National Academies Press: OpenBook

Permanent Signs Mounted on Median Barriers (2014)

Chapter: CHAPTER THREE Existing Sign and Barrier Combinations

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Suggested Citation:"CHAPTER THREE Existing Sign and Barrier Combinations." National Academies of Sciences, Engineering, and Medicine. 2014. Permanent Signs Mounted on Median Barriers. Washington, DC: The National Academies Press. doi: 10.17226/22344.
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Suggested Citation:"CHAPTER THREE Existing Sign and Barrier Combinations." National Academies of Sciences, Engineering, and Medicine. 2014. Permanent Signs Mounted on Median Barriers. Washington, DC: The National Academies Press. doi: 10.17226/22344.
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Suggested Citation:"CHAPTER THREE Existing Sign and Barrier Combinations." National Academies of Sciences, Engineering, and Medicine. 2014. Permanent Signs Mounted on Median Barriers. Washington, DC: The National Academies Press. doi: 10.17226/22344.
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Suggested Citation:"CHAPTER THREE Existing Sign and Barrier Combinations." National Academies of Sciences, Engineering, and Medicine. 2014. Permanent Signs Mounted on Median Barriers. Washington, DC: The National Academies Press. doi: 10.17226/22344.
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Suggested Citation:"CHAPTER THREE Existing Sign and Barrier Combinations." National Academies of Sciences, Engineering, and Medicine. 2014. Permanent Signs Mounted on Median Barriers. Washington, DC: The National Academies Press. doi: 10.17226/22344.
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Suggested Citation:"CHAPTER THREE Existing Sign and Barrier Combinations." National Academies of Sciences, Engineering, and Medicine. 2014. Permanent Signs Mounted on Median Barriers. Washington, DC: The National Academies Press. doi: 10.17226/22344.
×
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Suggested Citation:"CHAPTER THREE Existing Sign and Barrier Combinations." National Academies of Sciences, Engineering, and Medicine. 2014. Permanent Signs Mounted on Median Barriers. Washington, DC: The National Academies Press. doi: 10.17226/22344.
×
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Suggested Citation:"CHAPTER THREE Existing Sign and Barrier Combinations." National Academies of Sciences, Engineering, and Medicine. 2014. Permanent Signs Mounted on Median Barriers. Washington, DC: The National Academies Press. doi: 10.17226/22344.
×
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Suggested Citation:"CHAPTER THREE Existing Sign and Barrier Combinations." National Academies of Sciences, Engineering, and Medicine. 2014. Permanent Signs Mounted on Median Barriers. Washington, DC: The National Academies Press. doi: 10.17226/22344.
×
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Suggested Citation:"CHAPTER THREE Existing Sign and Barrier Combinations." National Academies of Sciences, Engineering, and Medicine. 2014. Permanent Signs Mounted on Median Barriers. Washington, DC: The National Academies Press. doi: 10.17226/22344.
×
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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.

