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8 CHAPTER 2. LITERATURE REVIEW Finding solutions for non-herbicide VMSs has been a challenge for transportation agencies. The management of vegetation at key locations such as guardrails, cable barriers, luminaire supports and poles, and other roadside appurtenances involves the safety of the traveling public and maintenance personnel, environmental impacts, roadside aesthetics, and budget constraints. A review of the available literature and an internet search gathered data regarding the usage of VMSs. This chapter covers research and DOT practices regarding the use of VMSs as designated by the project. The information presented provides identification and guidance for the selection of non- herbicide, long-term VMSs for roadsides and roadside appurtenances. The VMSs identified and presented do not present specific design guidance for highway safety appurtenances, nor are they a substitute for any other highway design practice. The user should refer to the AASHTO Roadside Design Guide (RDG), AASHTO MASH, and any specific state DOT practices for warrants, proper placement, and maintenance of roadside safety appurtenances when applying these VMSs (AASHTO 2011b, 2016). In addition, before applying any of the techniques described on a proprietary roadside safety hardware device (e.g., guardrail terminal, crash cushion, or breakaway sign support), the manufacturer should be contacted to discuss the potential for the treatment to adversely affect the performance of the manufacturerâs safety hardware device. The project team reviewed the 2011 AASHTO Guidelines for Vegetation Management (AASHTO 2011a), which lists numerous VMSs. Each has advantages and disadvantages for roadside and median applications. Much of the information contained in Chapter 12 of the AASHTO Guidelines for Vegetation Management came from the California Department of Transportationâs (Caltransâs) comprehensive website called the Roadside Management Toolbox. The current version of the Roadside Management Toolbox can be accessed on the Caltrans website (Caltrans 2017a). As a general statement, all VMSs are subject to surface accumulation of soil, seeds, and debris and may require spot treatments to control unwanted vegetation and debris removal. Textured surfaces may allow for more accumulation. However, the use of VMSs significantly reduces the need for recurring maintenance activities and herbicide treatments. The use of VMSs improves both worker and driver safety by minimizing worker exposure on the roadsides for recurring maintenance and by reducing traffic diversions and delays for maintenance activities. The VMS locations examined in this study and defined in Table 1 are: ⢠Guardrail ⢠Cable barrier ⢠Support posts and poles ⢠Edge of pavement ⢠Gore/median ⢠Mow edge/strip ⢠Slope/embankment
9 The VMSs have three basic categories that include impervious surfaces, pervious surfaces, and select vegetation establishment. VMSs are designed to cover the designated area and minimize maintenance activities, particularly adjacent to the travel lanes. Impervious surface VMSs are enduring and very effective but can be more expensive to install. Potential problem areas associated with impervious materials used as a VMS (i.e., concrete, asphalt, and others) are the leave-out areas or post deflection spaces around posts and other fixed objects in proximity to or around roadway safety devices that may accumulate unwanted vegetation. Nonetheless, leave- outs may be required for some VMS use locations due to the effect that the rigidity of the VMS material may have on altering the performance when placed around roadway safety devices. Pervious surface VMSs allow for stormwater runoff infiltration. Pervious surface VMSs are designed to block sunlight to inhibit plant growth. Some pervious surface VMSs may use a geosynthetic fabric (herbicide or non-herbicide treated) to inhibit root growth placed underneath the VMS. Several of the pervious VMS choices are advantageous for retrofit applications. The third category involves the establishment of specific plant material in designated areas. The vegetation used around roadside appurtenances should have a low mature height, require minimal maintenance, and be able to out-compete weed growth. This vegetation can be native or non-native and irrigated or non-irrigated depending on the site-specific requirements. VMSS AND PERFORMANCE OF HIGHWAY SAFETY APPURTENANCES Highway safety appurtenances reduce the severity of an errant motorist leaving the roadway. Many features of the roadway and roadside influence the ability of the safety device to perform properly. These features may include but are not limited to the slopes and ditches parallel to the roadside; discontinuities in the travel surface; the presence of other fixed objects; safety devices within the clear zone such as signs, luminaires, delineators, curbs, and sidewalks; and soil moisture. In addition, the geometric alignment of the roadway can affect the performance of safety devices located off and along the roadway. The roadway and roadside are an operating environment, and their performance has a symbiotic relationship with their features. The designer should always be cognizant of the effect that changes may have on the function of each of the individual components within the roadway environment. In addition, before applying any of the VMS techniques described herein to a proprietary roadside safety hardware device (e.g., guardrail terminal, crash cushion, or breakaway sign support), the manufacturer should be contacted to discuss if and how the treatment might adversely affect the performance of the manufacturerâs safety hardware device. The effects that VMSs could potentially have on the performance of roadside safety appurtenances are presented in the following sections. Appendix E provides more information regarding roadside safety. Roadside safety hardware design continues to evolve, and the testing of safety systems is an active area of research. Designers must stay abreast of the research and published MASH Tests to evaluate the safety performance of any configuration of roadside safety appurtenances and VMSs.
