time in which a rail is experienced. DESIGN TECHNIQUES Rhythm (i.e., the frequency of a repeated pattern) and sequence (i.e., the segmenting of distance and/or time) can be achieved through the following techniques: • Create contrasting surface reflectivity by varying the amount of light reflected from a surface. • Create a balanced discontinuity by introducing random- ness into a line, rhythm, sequence, or pattern. • Create an interesting pattern by using contrasting sur- face reliefs, textures, or colors to create vertical, hori- zontal, curvilinear, or angular shapes. Modifying the surface of a barrier wall entails introducing a different surface coating or deforming the surface itself. 17 Surface coatings can be cementitious coatings or pigmented coatings (i.e., paints and stains). Deforming a surface can be accomplished by sandblasting to change the color and reflectivity of a surface, or parts of a surface can be recessed or be made to protrude from the pri- mary surface. Receding or protruding surfaces are perceived by the shadow contrast created by their edges. Surface reflec- tivity and shadow contrast are the ways in which patterns are perceived. It may be possible to communicate patterns through the contrast in reflectivity brought about by small changes in surface angles. Adding aesthetic treatments to the interior of concrete bar- riers will entail the addition of vertical edges (i.e., lines per- pendicular to the line of the barrier) into the face of the bar- rier. Research and experience clearly confirm that almost any edge that is part of a surface perpendicular to the direction of traffic can negatively influence vehicle impact. The size of the relative change in the surface determines whether or not it may snag some part of an impacting vehicle. The issue, then, is the degree to which surface reflectivity and patterns can be introduced into a rail surface without negatively affect- ing impact performance. The key question is “how can vertical edges and other sur- face discontinuities be safely introduced into a rail surface design?” FACTORS AFFECTING DESIGN Parameters have already been established that greatly influ- ence the search for alternative barrier and rail designs. Three of the most critical parameters are (1) adequate rail height; (2) the need for a continuous solid surface (either rounded or flat) of adequate contact surface area at the point of impact; and (3) the absence or protection of any vertical edge that will snag a vehicle. The dimensions may vary, but a representative example is shown in Figure 10. Of critical note is the 356-mm impact sur- face (and its 330-mm vertical location) and the 115-mm set- back of the post. The post is set back from the face of the rail to prevent snagging of the vehicle tire/wheel. Snagging can lead to excessive occupant compartment deformation, high longitudinal deceleration, and/or vehicle instability. This example reflects the minimum considerations for a new barrier design for either concrete or steel materials. Aesthetic elements such as pipes and decorative forms can be added to this form as long as these basic parameters are respected. Vertical openings in a concrete rail present serious prob- lems. The Texas T411 railing is frequently mentioned as hav- ing a very desirable aesthetic. A cross section of the T411 in plan view is shown in Figure 11. The T411 is rated for NCHRP Report 350 TL-2 and is not crashworthy for high- speed applications. Using this as a model for performance enhancement, the width of the openings may be narrowed. However, at some point this approach results in a series of “slits” rather than “windows” and, in effect, becomes a more