12 CHAPTER THREE EXISTING SIGN AND BARRIER COMBINATIONS The primary objective of the synthesis study was to gather, compile, and disseminate information describing current practices for mounting permanent signs on rigid median barriers throughout the United States. A majority of the information was gathered from responses to a survey ques- tionnaire that was distributed to all voting members of the AASHTO Subcommittee on Design, which includes repre- sentation from all 50 state DOTs and the District of Colum- bia DOT. In addition, invitations to participate in the survey were sent to 11 toll road authorities that manage turnpikes and other divided highway networks. The survey question- naire is provided in Appendix A. A total of 51 completed responses to the survey question- naire were received: 46 from state DOTs and five from toll road authorities. Because the response rate from toll road authorities (five of 11 surveyed) was low for statistical sig- nificance, some of the data analyses apply only to DOT responses. Corresponding data received from the toll road authorities are discussed in the commentary. The information received varied greatly among the agen- cies. One agency reported having no barrier-mounted sign installations at all, while several reported having more than 1,300 within their jurisdictions. Many agencies provided access to standard plans, specifications, photos, and other information related to their practices. These materials and the questionnaire answers have been reviewed and com- piled, and the results are summarized in this chapter. RIGID BARRIER TYPES AND USAGE The vast majority of rigid median barrier designs are con- structed of reinforced concrete, with varying height, width, and cross-sectional shape. Because this study focuses on high-speed, median-divided highways, only TL-3 and higher rated rigid barriers are included in the data-gathering process. For this range of test levels, four basic cross-sectional bar- rier shapes are in widespread use: New Jersey safety-shape, F-shape, single-slope, and vertical wall. These shapes have been in use for many years, and their geometric details are available in AASHTO publications (AASHTO 1995, 2011). For median applications, the minimum width at the base is typically 24 in. (610 mm); however, the width can be increased as needed to provide increased protection for objects in the ZOI or to provide increased capacity for impact loading. Many agencies have set a standard width to meet the needs of their highway networks. Where extra-wide median bar- rier is needed, typical practice is to install back-to-back half- sections with earth backfill and a concrete cap between the sections. Figure 2b offers a general view of this configuration. The minimum nominal height for all four basic barrier shapes is 32 in. (813 mm) for TL-3/TL-4 ratings. Tradition- ally, this allows for up to 3 in. (76 mm) of pavement overlay adjacent to the barrier, making the overall minimum height 29 in. (737 mm) (AASHTO 2011). Raising the barrier section height is the primary means of increasing capacity. Forty-two inch (1,067 mm) high concrete barrier has been established as the minimum nominal height that satisfies TL-5 impact con- ditions. A number of ultra-high-capacity barriers have been developed to satisfy specific agency needs. Higher barriers are also used to provide a glare screen to block light from oncoming traffic. Figure 4 shows results from Question 2 of the survey, which asked agencies to identify all types of rigid median barriers currently in use in their divided highway network. New Jersey (NJ) and F-shape (FS) are the most common shapes, followed by single-slope (SS) and then vertical wall (VW). The most common nominal barrier heights are 32 in. (813 mm) and 42 in. (1,067 mm). The data do not include the quantity or mileage of each system installed in the United States; however, they indicate the distribution of quantities currently in service. For rigid barriers, height is the primary factor that affects ZOI for TL-3 and higher impact conditions. In general, higher barriers reduce vehicle roll during an impact and thus reduce the size of the ZOI by limiting overhang of the upper portions of the vehicle over the barrier. The shape of the barrier’s face has some effect on ZOI, and this effect likely varies with overall height. However, definitive relationships between barrier shape and ZOI have not yet been established. OVERHEAD SIGN SUPPORTS Figure 1 showed the typical configurations of overhead sign supports on divided highways. For the purpose of obtain-