10 MASH provides the following five categories or classes of highway safety appurtenances: 1. Longitudinal barriers 2. Terminals and crash cushions 3. Truck- and trailer-mounted attenuators and portable work zone traffic control trailers 4. Support structures, work zone traffic control devices, breakaway utility poles, and longitudinal channelizers 5. Roadside geometric features and pavement discontinuities Longitudinal Barriers Longitudinal barriers include roadside, median and temporary barriers, and bridge rails and transitions. Longitudinal barriers are all designed to contain, shield, or redirect an errant vehicle from a roadside hazard. Longitudinal barriers may be additionally divided into categories of rigid, semi-rigid, and flexible that are based on the stiffness/deflection characteristics when impacted. When barriers of different stiffness are joined, they are connected via a transition section of barrier. Transitions are commonly found where, for example, a rigid concrete bridge rail is joined to a semi-rigid W-beam guardrail. The performance of longitudinal barriers depends on their specific design features. All longitudinal barriers are designed for engagement of the errant vehicle within specific vehicle center-of-gravity (CG), weight, and impact angle envelopes along the barrierâs height profile. For example, a concrete safety shape barrier has a specific geometric profile that interacts with the bumper and CG of the vehicle. Any change in the height at which the vehicle engages the barrier could alter the barrierâs ability to safely redirect an impacting vehicle. A VMS should not alter the acceptable construction height tolerances of the barrier or eliminate or modify a feature of the geometric profile of the barrier (e.g., eliminate the toe of a concrete barrier or raise or lower a rail height), thus raising or lowering the vehicle impact point or the way an errant vehicle engages the barrier. Rigid and semi-rigid barrier performance depends on the structural stiffness and lateral displacement characteristics to perform properly. A rigid barrier does not experience lateral displacement during impact and should remain rigidly affixed to its foundation and the roadway surface. Rigid barriers are typically concrete barriers (e.g., concrete safety shape and vertical concrete wall). Use of VMSs around such barriers is acceptable provided the treatment does not extend above the pavement surface and does not alter the height of the barrier. Converse to the rigid barrier, the semi-rigid barrier does displace and rotate when impacted, and those specific characteristics should also not be changed. Examples of semi-rigid barriers are portable concrete barriers of various lengths and connection types and W-beam and thrie beam guardrail. The semi-rigid portable concrete barriers may be installed in temporary or permanent applications. Their performance is specific to the length of the barrier and the connection between barrier segments. The displacement and rotation characteristics of the semi-rigid concrete barrier are unique to the specific design configuration. A VMS applied to a portable semi-rigid concrete barrier should not alter the barrier installationâs ability to displace laterally or rotate upon impact. Any VMS treatment should be passive in nature with regard to weight and anchorage and not add to or subtract from the original design anchorage and connection performance for portable semi-rigid concrete barriers. Additionally, the designer should be
