National Academies Press: OpenBook

Permanent Signs Mounted on Median Barriers (2014)

Chapter: CHAPTER TWO Sign and Barrier Design Criteria and Guidelines

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Suggested Citation:"CHAPTER TWO Sign and Barrier Design Criteria and Guidelines." 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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Page 10
Page 11
Suggested Citation:"CHAPTER TWO Sign and Barrier Design Criteria and Guidelines." National Academies of Sciences, Engineering, and Medicine. 2014. Permanent Signs Mounted on Median Barriers. Washington, DC: The National Academies Press. doi: 10.17226/22344.
×
Page 11
Page 12
Suggested Citation:"CHAPTER TWO Sign and Barrier Design Criteria and Guidelines." National Academies of Sciences, Engineering, and Medicine. 2014. Permanent Signs Mounted on Median Barriers. Washington, DC: The National Academies Press. doi: 10.17226/22344.
×
Page 12
Page 13
Suggested Citation:"CHAPTER TWO Sign and Barrier Design Criteria and Guidelines." 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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8 CHAPTER TWO SIGN AND BARRIER DESIGN CRITERIA AND GUIDELINES Issues related to mounting signs on rigid median barriers are discussed, to varying degrees, in a number of pertinent national highway design specifications, policies, manu- als, and guidelines. These documents provide a wealth of information to highway designers and other practitioners, and promote sound and consistent application of the latest technology throughout the national transportation infra- structure. In general, their contents have developed over many years and are revised on a regular basis in the form of supplements and new editions. This evolutionary process ensures that the latest editions incorporate recent research results and evaluations from users in the field. On the national level, the use and placement of highway signs is addressed extensively in the following documents: • FHWA Manual of Uniform Traffic Control Devices (MUTCD) (FHWA 2009) • AASHTO Roadside Design Guide (RDG) (AASHTO 2011) • AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals (LTS) (AASHTO 2009b). These documents are valuable references for highway designers and provide a foundation for individual transpor- tation agencies to develop policies that meet their needs. The following is a brief overview of the sections of the docu- ments that relate to the use of permanent signs mounted on rigid median barriers. SIGN USAGE AND PLACEMENT The MUTCD is the national standard for all traffic control devices, including signs, installed on any roadway open to public travel in the United States (FHWA 2009). The 2009 edition sets minimum standards and provides guidance on practices intended to ensure uniformity across the nation. Part 2 addresses signs, which are classified by function; the primary categories are regulatory, warning, and guide signs. All three classes of signs may be mounted on either overhead or roadside supports. Overhead signs are typically used on multilane divided highways where some degree of lane-use control is needed and in locations where adequate space is not available on the roadside. If a sign cannot be mounted on a grade-separa- tion structure (bridge), a separate support structure, such as those shown in Figure 1, is required. The minimum height of an overhead sign panel is 17 ft (5.2 m), measured from the pavement surface to the bottom of the sign. The minimum lateral offset from the edge of the shoulder (or, if no shoulder exists, from the edge of the pavement) to the near edge of the vertical support is 6 ft (1.8 m) (FHWA 2009). Because breakaway or yielding supports are not applicable to over- head sign supports, they must be shielded with a longitu- dinal barrier (crash cushion) if they are located within the roadway clear zone (FHWA 2009; AASHTO 2009b, 2011). Roadside signs should be located on the right-hand side of the roadway where they are easily recognized and under- stood by road users (FHWA 2009). However, standardiza- tion of sign position cannot always be attained in practice; in some circumstances, it is necessary to place signs on the left-hand side of a travel way. These circumstances include the following: 1. To enhance conspicuity by placing identical signs on both sides of the roadway. 2. On some curves to the right, where sign visibility on the left-hand side is significantly better than that the right-hand side. 3. On multilane roads, where traffic in a lane to the right might obstruct the driver’s view to the right. 4. On left exit ramps. 5. At median openings. 6. At the beginning of divided highway sections. 7. For preferential and managed lanes; for example, high-occupancy vehicle (HOV) lanes. When any of these circumstances occur on a divided highway section with a rigid median barrier, it is usually necessary to mount the sign on top of the barrier. In some cases, small signs can be mounted on an existing feature, such as a luminaire pole or an overhead sign support; how-