13 ing information on median-barrier-mounted applications, the survey questionnaire did not differentiate between span/ bridge and cantilever supports. Question 3 of the survey asked agencies approximately how many existing overhead sign supports, mounted within or on top of rigid median bar- riers, are installed throughout their agency’s total divided highway network. The intent was to provide a general indi- cation of quantities rather than precise values, as the level of detail in roadway inventory systems varies considerably among agencies. In addition, there is a wide range in the size of divided highway networks among the agencies. Table 2 shows the results from the 46 DOTs that responded to the survey questionnaire. Twenty-one of the 46 DOTs (45.7%) and one of the five toll road authorities reported having at least 100 overhead sign supports mounted within or on top of rigid median barriers. FIGURE 4 Rigid median barrier usage by height and shape. Key: Number—Nominal barrier height in inches NJ—New Jersey safety-shape FS—F-shape SS—Single slope VW—Vertical wall TABLE 2 NUMBER OF EXISTING OVERHEAD SIGN SUPPORTS MOUNTED ON RIGID MEDIAN BARRIERS No. of Supports in DOTs Total Divided Highway Network No. of DOTs Percent of Total DOTs 300+ 12 26.1 100–299 9 19.6 10–99 11 23.9 Less than 10 6 13.0 None 1 2.2 Unknown 7 15.2 Thirty-two of the 51 total agencies that responded reported availability of either standard or project-specific construction plans. These plans have some common fea- tures, but details vary considerably. A majority of agencies use round pipe supports, and a majority of those use a rela- tively large single-pipe support mounted within or on top of the barrier. Figure 5a shows photos of large single-pipe supports. Other round pipe designs use two pipe supports connected by lighter diagonal members to form the basis for a truss support structure. The truss support creates a larger ZOI in the longitudinal direction because it consists of two rigid supports spaced a short distance apart in the longitu- dinal direction. Figure 5b shows photos of truss configu- rations. Structural square tube sections are less common; examples are shown in Figure 5c. A review of plans submitted with the survey question- naire shows that approximately half of the agencies build foundations for overhead sign supports as an integral com- ponent of the barrier, which is formed to match the barrier profile on each side of the support. The remaining agencies construct the support foundation in between precast bar- rier half-sections, which transition to standard barrier over tapered sections on both longitudinal sides of the support. Flare rates for the tapered sections are typically a minimum of 1:20; several agencies specify 1:30 or more gradual taper rates. Widening the median barrier section in the vicinity of fixed objects is one method of reducing exposure within the ZOI, whether the widening is required to accommodate the foundation or done solely for safety improvement. Question 8 of the survey questionnaire listed six design characteristics that can be used to improve the safety perfor- mance of overhead median-barrier-mounted sign supports and asked the agencies which they have used for installations in their highway networks. Respondents were asked to check all that apply. Table 3 presents the results from the 46 DOTs that responded to this question. Responses from the five toll road authorities that com- pleted this question followed a similar distribution—lateral widening of the barrier cross-section in the vicinity of a sign support was the most common design characteristic incorporated into installations within their jurisdictions. Several agencies commented that lateral widening is often required to accommodate foundations for overhead sup- ports, and safety benefits were not the primary motive. A review of the plans that were submitted shows that the mini- mum lateral offset distance from the top face of the barrier to the outside edge of the support ranges from 0 in. (0 mm) to 18 in. (457 mm). Two agencies specify a minimum offset distance of 16 in. (406 mm) and 18 in. (457 mm), specifi- cally to place the support farther away from the face of the barrier to improve ZOI impact performance. Some respon- dents noted that widening the barrier also reduces the inside shoulder width, which may be a concern in very narrow medians. Designers often have to weigh benefits and con- sequences of conflicting treatments.

14 Ten agencies reported vertical tapering of the top of the barrier profile in the vicinity of the sign support. This treat- ment is more applicable to barriers less than 42 in. (1,067 mm) in height. One agency noted that it uses this treatment at locations where existing conditions do not allow widening of the barrier. Another agency uses the practice in conjunc- FIGURE 5 Examples of typical overhead sign supports from survey responses.