11 cognizant of potential effects between a VMS that is placed on the non-traffic side of the barrier and any other safety feature that is installed behind the barrier. For example, pavers placed between a barrier and a signpost will have some mass and/or rigidity that can be displaced by an errant vehicle impact. The pavers can potentially restrain the movement of the barrier, affecting its safety performance, and/or can be displaced into a slip base sign support, activating the sign support. Metal beam guardrail/guard fence (e.g., W-beam and thrie beam) can be configured as a semi- rigid or flexible longitudinal barrier. Within this category of metal beam longitudinal barriers, the rail element may be supported by either strong posts (e.g., W6x8.5 or W6x9) to provide a semi-rigid barrier or weak posts (S3x5.7) to provide a flexible barrier. In MASH strong post W-beam or thrie beam installations, the posts are typically steel W6x8.5 or W6x9 posts that are augured and backfilled or driven into the soil. The proper lateral displacement and rotation of the posts reduce the chances of rail rupture and are critical to the proper operation of the barrier when impacted. The AASHTO RDG and FHWA eligibility letters provide information about VMSs, such as mow strips, rock formations, and other rigid structures that are acceptable for use with these barriers. Additional information from the AASHTO RDG regarding strong post installation in mow strips and similar installations is presented hereafter. Mow edges, also referred to as mow strips by AASHTO, prevent vegetation growth several feet around guardrail installations, including cable barriers, W-beam guardrail, guardrail transitions, and guardrail end treatments. W-beam guardrail posts, guardrail transition posts, and guardrail end treatment posts are treated equally with regard to the application of VMSs. However, a VMS should not be applied to any proprietary guardrail end treatment without first consulting with the product manufacturer. Mow strips are typically asphaltic or concrete pavement and vary in thickness from several inches up to 200 mm (8 inches) maximum. Strong post W-beam guardrail posts in mow strips and rock formations face similar problems regarding facilitating rotation of the strong posts. The AASHTO RDG provides guidance regarding the use of mow strips and posts in rock for strong post W-beam guardrail systems. This information is based on research and crash testing conducted under NCHRP Report 350 (Ross et al. 1993). More recent research and full-scale MASH testing have been conducted for strong post W-beam guardrail in mow strips with leave- outs, the portion of the mow strip omitted around the base of the post to allow for post rotation. The 31-inch W-beam guardrail system with steel posts in a concrete mow strip performed acceptably for both MASH Tests 3-10 and 3-11, and therefore the steel post W-beam system in a concrete mow strip is considered acceptable for MASH Test Level 3 (TL-3) longitudinal barrier (Sheikh et al. 2019). In this design, the critical measurement of the leave-out installation is from the back of the post to the edge of the mow strip; this measurement should be a minimum of 175 mm (7 inches). Wood post W-beam guardrail did not meet MASH TL-3 safety requirements in all cases. Standard strong post guardrail with 6-inch by 8-inch rectangular wood posts and 7½-inch-diameter round wood posts embedded 40 inches both failed to meet the MASH TL-3 criteria when installed in mow strips with leave-outs (Sheikh et al. 2019, Bligh et al. 2020). However, a modified 7½-inch-diameter round wood post system with 36-inch embedment (Moran et al. 2020) was full-scale crash tested by the Texas Department of Transportation (TxDOT) in a mow strip with leave-outs and did meet MASH TL-3 safety requirements. A 6-inch by 8-inch rectangular wood post system with 35-inch embedment also met MASH TL-3