uniform surface. The T411 was modified to perform in accor- dance with NCHRP Report 350 TL-3 by providing a flat, smooth, vertical traffic face for a height of 457 mm along the lower portion of the barrier and forming the openings above that height. In the case of steel see-through rails, the most limiting fac- tors are (1) the allowable deformation of the horizontal ele- ment and (2) the exposure of the vertical supports to an impact- ing vehicle. The question becomes “how far back can the supports be placed and still achieve the requisite support for the horizontal elements?” The vertical opening distance or clear space between rail elements must be considered, as well as the size of the horizontal elements that define the contact area of the rail. An easy way to help ensure proper impact per- formance is to reduce the clear opening between the horizon- tal elements to a point that an impacting vehicle cannot reach the vertical support. At some point we approach an increas- ingly uniform surface, one for which concrete is a more suit- able material. 18 The design of see-through rails reaches a point where a key question must be posed: “How much investment for aesthetic purposes is appropriate for a railing that should attract little attention?” (It is supposed that the scene beyond is the reason for seeing past the railing.) There will be a point of diminish- ing returns in fashioning a see-through rail, given the con- textual issues of how they are seen. It may be possible to achieve acceptable visual access with small openings in the rail. Answering this question should be a major goal of the visual preference studies. An aspect not demonstrated in the graphic studies but clearly apparent in existing examples of concrete barriers is the issue of the shape and location of the view window. Concrete barri- ers such as the Texas T411 have vertical openings between deep posts. A contrast to this is the concrete post-and-beam (P&B) type of rail (e.g., Kansas Corral), which has a horizon- tal opening. The depth of the T411 openings makes it impos- sible to see through the rail unless the viewer is nearly perpen- dicular to the rail. Until then, on approach to the rail, the surface appears as a solid, although textured, barrier. The rail offers lit- tle in the way of functional visual access. The horizontal open- ing of the P&B rail, however, affords a wide, continuous view window that is easily discernable on approach to the rail. Even though the concrete beam provides a significant visual screen, the viewers’ eyes can easily connect the upper and lower scenes into an understandable image. APPLICATIONS A key question in aesthetic design is “how much visual impact will the barrier modification have and why?” The degree of visual impact will depend heavily on the visual prominence of the barrier relative to the background. How does this relationship affect the design of rails? To investigate this question, the researchers developed a series of barrier examples. These were used in a graphical study to explore the relationship between barrier and background. Four images of each study alternative are presented in four settings. The first is the barrier alone, the second is with a rural background, the third is with an urban background, and the fourth is with both a rural and urban background. The back- ground imagery is stylized to represent a visually complex urban backdrop and a simpler, flatter, rural scene. Since this study was to determine the effects of contrasting shapes, the graphics are prepared in shades of gray. The graphics show a true-to-scale (with the road) image of a 914-mm-tall, single- slope wall. The shoulder is 1.8 m wide and the travel lane is 3.7 m wide. The joints shown in the barrier are spaced 7.6 m. Alternative A—Untreated and Recessed Panels Alternative A1 is an untreated single-slope barrier (Fig- ure 12). Alternative A2 has recessed panels of contrasting color (Figure 13). Figure 10. Dimensions of typical bridge railings (in millimeters). Figure 11. Plan view of Texas T411 bridge rail cross section (in millimeters).

19 This study explores how much surface contrast is neces- sary to achieve the effects noted in Alternative A. A com- parison of the two barriers without background clearly indi- cates the dashed line reduces the apparent length of the rail. The effects found in the two backgrounds in Alternative A are the same as well. This suggests that it may be possible to achieve a significant visual effect while limiting the segment- imparting elements to the top portion of the rail. Alternative C—Arches Alternative C1 is a 7.6-m arch pattern (Figure 16). Alternative C2 is a 15.2-m arch pattern (Figure 17). The contrast between these two rails without the back- ground is significant. The untreated barrier reinforces the line of the roadway, while the segments of the panel design reduce this effect dramatically. In the rural set, A1 mimics the lin- earity of the background, while A2 appears more static by comparison. In the urban set, A1 stands in strong contrast to the numerous lines behind it, but A2 starts to blend with the background. In the combined rural/urban set, each rail appears more visually balanced with the background. Alternative B—Recessed Line Alternative B1 is a single, recessed line about 102 mm wide with a 13-mm-wide line above (Figure 14). Alternative B2 is a recessed line alternately broken into 7.6-m and 15.2-m segments (Figure 15). Figure 13. Alternative A2—recessed panels. Figure 12. Alternative A1—single-slope barrier.

This study was used to compare the effect of lengthening a continuous pattern, in this case an architectural pattern. The curves of the arches seem to be in character to the rounded forms of the rural background. The shorter version feels more “architectural” than does the long arch alternative. The longer pattern appears to be in higher contrast to the back- ground than the shorter version due to the reduced numbers of lines on the surface. However, each may tend to blend too much with the urban background. Alternative D—Copings Alternative B incorporated a design that added a contrast- ing detail near the top of the barrier. This created a more 20 prominent edge and caused the rail to be more distinct against all backgrounds. Copings, shown in Figure 18, may provide an economical technique to improving the look of a barrier without affecting impact properties of the structure. Alter- native D provides some coping options. Two (D1 and D2) are very simple; three (D3, D4, and D5) are more complex, with more edges. Alternative D1 is a coping without any additional surface treatment (Figure 19). Alternative D2 is a coping with an added protruding sur- face spaced at 15.2 m and with the same surface color as the barrier (see Figure 20). Alternative D3 is a contrasting color added to the protrud- ing surface (Figure 21). Figure 14. Alternative B1—recessed lines. Figure 15. Alternative B2—segmented line.

21 Figure 16. Alternative C1—7.6-m arch. Figure 17. Alternative C2—15.2-m arch. Figure 18. Alternative D—copings. Alternative D4 is a protruding surface lengthened and matching the barrier color (Figure 22). Alternative D5 is a contrasting color added to the protrud- ing surface (Figure 23). The effect of adding a spaced, protruding surface is the same as was found in alternatives A, B, and C. Alternative E—Open Metal Rail The rail shown in Figure 24 relies on collapsible steel pan- els that on impact would form a smooth, steel barrier. The see- through character of this design tends to blend the barrier with the background in both rural and urban settings. The effect is more pronounced in the urban example. Barriers that allow a lot of the background to be seen may run the risk of losing a necessary degree of visual prominence. The visual prominence of these types of barriers may be increased through the use of strong colors.