15 Five agencies reported attaching metal beam rail to the traffic face of the barrier in the vicinity of sign supports to reduce vehicle climb during impact and thereby reduce the size of the ZOI. This treatment is shown in Chapter 6, Figure 6-12 of the AASHTO Roadside Design Guide and is used extensively in at least one state to increase protection against impacting bridge piers that are immediately behind rigid barriers. Its use at sign support locations is more limited, although five agencies have reported using it. One agency noted that this treatment is in its toolbox of options but has not yet been installed in the jurisdiction. Another agency reported placing precast barrier sections on each longitudi- nal side of a support, then bridging the gap with metal beam rail attached on the lateral sides of the barrier. However, this was an isolated case. Twenty-seven agencies reported eliminating the need for new sign supports by changing their location and mounting them on existing structures. This is a preferred treatment, and one agency noted that it is included in the analysis before installing any support in the median. Several agencies stated that they are phasing out median-barrier-mounted overhead sign supports unless no other option is available. Many pre- fer to span the entire roadway (both directions) with an over- head support where possible, as shown in Figure 1a. The “Other” category was checked by four agencies, and they described combinations of raising the barrier, wid- ening, and transitioning to a different barrier type in the vicinity of overhead sign supports. In the “None” category, comments primarily related to not allowing any median-bar- rier-mounted sign supports or not needing special treatments because the standard barrier is expected to be high enough to sufficiently reduce the ZOI. SMALL BARRIER-MOUNTED SIGN SUPPORTS Typical configurations of small barrier-mounted sign sup- ports on divided highways were shown in Figure 2. To obtain information on median-barrier-mounted applica- tions, the survey questionnaire did not differentiate among regulatory, advisory, guide, and mile marker sign functions. The primary criterion for including signs in this category is that they must be supported by a separate vertical support or post. Very small, lightweight, self-supported panels— such as some mile markers and barrier delineators—are not included because they are far less likely to affect crash per- formance of the host barrier. Panel sizes for this category of sign fall into the range of 1.0 ft2 (0.1 m2) to 49 ft2 (4.5 m2). Question 11 asked agencies approximately how many permanent, small sign supports mounted on top of rigid median barriers are installed throughout their total divided highway network. The intent was to elicit a general indica- tion of quantities rather than precise values, as the level of tion with the next treatment in the list: transitioning to a different type of barrier in the vicinity of sign supports. For appropriate conditions, this agency transitions from 32-in. (813-mm) high F-shape to 54-in. (1,372-mm) high vertical wall over a length of 15 ft (4.6 m). The barrier remains 24 in. (610 mm) wide, with zero offset between the face of the barrier and the edge of the support base plate, because it is located 54 in. (1,372 mm) above the pavement surface. Six- teen agencies reported using transitions to a different type of barrier at overhead sign support locations. TABLE 3 DESIGN CHARACTERISTICS USED TO IMPROVE SAFETY PERFORMANCE OF MEDIAN BARRIER-MOUNTED OVERHEAD SIGN SUPPORTS Method and Description No. of DOTs Percent of Total DOTs LATERAL WIDENING OF THE BARRIER CROSS-SECTION IN THE VICINITY OF THE SIGN SUPPORT: This treatment is typically used to move the support laterally farther from the traf- fic face of the barrier to remove it from, or reduce the severity of its position within, the zone of intrusion (ZOI). 30 65.2 ELIMINATING THE NEED FOR A SIGN SUPPORT BY MOVING THE SIGN LOCATION SUCH THAT IT IS SUPPORTED BY AN EXISTING STRUCTURE (BRIDGE, LUMINAIRE, OTHER SIGN SUPPORT, ETC.): 24 52.2 TRANSITIONING TO A DIFFERENT TYPE OF BARRIER IN THE VICIN- ITY OF THE SIGN SUPPORT: For example, transitioning from safety-shape barrier to vertical wall at the support loca- tion, and then back to safety shape. This treatment is also intended to reduce the ZOI at the support location. 15 32.6 VERTICAL TAPERING OF THE TOP OF THE BARRIER PROFILE IN THE VICINITY OF THE SIGN SUP- PORT: This treatment gradually raises the height of the barrier on both sides of the sign support in an effort to reduce the ZOI of an impacting vehicle in the vicin- ity of the support. 9 19.6 MODIFYING THE BARRIER BY ATTACHING A METAL BEAM RAIL TO THE TRAFFIC FACE OF THE BARRIER IN THE VICINITY OF THE SIGN SUPPORT: This type of treatment is also intended to reduce the ZOI at the support location. 5 10.9 OTHER: Treatments that have been incorporated into existing installations that are intended to improve safety per- formance of a barrier/sign support installation. 3 6.5 NONE or UNKNOWN: No known treatments have been incorporated into existing installations that are intended to improve safety performance of a barrier/ sign support installation. 9 19.6