12 criteria (Moran et al. 2021). These recent findings suggest that leave-outs remain a viable method for use in mow strips with both steel and wood post W-beam guardrail systems. Wood post W-beam guardrail systems appear to be more sensitive to the use of mow strips, and care should be taken to ensure the proper post configuration and embedment depth. As previously discussed, MASH testing is an active area of research, and design evolution is occurring. As a result, the RDG is out of date for some applications. Designers should always check that a mow strip design has been evaluated in accordance with MASH. For illustrative purposes, Figure 2 shows the details from Figures 5-52(a) and 5-52(b) of the AASHTO RDG (AASHTO 2011b). Leave-outs can be filled with low-strength grout, two-part polyethylene foam, or other material that has a compressive strength of 0.85 MPa (120 psi) or less. During an impact, the leave-out material allows for some degree of post rotation by deforming or crushing prior to generating sufficient force to cause post failure. Failure of the sacrificial leave-out backfill material also minimizes damage to the surrounding mow strip. Figure 2. AASHTO RDG Guardrail Post Detail in Mow Strip Application (AASHTO 2011b). Posts in high-tension cable barriers and other weak post (e.g., S3x5.7 steel post) guardrail systems do not need a leave-out in the mow strip. Weak post barrier systems do not rely on the displacement and rotation of the post, like a strong post system does, to perform successfully. During an impact, weak posts bend at or near the groundline and thus may be more rigidly constrained by the applied VMS. For strong steel post W-beam guardrail posts installed in asphalt or concrete surfacing that is thicker than 200 mm (8 inches), Figure 3 shows AASHTO RDG Figures 5-5l (a) and 5-5l (b) for installation in rock formations (AASHTO 2011b). For these installations, the backfill around the posts is typically a coarse aggregate material. In some locations, it may be beneficial to seal the surface with an asphaltic crack sealant or other similar material to reduce water infiltration.
13 Figure 3. Guardrail Post Details in Rock Formation (AASHTO 2011b). A strong steel post W-beam guardrail system installed in simulated rocky terrain following the guidance provided in Figure 3 was successfully tested to MASH TL-3 (Bligh et al. 2019). A round wood post W-beam guardrail system tested in a similar configuration did not satisfy MASH criteria (Bligh et al. 2019). Terminals and Crash Cushions Terminals and crash cushions both serve similar functions, which is to reduce the severity of a motorist impacting the terminus of a longitudinal barrier, bridge pier, gore area, or other rigid or semi-rigid structure. Terminals shield the end of a longitudinal barrier and reduce the impact severity of an errant vehicle striking the end of the barrier by absorbing the kinetic energy of the vehicle, permitting the vehicle to gate behind the barrier, or redirecting the vehicle if the barrier is struck along its side. Likewise, crash cushions perform in much the same way by attenuating the crash energy of the impact. However, not all crash cushions are re-directive if impacted along their side. MASH-tested terminals and crash cushions are all proprietary to date. As previously presented, before applying any of the VMS techniques described herein to a proprietary roadside safety hardware device (e.g., guardrail terminal, crash cushion, or breakaway sign support), the manufacturer should be contacted to discuss if and how the treatment might adversely affect the performance of the manufacturerâs safety hardware device. VMS treatment may be applied to the area leading to and around the safety device to the extent the treatment does not create discontinuities or a change in elevation in or around the approach and departure areas of the device. Much like longitudinal barriers, crash cushions and terminals are designed and tested for engagement of an errant vehicle within specific vehicle CG, weight, and impact angle envelopes along the safety deviceâs height profile. Curbs and VMS treatments that create lift or upset the vehicle and its suspension when struck should not be introduced in and around these devices unless crash tested and accepted for use in the tested (as-implemented) configuration.