9 ever, the availability of existing structures at desired place- ment locations is typically very limited (AASHTO 2011). It is not recommended to place ground-mounted break- away supports on each side of a median barrier, creating a two-post sign that straddles the barrier. This configura- tion inhibits the proper function of both the barrier and the breakaway support because of the unpredictable interac- tion of these systems when struck by a vehicle. In addition, components of the sign that are detached by an impact can impose a secondary hazard to traffic traveling on either side of the barrier. It is also not recommended to install breakaway supports on top of median barriers. This issue is addressed on the FHWA Roadway Departure Safety website (FHWA 2013) as follows: Q: Should we use breakaway bases for sign and light poles mounted on concrete median barriers? A. No, breakaway bases should not be used. Mounting any pole on top of a median barrier should be avoided because trucks will lean over the barrier upon impact and hit what- ever is on top. A rigid pole may or may not break off, but there is no safety advantage in making it easier for the pole to break away and fly into the opposing travel lanes. The potential for a pole being struck by the box of the truck can be minimized by making the barrier wider. If you transition to a vertical face and/or taper the width of the barrier, you can provide additional offset to the pole. The point is to minimize the potential for broken poles to fly into the opposite roadway. Larger passenger vehicles as well as their occupants may contact objects on top of barriers under severe impact conditions. The use of small median-barrier-mounted signs is generally limited to locations where they are critical for safety. These signs may include those in the regulatory and warning classes, and they are typically recommended only where the safety benefit of the sign outweighs its potential hazard. For example, the following common signs might meet these criteria: • “No U Turn” and other signs used for barrier openings. • Preferential and managed lane signs; for example, for HOV lanes. • “Reduce Speed Ahead.” • “Slippery When Wet.” • Chevrons. • Post-supported mile markers. Panel sizes for median-barrier-mounted signs are limited by lateral clearances to other highway elements. The MUTCD provides standards and guidelines for placement of road- side signs; however, those specifically applicable to barrier- mounted signs are limited. This issue is addressed primarily in Chapter 2G, Preferential and Managed Lane Signs (FHWA 2009). The following are pertinent excerpts from this chapter: Section 2G.03 Regulatory Signs for Preferential Lanes— General, and also appears in Section 2G.08, Warning Signs on Median Barriers for Preferential Lanes: Guidance: The edges of Preferential Lane regulatory (and Warning) signs that are post-mounted on a median barrier should not project beyond the outer edges of the barrier, including in areas where lateral clearance is limited. Option: Where lateral clearance is limited, Preferential Lane regulatory (and Warning) signs that are post-mounted on a median barrier and that are 72 inches or less in width may be skewed up to 45 degrees in order to fit within the bar- rier width or may be mounted higher, such that the vertical clearance to the bottom of the sign, light fixture, or struc- tural support, whichever is lowest, is not less than 14 feet above any portion of the pavement and shoulders. Standard: Where lateral clearance is limited, Preferential Lane regulatory (and Warning) signs that are post-mounted on a median barrier and that are wider than 72 inches shall be mounted with a vertical clearance that complies with the provisions of Section 2A.18 for overhead mounting. Although these provisions do not apply to all barrier- mounted signs, they provide general guidance for panel width and height restrictions that are unique to this applica- tion. The following three general recommendations can be summarized from the available sources: 1. Panel edges are to be located as far as possible from travel lanes. 2. The sign panel may not be wider than the width of the barrier, unless it is skewed or raised to a minimum height of 14 ft (4.3 m). 3. Panels wider than 72 in. (1.8 m) must be mounted at the same height as overhead signs. TYPES OF RIGID MEDIAN BARRIERS AND THE ZONE OF INTRUSION A variety of rigid median barrier designs are in widespread use throughout the United States. Many of the designs have remained essentially unchanged for many years, while oth- ers have been developed more recently. Current crash per- formance criteria for barriers are contained in the AASHTO Manual for Assessing Safety Hardware (MASH) (AASHTO 2009a). However, a majority of the barriers in service today have been crash-tested and accepted in accordance with