16 2. Saddle-style base plate that is bent to straddle the top of the barrier with various anchor bolt configu- rations: horizontal bolts that extend through the bar- rier, with nuts on both sides; horizontal expansion or adhesive anchor bolts extending into the barrier from both sides; or a combination of horizontal and vertical anchor bolts. Component size ranges and typical sizes are provided here: Base plate thickness: 1/4 in. (6.3 mm) to 3/4 in. (19 mm); 1/2 in. (15.9 mm) typical. Base plate width at top: 6 in. (152 mm) to 8 in. (203 mm); 6 in. (152 mm) typical. Base plate length: 6 in. (152 mm) to 38 in. (965 mm). Round pipe support: 2.5 in. Schedule 40 to 4 in. Schedule 80. Square tube support: 2 in. x 2 in. Perforated to 6 in. x 4 in. x 3/8 in. Structural tube. Figure 6 shows typical details of primary configurations used for small sign supports. Figure 6a shows the flat base plate type with the post welded directly to it, while Figure 6b shows this type with a sleeve welded to the base plate. This is common for relatively light perforated square tube supports, in which the post is inserted into the sleeve and secured with bolts that extend through the post and sleeve. Figure 6c shows general details of a saddle-style base plate that wraps over the top of the barrier and is secured with hor- izontal bolts. Some of the larger saddle-style bases include vertical anchor bolts that attach the top of the saddle to the barrier in addition to horizontal bolts. A majority of agen- cies that provided plans specify the use of chemical adhesive anchors for vertical bolts that secure base plates to existing barriers. Adhesive anchor bolt diameters range from 1/2 in. (13 mm) to 1 in. (25 mm), and adhesive specifications typi- cally reference those they have in place for a variety of other applications. In general, adhesive anchors must be installed in strict conformance with the manufacturer’s recommenda- tions, and they are intended to develop the full strength of the steel bolt material that is specified. Less reliable expan- sion or wedge anchors are used by some agencies for applica- tions using 1/2 in. (13 mm) to 3/4 in. (19 mm) anchor bolts for generally smaller signs compared with those using adhe- sive anchor bolts. Material sizes of the base plate, post, and anchor bolts vary significantly, depending on the size and height of the supported sign panel. The perforated sleeve and post design shown in Figure 6b is used for smaller and lower signs, which include mile markers. Larger and higher signs use one of the base types shown in Figure 6, a and c. All the agencies that provided details of their supports use steel posts, and detail in roadway inventory systems varies considerably among agencies. Table 4 shows the results from the 46 DOTs that responded to the survey questionnaire. There is far less certainty about the number of small barrier- mounted signs compared with overhead sign supports. Because of the large number of small signs, they are more likely to be overlooked in roadway inventory systems, whereas overhead signs are more likely to be cataloged and inspected on a regular basis. Thirteen of the 46 DOTs (28.3%) and two of the five toll road authorities reported having at least 100 small sign supports mounted on top of rigid median barriers. Two of the five toll road authorities reported having fewer than 10, and one reported having between 10 and 99. TABLE 4 NUMBER OF EXISTING SMALL SIGN SUPPORTS MOUNTED ON RIGID MEDIAN BARRIERS No. of Supports in Agency’s Total Divided Highway Network No. of DOTs Percent of Total DOTs 1,000+ 6 13.0 500–999 3 6.5 100–499 4 8.7 10–99 5 10.9 Less than 10 2 4.3 None 7 15.2 Unknown 19 41.3 Twenty-six of the 51 agencies that responded made their standard or project-specific construction plans available. Review of the drawings showed that details of supports vary considerably among agencies; however, they generally fall into two primary categories: 1. Flat base plate mounted to the top horizontal sur- face of the median barrier with vertical anchor bolts. Physical sizes of support components vary consider- ably among the agencies that responded to the survey questionnaire. Component size ranges and typical sizes are provided here: Base plate thickness: 1/4 in. (6.3 mm) to 1 in. (25.4 mm); 5/8 in. (15.9 mm) typical. Base plate width: 4 in. (102 mm) to 7 in. (178 mm); 6 in. (152 mm) typical. Base plate length: 6 in. (152 mm) to 28 in. (711 mm). Round pipe support: 2.5 in. Schedule 40 to 4 in. Schedule 40. Square tube support: 1.75 in. x 1.75 in. Perforated to 4 in. x 4 in. x 3/8 in. Structural tube.