Alternative F—Open Tube Rail This rail, shown in Figure 25, exhibits the same character- istics as Alternative E. It is similar in concept to the widely used Wyoming Rail that features square tubing. The issue with railings that allow a lot of visibility through their structure is the question of “what is being seen?” The goal of any see- through barrier is to give visual prominence to the background scene. This suggests that the aesthetic character of these rail types is of less importance than that of solid barriers since we are intentionally making it less visible. If this is the case, the form of the rail may be less important than its finish. Alternative G—Concrete Post and Beam This alternative is a modified version of a typical concrete post-and-beam design. In this alternative, shown in Figure 26, 22 the face has been converted to a curved recess. The effect will add depth to the face by imparting subtle shading to the bar- rier face and may have some “directive” capabilities regard- ing impacting vehicles. This is a visually prominent barrier, but the small amount of open space beneath the beam makes it appear less massive. Even though there is little view shed available through the openings, it is enough to complete the lower portion of the view above the rail. Despite their small size, the openings also reduce some of the linear emphasis of the rail. Discontinuous Element Concepts Introducing discontinuous elements into the face of a bar- rier treats the barrier as a static unit over distance and time. A feature of some designs is that simple aesthetic elements Figure 19. Alternative D1—coping only. Figure 20. Alternative D2—coping with protruding surface.

are placed in only some or a few of the concrete barrier sec- tions through the length of the entire installation. Fewer aes- thetic sections permit costs to be lower. An example of this approach is to use key aesthetic elements to highlight the beginning and end of a bridge only and use a standard barrier shape between the aesthetic elements. An underlying premise about this approach is that barriers do not have to be a repeatable cross section. This opens other options for creating balance, uniqueness, and innovation into the design. This application may be most appropriate for shorter spans where the driver sees the entire length at once, such as spans found in urban areas. 23 VIEWER PREFERENCE SURVEY The viewer preference survey was performed in preparation for establishing the design guidelines for aesthetic concrete barriers and see-through bridge rails. The barriers selected for the study met fundamental aesthetic principles of attractive form, line, balance, and proportion and were studied in differ- ent background settings (i.e., rural and urban). It was noted in the study that most people respond favorably to a rail aesthetic if it is different than what they are used to seeing. Of course, this does not provide suitable guidance to designers in how to design an aesthetically pleasing rail. The Figure 21. Alternative D3—coping with protruding surface in contrasting color. Figure 22. Alternative D4—coping with lengthened protruding surface.

experience gained by the researchers indicates that most aes- thetic rail designs will meet with the favor of the general pub- lic. If it can be assumed that people will like a particular design, the questions, then, are “will people even notice the rail?” and “is there a preference for a particular rail or barrier design?” The researchers performed a viewer preference sur- vey using a controlled photographic evaluation process to aid in answering these questions. The goals of the survey were to assess the following: • Will people notice changes to a scene due to barrier design? 24 • To what degree does barrier design determine how a viewer feels about a scene? • Is barrier design likely to change the way a viewer feels about a scene? • Is there a preference for a particular barrier design? Barrier rails are perceived in a distance/time/setting frame- work. Modeling all these conditions to achieve a real-time test condition is economically prohibitive. The researchers believed that sufficient insight into viewer preferences could be gained through a static image survey. The survey attempts to identify the gross characteristics of rail design that are noticed and/or Figure 23. Alternative D5—coping with lengthened protruding surface in contrasting color. Figure 24. Alternative E—open metal rail.

preferred. The gross characteristics incorporated are the pre- viously discussed design techniques that modify rhythm and sequence impressions. The viewer preference photographic survey was structured as follows: • Five barrier/rail designs, shown in Figure 27, were used. Three of these were concrete, and two were steel see- through bridge rails. A plain, single-slope concrete bar- rier and W-beam guardrail were used as a control. Com- puter models of the rails were created and then inserted into photographs of rural and urban background scenes. This resulted in 30 scenes. • Two hundred and fifty randomly selected individuals were shown a series of three scenes in rapid succes- sion (2 to 3 seconds per scene) and then asked to rank the scenes in terms of their visual quality, from mem- ory. A descriptor term was provided to aid the respon- dent in describing the feeling or emotion sensed when the photograph of the barrier/rail and scene were viewed together. Sets of three different descriptors were used depending on the scene. The descriptor sets were: – Photograph Set 1—Rural 1 (Figure 28) ▪ Architectural feeling ▪ Rural feeling ▪ Interesting feeling – Photograph Set 2—Urban bridge (Figure 29) ▪ Upscale feeling 25 Figure 25. Alternative F—open tube rail. Figure 26. Alternative G—concrete post and beam.