14 Truck- and Trailer-Mounted Attenuators and Portable Work Zone Traffic Control Trailers Truck- and trailer-mounted attenuators (TMAs) and work zone traffic control trailers are portable devices that are normally deployed in temporary applications. TMAs are used to shield stationary or slow-moving trucks that are used in maintenance and work zones. Portable work zone traffic control trailers are also used in maintenance and work zones and commonly parked along the roadside. Work zone trailers are normally equipped with variable message signs and arrow boards. These devices will not be addressed with regard to the application of VMSs because they are portable and mobile in nature and are moved either continuously or frequently. Support Structures, Work Zone Traffic Control Devices, Breakaway Utility Poles, and Longitudinal Channelizers Support structures include sign supports, luminaire supports, emergency call box supports, and road closure gates. These supports, along with work zone traffic control devices and breakaway utility poles, all use some form of a vertical structural support that poses a potential hazard if an errant vehicle impacts it. Additionally, these devices commonly use a breakaway feature (e.g., slip base) or are fabricated of a frangible or yielding material that permits the support structure to yield, displace, or be frangible (e.g., frangible cast aluminum luminaire base) when impacted and thus reduce the severity of a vehicle impact. The AASHTO RDG provides examples of these types of breakaway devices. Figures 4, 5, and 6 from the AASHTO RDG show examples of a small sign support slip base, a frangible aluminum luminaire base, and a luminaire attached to a frangible coupling base, respectively. As the figures show, all the foundations in these examples are rigidly anchored to the ground and are designed not to move to facilitate activation of their safety feature. The term breakaway refers to all supports that are designed to incorporate a feature that permits it to yield, fracture, or separate from its ground anchor or foundation when impacted by a vehicle. When an impact to a support occurs by an errant vehicle, the support and what is attached to the support such as a sign panel or luminaire, for example, should yield to the vehicle. Depending on the design, the vehicle will either pass over the installation, or the system will release and separate from its foundation and rotate over the vehicle without causing excessive deformation or intrusion into the occupant compartment. The support and its attachments rotating over the vehicle should make as little, if any, contact as possible with the windshield and roof. The weight of the support, the features attached to the support, and the location where they attach vertically and horizontally on the support all play a vital role in the proper operation of the safety device when impacted. Therefore, any modifications to the safety device can alter its performance. When a VMS is applied to one of these devices, it is necessary that the treatment does not infringe on the operation of the device when impacted. In the case of a slip-base-type sign support (Figure 4), the applied VMS must not interfere with the ability of the support to displace the bolts clamping the support to the ground stub anchor plate or interfere with the ability of the support to rotate away from the ground stub anchor plate when impacted.
15 Figure 4. Multidirectional Slip Base for Small Signs (AASHTO 2011b). If a luminaire base is being evaluated for use with a VMS, the designer should be cognizant of not changing the interaction height between the errant vehicle and the frangible base or couplings. This statement also applies to slip base supports. Slip base, cast aluminum frangible luminaire base (Figure 5), and other types of breakaway sign support systems (Figure 6) may be sensitive to the impact height of the vehicle bumper. The typical automobile bumper height is approximately 51 mm (20 inches). Slip base supports are designed to activate in shear and not in bending stress. Slip base supports and other breakaway safety devices may not activate properly if the impacting height of the vehicle is affected by design elements such as super-elevation, slopes, and ditches. Foreslopes of 1V:10H or flatter between the roadway and the support help ensure consistent approach conditions and vehicle interaction height. The acceptable foreslope is limited to 1V:6H or flatter between the roadway and the support if the vehicle bumper height can interact with the support at the appropriate height. In addition, sign mounting height is typically 2.1 m (7 feet) to the bottom of the sign. This height, whether it is 2.1 m (7 feet) or some other tested height, is often critical to the performance of the support when struck by an errant vehicle and subsequently the way the support and its attached components rotate over or interact with the vehicle. The preceding statements also apply to luminaire supports that are currently up to 18.3 m (60 feet) tall. In summary, when applying a VMS to a support structure, the designer must avoid interfering with the activation and release of the system for it to perform properly. This means keeping the applied VMS from obstructing the displacement of bolts from slip bases, being in the path of the rotating support after it released from its foundation or anchor, or significantly altering the impact height of the errant vehicle by altering the effective ground height around the structure and the vehicle approach to the structure.