17 FIGURE 6 Typical details of small sign supports mounted on rigid median barriers.

18 the types and sizes of posts cover a wide range, as shown in Table 5. U-channel and perforated square tubes are the lightest, and the weight range progresses up through vari- ous structural shapes. The heaviest post identified is a 6 in. x 4 in. x 3/8 in. structural square tube; it is used to support a 49 ft2 (4.5 m2) sign panel with a 14 ft (4.3 m) minimum mounting height, measured from the pavement surface to the bottom of the panel. TABLE 5 RANGE IN SIZE OF SUPPORTS FOR SMALL SIGNS MOUNTED ON RIGID MEDIAN BARRIERS Support Description Weight per Unit Length (lb/ft) Mass per Unit Length (kg/m) 2-lb U-Channel 2.00 2.98 1.75" x 1.75" Perforated Square Tube 2.09 3.11 2" x 2" Perforated Square Tube 2.44 3.63 3-lb U-Channel 3.00 4.48 2.5" x 2.5" Perforated Square Tube 3.14 4.69 3" x 3" x 1/8" Structural Square Tube 4.75 7.07 2.5" Schedule 40 Round Pipe 5.80 8.63 3" Schedule 40 Round Pipe 7.58 11.28 4" x 3" x 3/16" Structural Square Tube 8.15 12.12 W6 x 9 I-Beam 9.00 13.39 3.5" Schedule 40 Round Pipe 9.12 13.57 4" Schedule 40 Round Pipe 10.79 16.05 W6 x 12 I-Beam 12.00 17.85 4" Schedule 80 Round Pipe 14.98 22.28 4" x 4" x 3/8" Structural Square Tube 17.27 25.69 6" x 4" x 3/8" Structural Square Tube 22.37 33.28 Figures 7 and 8 show examples of various sizes and config- urations of small sign supports mounted on rigid median bar- riers. These examples represent some of the post sizes shown in Table 5. Heavier posts are shown in Figure 7, and examples of significantly lighter perforated square tube supports are shown in Figure 8. Because the perforated square tube sup- ports are typically used for mile markers, they are found in more existing installations throughout the United States than the nonperforated post types. When mile markers are placed every 0.1 mi (0.16 km) along a highway section, the number of installations adds up to significantly greater numbers than other types of signs that normally have larger panels. When signs are mounted within the barrier ZOI, post weight and structural characteristics of the base anchorage may have an effect on impact response. A heavier post with a more substantial anchorage design is more likely (though not certain) to behave as a fixed object in response to passenger car impacts. In such cases, the primary concern is the risk of injury for occupants of the impacting vehicle if it snags on the support. Lighter posts may fully or partially yield during a ZOI impact and may or may not completely detach from the barrier. In these cases, risk to occupants of the impacting vehicle may be reduced; however, debris from the detached sign may pose a risk to adjacent traffic. Whether the support remains fixed or yields during an impact is influenced by both the structural integrity of the support and the impact condi- tions (vehicle size and weight, speed, and impact angle). Question 16 of the survey questionnaire listed 10 design characteristics that can be used to improve the safety perfor- mance of small median-barrier-mounted sign supports and asked which ones the agencies have used in their highway networks. Respondents were asked to check all that apply. Table 6 shows the results from the 46 DOTs. The survey shows that fewer of these treatments are used for small sign supports compared with those used for overhead supports. Treatments that require modifications to the barrier at the sign location are typically expensive and impractical to be incorporated at a large number of small sign locations. At least one agency indicated that it has a barrier design that is wider at the top than its standard design, specifically for use where barrier-mounted signs will be installed in the future. However, this agency noted that it discourages the practice of mounting objects on top of barriers unless there are no other options. Using a wider barrier also allows the use of wider sign panels while still conforming to the MUTCD recom- mendation that sign panels be no wider than the barrier. Ten agencies reported the use of breakaway supports for barrier-mounted signs; however, none of the plans provided in response to the questionnaire showed details of break- away supports on top of barriers. The photo in Figure 7a is the only item received that shows a breakaway support for this application. Some agencies reported making relatively minor modifi- cations to supports with the intent of improving impact safety performance. Removing sharp edges and reducing the projec- tion height of anchor bolts can reduce snagging of an impact- ing vehicle. Crash testing has shown that some horizontal anchor bolts used in saddle bases project off the side face of the barrier far enough to cut into and tear vehicle bodies dur- ing an impact event (Caldwell 2011). Six agencies reported extra strengthening of the support to reduce the likelihood that it would be dislodged. This practice could reduce the number of designs an agency needs to detail and maintain, by having one support design for a wide range of panel sizes. Nineteen agencies reported eliminating the need for new sign supports by moving the signs and mounting them on existing structures nearby. This is a preferred treatment. One agency noted that it attaches signs to luminaire poles on or

19 between median barrier sections to eliminate the need for additional supports. Several agencies stated that they either prohibit or are phasing out median-barrier-mounted sign supports unless no other option is available. Five agencies checked the “Other” category. Three stated that they raise the mounting height of sign panels to reduce the likeli- hood of their being struck by trucks. One stated that it specifies a minimum mounting height of 14 ft (4.3 m) in its standards. In FIGURE 7 Examples of small sign supports from survey responses.