10 NCHRP Report 350 (Ross et al. 1993; FHWA 2013). Indi- vidual barrier designs meet specific test levels in the crash performance criteria, which define the level of containment provided by the barrier system. In general, the higher test levels contain and redirect heavier vehicles. The more com- mon rigid median barriers have passed NCHRP Report 350 Test Level 4 (TL-4) or TL-5 (AASHTO 2011). Table 1 identifies some of the more common rigid median barrier designs and shows their overall height and test level. TABLE 1 COMMON RIGID MEDIAN BARRIER DESIGNS Barrier Description Height (mm) Test Levela F-Shape Median Barrier, Designation SGM10a 32 [813] TL-4 F-Shape Median Barrier, Designation SGM10b 42 [1,067] TL-5 NJ-Shape Median Barrier, Designation SGM11a 32 [813] TL-4b NJ-Shape Median Barrier, Designation SGM11b 42 [1,067] TL-5 Tall-Wall Median Barrier, Designation SGM12 42 [1,067] TL-5 Single-Slope Median Barrier 32 [813] TL-4 Single-Slope Median Barrier 42 [1,067] TL-5 Vertical Wall Median Barrier 32 [813] TL-4 aIn accordance with NCHRP Report 350 and/or AASHTO MASH testing criteria. b32-in. (813-mm) NJ-shape barrier failed TL-4 testing per AASHTO MASH criteria (AASHTO 2011). Barriers that satisfy TL-4 test conditions can contain and redirect vehicles up to and including a single-unit box truck weighing approximately 17,650 lbs (8000 kg), traveling at 50 mph (80 kph), and impacting the barrier at 15 degrees. Bar- riers that satisfy TL-5 test conditions are able to contain and redirect vehicles up to and including a tractor-van trailer truck weighing approximately 79,400 lbs (36,000 kg), traveling at 50 mph (80 kph), and impacting the barrier at 15 degrees. For more details on impact testing criteria, see NCHRP Report 350 (Ross 1993) and MASH (AASHTO 2009a). Measured from the surface of the adjacent pavement to the top of the barrier, nominal height values include a provi- sion for a 3-in. (76-mm) pavement overlay. Because overall height is critical to a barrier’s structural capacity and perfor- mance, its height may be extended if thick overlays are antici- pated or increased vehicle containment capacity is desired at existing installations. Height extensions must be structurally connected to the host barrier. In addition to higher structural capacity, taller barriers reduce the ZOI, which is defined as the region measured above and beyond (behind) the face of the barrier system where an impacting vehicle or any major part of the system may extend during an impact (AASHTO 2011). If an object is located in the ZOI, it will be struck by part of an impacting vehicle if the impact point on the barrier is in the vicinity of the object. ZOI is analogous to the lateral distance behind a flexible or semi-rigid barrier, defined by the lateral dynamic deflection that occurs during a standardized crash into the barrier. For these systems, the lateral distance defines the “working width” that is needed for the barrier to function; placement of fixed objects within the working width behind the barrier is generally not recommended. Figure 3 illustrates the general configuration of the ZOI for a 32-in. (813-mm) high safety-shape barrier and TL-3 and TL-4 impact conditions (AASHTO 2011). However, these are based on limited crash test and research data. The amount of intrusion above and behind a barrier system is directly related to its height and profile, as well as the vehicle’s size, speed, and angle of impact (AASHTO 2011). For a particular barrier design, truck impacts have significantly larger ZOI areas than those created by passenger vehicle impacts. TL-4 and TL-5 barriers may have some level of ZOI for passen- ger vehicle impacts, as well. For example, a 32-in. (813-mm) high (or higher) TL-4 barrier may have a significant ZOI for TL-3-level passenger vehicle impacts. This may become a concern if sign supports are mounted on the barrier and posi- tioned within its ZOI for a wide range of impact conditions that are likely to occur at the installation location. FIGURE 3 Zone of intrusion (ZOI) for 32-in. sloped-face concrete barrier [adapted from AASHTO (2011)].