16 Figure 5. Example of a Cast Aluminum Frangible Luminaire (AASHTO 2011b). Figure 6. Example of a Frangible Coupling Design (AASHTO 2011b). Roadside Geometric Features and Pavement Discontinuities As presented in MASH, roadside geometric features and pavement discontinuities are any feature of the roadside that deviates from a flat surface, such as ditches, curbs, embankments, driveways, depressed or elevated medians, drainage structures, and rock cuts. These features should be designed to be traversable by errant vehicles. The AASHTO RDG provides some guidance for the designer for these features. If applying a VMS to these features, the designer should consult the RDG to ensure the geometry of the feature is not adversely changed. For example, by adding a VMS, the slope, any slope rounding present, or the continuity of the surface should not alter from its designed parameters. This means the addition of a VMS should not make a traversable slope non-traversable or create other discontinuities. In addition, as previously discussed, the VMS should not change the approach height or grading leading to and surrounding a highway safety appurtenance such that it would alter the way an impacting vehicle engages with the safety feature. Summary VMSs applied in and around highway safety appurtenances should consider their effect on the performance of everything in the highway design environment. If a VMS is thought to possibly influence the performance, then consideration should be given to crash testing the VMS and
17 safety appurtenance together as a system. As of January 1, 2011, all newly developed hardware must be tested following MASH criteria. Of particular interest to the application of VMSs, FHWA also issued a memorandum dated January 7, 2016, regarding the federal-aid eligibility of highway safety hardware. The following applies to VMSs (FHWA 2016): ⢠FHWA will no longer issue eligibility letters for highway safety hardware that has not been successfully crash tested to the 2016 edition of MASH. ⢠Modifications of eligible highway safety hardware must use criteria in the 2016 edition of MASH for reevaluation and/or retesting. ⢠Non-significant modifications of eligible hardware that have a positive or inconsequential effect on safety performance may continue to be evaluated using finite element analysis using simulation models. LEAVE-OUTS Strong post (both wood and steel posts) W-beam guardrail is designed to absorb some crash energy through post rotation in the soil. Mow strips or other forms of VMSs can restrict this rotation and adversely affect the impact performance of the guardrail system. A properly designed leave-out area in the VMS around a post permits the required rotation. The leave-out area allows roadside appurtenances such as guardrail to perform properly by permitting the posts to move upon impact without encountering the surrounding, more rigid material surface and enables easier maintenance and repair of the guardrail. These leave-out areas can accumulate litter, debris, sand from winter operations, and windblown soils. Seeds deposited in leave-out areas can take root and create problems. Similar problems can be seen where the VMS abuts the edge of the pavement, as shown in Figure 7. To minimize unwanted vegetation, the leave-out areas are filled with a low-strength cover material to inhibit vegetation growth. Typical leave-out materials include grout, hand-stamped hot-mix asphalt, gravel, emulsified asphalt on gravel, spray foam, Styrofoam, or other similar impervious material. Figure 7. Weed Growth at the Edge of Pavement (Barton and Budischak 2013). Some of the first research performed to investigate the design of leave-outs is documented in a Midwest Roadside Safety Facility (MwRSF) report Development of Guidelines for Placement of Steel Guardrail Posts in Rock (Rohde 2003). MwRSF studied and documented post deflection characteristics in various embedment configurations using dynamic impact testing and simulation and thereafter developed a steel post W-beam guardrail system for installation in
18 rocky soils. The posts were installed in holes drilled in concrete, constructed by drilling three 203-mm diameter holes on 165-mm centers to a depth of 610 mm. The drilled holes were backfilled with compacted ASTM C33 coarse aggregate, size no. 57. The steel post W-beam guardrail system installed in rock-soil foundations was successfully tested to NCHRP Report 350 Test 3-11. During the same time, the Texas A&M Transportation Institute (TTI) examined the effect of pavement post encasement on the crashworthiness of strong post guardrail systems to determine the performance of these systems using dynamic impact testing, numerical simulation, and full- scale crash testing (Bligh et al. 2004). Mow strip dimensions, materials, and depths were considered in addition to the presence of leave-out sections around posts. Along with enhanced impact performance and vegetation management, it was suggested that mow strip configurations featuring leave-outs facilitate ease of repair after an impact. Crash tests of a steel post guardrail system and a round wood