26 Figure 27. Barrier/rail designs used in viewer preference survey. Figure 28. Photo Set 1—rural 1.

▪ Busy feeling ▪ Historic feeling – Photograph Sets 3 and 6—Rural and urban control sets, respectively (Figure 30) ▪ Scenic feeling ▪ High-speed feeling ▪ Boring feeling ▪ Congested feeling ▪ Typical feeling ▪ Stressful feeling – Photograph Set 4—Urban at grade (Figure 31) ▪ Historic feeling ▪ Common feeling ▪ Cluttered feeling – Photograph Set 5—Rural 2 (Figure 32) ▪ Country feeling ▪ City feeling ▪ Anywhere feeling The three scenes each contained different barrier/rail designs in different settings. The process of presenting photographic scenes to the respondent was repeated five times. Each time a different design and setting combination was presented to the respondent. Each design was tracked for its ranking in differ- ent settings and against different rail choices. Additionally, the participants were asked to rank the barriers and rails as to “best” and “worst” designs. Results of the Viewer Preference Survey The researchers found a preference among the respondents for the alternative designs over the common rail or barrier in all settings, but perhaps less so in complex urban backgrounds. Additionally, the researchers found smaller differences in pref- erences between the new alternatives themselves. It is hypo- thesized that much of the difference is due to the character of background influence. Regardless of the barrier or rail used, the urban setting consistently elicited a busy or cluttered response. In urban environments, with enormous amounts of background clutter, the barrier or rail had very little effect on the respon- dents, which suggests that providing an aesthetic application to the barrier is unwarranted. Responses were more positive to changing the aesthetics of the barrier in the rural settings, where the barriers were more prominent in the scene and did not compete for the viewer’s attention with other background images. With the exception of the W-beam guardrail, all the barrier designs had nearly the same effect on the rating of the scene. The aesthetic preference produced two components: the quality of the beauty and the quality of the experience. A significant bias regarding a particular alternative design may reflect a subjective bias on the part of the viewer or may be simply due to the barrier’s visual prominence (i.e., contrast) in a given setting. Designing for individual subjectivity will most probably be inconsistent in most cases. Of more value to the engineer or designer will be the question of how much 27 Figure 29. Photo Set 2—urban bridge.

28 Figure 30. Photo Sets 3 and 6—rural and urban control images. Figure 31. Photo Set 4—urban at grade.

visual prominence is necessary to achieve a noticeable change in the way a barrier or rail is perceived by the viewer. ASSESSMENT The study designs suggest that: • Background plays a significant and critical role in how the barrier is perceived. • The linear character of barrier rails can be significantly modified through the use of line and contrasting forms. • Relatively small elements or openings near the top or near the bottom of the barriers can significantly change the character of the barrier. • The aesthetic character of see-through rails should be secondary to the background scene. The degree that these findings can be applied is determined by established, critical parameters. AESTHETIC DESIGN DISCUSSION There can be a very large number of design variations for barriers and rails. The most important question to be answered in this and all other designs is “how significant of a surface change (either recessed or protruding) can be accomplished without affecting the behavior of impacting vehicles?” In all the graphic studies, the features that most affect impacting vehicles appear to be: • Depth and frequency of shadow forming by vertical ele- ments in the barrier face. • Thickness of protruding elements that form patterns in the barrier face. • The angle of the edges of shadow-producing elements. Determining these features will allow the development of detail-specific guidelines that, when coupled with context- sensitive design rules, can be applied to any alternative design in any situation. The previous sections of this report have provided informa- tion on the state of the practice regarding defining aesthetics for barriers and assessing and using aesthetic treatments on concrete barriers. This information was obtained by review- ing the literature and surveying state DOTs, roadside safety researchers, and crash-testing laboratories in the United States and internationally. The remainder of this report presents the methodology used for developing design guidelines for aes- thetic concrete safety shape barriers based on finite element simulation and full-scale crash testing of specific asperities cast into the face of concrete safety shape barriers. 29 Figure 32. Photo Set 5—rural 2.

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 554: Aesthetic Concrete Barrier Design provides guidance for the aesthetic treatment of concrete safety shape barriers.

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