11 In many cases, the effective test level of the barrier is questionable in the vicinity of a sign support that is mounted within a ZOI. For example, a normally rated TL-4 barrier may effectively be reduced to TL-2 where a sign is mounted within the ZOI, because the support could cause failure of standard TL-3 and TL-4 tests at that location along the barrier. This is a particular concern when performance is reduced for passenger car impacts, which are more common events and potentially cause greater occupant risk. Because barriers are not typically crash-tested with mounted sign supports, the degradation in performance caused by the sign is often overlooked. Additionally, detached elements of the sign could cause failure of a standardized crash test, as this is an evaluation criterion that considers potential for secondary hazards caused by debris flying into adjacent traffic lanes. Overhead signs require substantially larger and heavier support structures owing to the heavier loads they must bear. They are considered fixed objects, because they are not likely to be displaced even with the most extreme impact-loading conditions. Small signs vary considerably in size and mount- ing height, so the size and weight of their supports typically vary to match the magnitude of environmental loading. On the high end of the range, supports that carry relatively large signs may also act as true fixed objects during typical impact conditions. On the small end of the range, supports are less robust and may deform, deflect, or detach during a vehicle impact, even if they are not designed as a breakaway feature. Although not included in the scope of this synthesis study, luminaire and equipment support poles are often mounted on top of rigid median barriers and may present similar concerns for ZOI impacts. As with signs, this type of installation should not use breakaway supports because of the risk a downed pole might present to opposing traffic (AASHTO 2011). Because all barrier-mounted features have potential for reducing impact performance, consideration of crashworthiness during the design process may help mini- mize adverse effects associated with barrier attachments. TECHNIQUES FOR REDUCING ZONE OF INTRUSION EXPOSURE This section includes practices that attempt to reduce the consequences of impacts involving structural supports located within a barrier’s ZOI. Some of these techniques are described in the national design guidelines cited earlier; oth- ers were identified in responses to the survey questionnaire. Techniques vary from relatively minor incidental treatments to extensive modifications of the barrier in the vicinity of sign supports. They may apply to either overhead or small sign sup- ports with varying degrees of practicality and effectiveness. • Eliminate the need for a separate sign support by locating the sign where it can be supported by an existing bridge structure, luminaire pole, or other sign support. Where possible, this treatment is preferred and encouraged in the AASHTO RDG; it reduces the number, and thus exposure, of supports located in the ZOI (AASHTO 2011). • Widen the barrier cross-section laterally in the vicin- ity of the sign support. This treatment has the effect of placing the support farther away from the traffic face of the barrier and removing it from the ZOI, or at least reducing the extent to which it impinges on the ZOI. Recommended maximum flare rates may be followed on both approach sides where the barrier transitions from its normal width to the widened section (AASHTO 2011). • Vertically taper the top of the barrier profile in the vicinity of the sign support. This treatment was identified from responses to the survey questionnaire. It gradually raises the height of the barrier on both sides of the sign support to reduce the ZOI of a vehicle impacting from either direction. • Transition to a different type of barrier in the vicin- ity of the sign support. For example, transition from safety-shape barrier to vertical wall at the support loca- tion and then back to safety-shape on both sides. This treatment is also intended to reduce the size of the ZOI at the support location. Vertical wall barriers have a reduced ZOI compared with other barriers of the same overall height (AASHTO 2011). • Modify the barrier by attaching a metal beam rail to the traffic face in the vicinity of the sign support. As shown in Chapter 6, Figure 6-12 of the AASHTO RDG, the New York State DOT designed and tested a box beam attached near the top of the upper face of a typical safety-shape barrier to limit vehicle climb and improve performance (Phillips and Bryden 1984; AASHTO 2011). • Chamfer or round off sharp edges of the sign sup- port. This treatment was identified from responses to the survey questionnaire; it can be used to reduce the potential for snagging of the impacting vehicle. Reduced snagging may help reduce the severity of the primary impact as well as the likelihood that the sign structure will become dislodged and present a second- ary hazard to adjacent traffic. • Limit projection height of sign support anchor bolts. This treatment can also be used to reduce the potential for snagging of the impacting vehicle, reducing the severity of the primary impact and the likelihood that the sign structure will become dislodged and present a secondary hazard to adjacent traffic (Caldwell 2011). • Provide extra strengthening of the sign support beyond that required to resist wind loads. This treatment was identified from responses to the survey questionnaire and applies primarily to small sign sup- ports. It can be used to reduce the likelihood of the sign becoming dislodged during a ZOI impact and creating secondary hazards to adjacent traffic.

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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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