post guardrail system encased in a concrete mow strip with grout-filled leave-outs were successful performed following NCHRP Report 350 criteria. The Bligh et al. report provides recommendations regarding the acceptable ranges for some key mow strip parameters, like mow strip material and dimensions, leave-out dimensions, leave-out backfill material, and guardrail post location (Bligh et al. 2004). The research performed by MwRSF and TTI led FHWA to publish memorandum B-64-b, âW-Beam Guardrail Installations in Rock and in Mowing Stripsâ on March 10, 2004. The memorandum offers guidance developed from the research performed for installing W-beam installations in rock and mow strips. The AASHTO Task Force for Roadside Safety reviewed the research and later included the research findings in the 2011 AASHTO RDG. The RDG information was developed under NCHRP Report 350 guidance. FHWAâs âFrequently Asked Questions: Barriers, Terminals, Transitions, Attenuator, and Bridge Railingsâ currently provides direction to the B-64-b memorandum for guidance (FHWA 2020). Research conducted by Arrington et al. focused on testing alternative materials for the leave-out sections around guardrail posts encased in a pavement mow strip (Arrington et al. 2009). A two- sack grout mixture that had been successfully crash tested was used as a baseline reference for acceptable impact performance. Static laboratory and dynamic impact testing was conducted to evaluate the various products. The long-term durability of these products was not evaluated. The products that were investigated include a two-part urethane foam, a molded rubber product that has an insert fabricated to match the size of the leave-out, a flat recycled rubber mat that rests on top of a leave-out backfilled with soil, and a pop-out concrete wedge. All of the products except the flat rubber mat are considered to have acceptable impact performance. The acceleration levels associated with the flat rubber mat significantly exceeded the baseline threshold established from the test results of the two-sack grout mixture. The compacted soil confined within the leave-out was responsible for the higher acceleration. The other tested products were acceptable alternatives for the two-sack grout mix from an impact performance and vegetation control standpoint. However, the advantages and disadvantages regarding cost, availability, ease of installation, and inspection should be considered before selecting a product. A 2019 study evaluated the MASH performance of a 31-inch-tall W-beam guardrail system with steel and wood posts installed in a concrete mow strip (Sheikh et al. 2019). The 31-inch W-beam
19 guardrail system with steel posts in a concrete mow strip performed acceptably for both MASH Tests 3-10 and 3-11. The steel post W-beam guardrail system in a concrete mow strip is considered to have acceptable performance in accordance with the criteria for MASH TL-3 longitudinal barriers. The 31-inch W-beam guardrail system with 6-inch by 8-inch rectangular wood posts in a concrete mow strip performed acceptably for MASH Test 3-10. However, during MASH Test 3-11, the W-beam rail element ruptured, allowing the 2270P vehicle to penetrate the installation (Sheikh et al. 2019). Similarly, a 31-inch W-beam guardrail system with 7½-inch round wood posts installed in a concrete mow strip with grout-filled leave-outs also failed to meet the MASH TL-3 criteria (Bligh et al. 2020). Both wood post systems were tested with a 40-inch embedment depth. However, a modified 7½-inch-diameter round wood post system with a reduced embedment of 36 inches was successfully full-scale crash tested in accordance with MASH TL-3 requirements (Moran et al. 2020). A 6-inch by 8-inch rectangular wood post system with 35-inch embedment also met MASH TL-3 criteria (Moran et al. 2021). Therefore, both steel and wood post W-beam guardrail configurations can be used in mow strips with leave-outs, but care should be taken to ensure the proper post configuration and embedment depth. The maximum strength of the grout in the leave-out was increased during this testing from 120 psi to a maximum of 230 psi, which should provide more installation flexibility (Moran et al. 2020). A steel post W-beam guardrail system installed in simulated rocky terrain following the guidance from the previously cited research (Rohde 2003) and FHWA memorandum B-64-b was successfully tested to MASH TL-3 (Bligh et al. 2019). A round wood post W-beam guardrail system tested in a similar configuration did not satisfy MASH criteria (Bligh et al. 2019). NON-HERBICIDE, LONG-TERM VMSS Impervious VMS Treatments Minor Concrete Pavement Minor concrete for vegetation control is used in areas requiring a roadside management strategy that eliminates or minimizes the growth of unwanted vegetation. Minor concrete can contain additives such as crumb rubber and polypropylene fibers (see Figure 8). Color can be added to minor concrete. The Caltrans Construction Policy Bulletin, Minor Concrete Vegetation Control, indicates the use of this VMS with Midwest guardrail systems and thrie beam barriers (Caltrans 2017b).