20 the “None” category, comments primarily related to not need- ing special treatments because the standard barrier is presumed to be high enough to sufficiently reduce the ZOI or the agency simply does not allow median-barrier-mounted sign supports. The five toll road authorities that responded to the sur- vey questionnaire reported using some of the same practices used by DOTs. Three of the five reported that their pre- ferred practice for both overhead and small sign supports FIGURE 8 Examples of small sign supports from survey responses.

21 is to eliminate the need for barrier-mounted sign supports by moving the signs. Two toll road authorities reported hav- ing 500 to 999 small sign supports installed on rigid median barriers within their jurisdiction. Although details were not provided, it is assumed that these are primarily mile marker signs. Two toll road authorities reported that they use sign- ing standards from their state DOT and follow the same practices for addressing the ZOI. TABLE 6 DESIGN CHARACTERISTICS USED TO IMPROVE SAFETY PERFORMANCE OF SMALL MEDIAN BARRIER-MOUNTED SIGN SUPPORTS Method and Description No. of DOTs Percent of Total DOTs ELIMINATING THE NEED FOR A SIGN SUPPORT BY MOVING THE SIGN LOCATION SUCH THAT IT IS SUPPORTED BY AN EXISTING STRUCTURE (BRIDGE, LUMINAIRE, OTHER SIGN SUPPORT, ETC.): 17 37.0 LATERAL WIDENING OF THE BARRIER CROSS-SECTION IN THE VICINITY OF THE SIGN SUPPORT: This treatment is typically used to move the support laterally farther from the traffic face of the barrier to remove it from, or reduce the severity of its position within, the zone of intrusion (ZOI). 11 23.9 USE OF BREAKAWAY SIGN SUPPORTS: Although this treatment is not typically recommended for barrier-mounted signs, there may be locations for which a designer determines that the benefits outweighed the risks for a unique situation. 10 21.7 VERTICAL TAPERING OF THE TOP OF THE BARRIER PROFILE IN THE VICINITY OF THE SIGN SUPPORT: This treatment gradually raises the height of the barrier on both sides of the sign support in an effort to reduce the ZOI of an impacting vehicle in the vicinity of the support. 6 13.0 TRANSITIONING TO A DIFFERENT TYPE OF BARRIER IN THE VICINITY OF THE SIGN SUPPORT: For example, transitioning from safety-shape barrier to vertical wall at the support location, and then back to safety shape. This treatment is also intended to reduce the ZOI at the support location. 6 13.0 LIMIT PROJECTION HEIGHT OF ANCHOR BOLTS: This treatment may be used to reduce snagging of the impacting vehicle, which may reduce the severity of the primary impact, as well as reduce the likelihood of the sign structure becoming dislodged. 5 10.9 EXTRA STRENGTHENING OF THE SIGN SUPPORT BEYOND THAT REQUIRED TO RESIST WIND LOADS: This treatment may be used to reduce the likelihood of the sign structure becoming dislodged during a ZOI impact, and therefore reduce the potential for creating secondary hazards. 5 10.9 CHAMFER OR ROUND OFF SHARP EDGES OF THE SIGN SUPPORT: This treatment may be used to reduce snagging of the impacting vehicle, which may reduce the severity of the primary impact, as well as reduce the likelihood of the sign structure becoming dislodged. 2 4.3 MODIFYING THE BARRIER BY ATTACHING A METAL BEAM RAIL TO THE TRAFFIC FACE OF THE BARRIER IN THE VICINITY OF THE SIGN SUPPORT: This type of treatment is also intended to reduce the ZOI at the support location. 1 2.2 OTHER: Treatments that have been incorporated into existing installations that are intended to improve safety performance of a barrier/sign support installation. 5 10.9 NONE or UNKNOWN: No known treatments have been incorporated into existing installations that are intended to improve safety performance of a barrier/sign support installation. 16 34.8

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TRB’s National Cooperative Highway Research Program (NCHRP) Synthesis 465: Permanent Signs Mounted on Median Barriers reports on the current state of practice for mounting permanent highway signs on top of rigid median barriers throughout the United States.

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