20 Figure 8. Minor Concrete Used for Vegetation Management (Caltrans 2017b). A further benefit of using minor concrete as a VMS is easy installation using standard equipment during new construction. The RDG (AASHTO 2011b) and MASH (AASHTO 2016) provide leave-out compliance requirements around strong post W-beam guardrail and other systems. Retrofit installation at existing structures may be impractical and not cost-effective due to grading and excavation requirements for this VMS. Crumb Rubber Modified Concrete Crumb rubber modified concrete (CRMCrete) is a concrete-based product using a slurry blend of recycled scrap tire crumb rubber material and homopolymer polypropylene high-performance reinforcing fibers. The CRMCrete weed barrier system is installed like minor concrete and can be used with color and texture for increased aesthetic value. Formwork is not always necessary, and this VMS has a higher daily production rate than other surface treatments. Installation is easy and uses standard equipment and concrete mixes. However, because of the consistency of the mix, CRMCrete is not appropriate for steeper slopes. There is a limited history of maintainability and life cycle costs. As with other colored and/or textured materials, when making repairs, it is difficult to match the original color. The RDG (AASHTO 2011b) and MASH (AASHTO 2016) provide leave-out compliance requirements around strong post W-beam guardrail and other systems (Malcolm 2009). Caltrans used CRMCrete in a median location with many obstructions for mowing crews such as raised drop inlets, culvert pipes sticking out, survey monuments, signage, posts, and pullboxes. Caltrans made the obstacles flush and reset around the appurtenances using CRMCrete. This made mowing easier and thus reduced employee exposure time in dangerous locations. Caltrans used CRMCrete under the guardrail and around signposts. Caltrans gained approval from FHWA for the use of CRMCrete under the W-beam guardrail system. Standard Concrete Pavement Standard concrete is commonly used as a VMS. Concrete can be colored or stamped, making it an option for use in areas with greater aesthetic concerns (shown in Figure 9). The patterns are stamped into the concrete before curing using a patterned form. The color can be surface-applied or integral to the concrete mixture. Leave-outs may be required for some applications due to the rigidity of the material, particularly when used with strong post W-beam guardrail and other
21 systems. The RDG (AASHTO 2011b) and MASH (AASHTO 2016) provide leave-out compliance requirements. Although the relative cost of patterned concrete is greater than many other treatments, it is adaptable to different aesthetic goals, provides a long-term solution, and can be used in slope conditions. Patterned materials with seams are subject to surface accumulation of soil, seeds, and debris (Caltrans 2012). Figure 9. Standard Concrete Pavement Uses (Sheikh et al. 2019). Asphalt Concrete Pavement Asphalt concrete pavement used as a VMS is an applicable strategy in medians and gore areas, under guardrails and cable barriers, and as a mow edge adjacent to structures such as sound walls and retaining walls (Figure 10). Photo courtesy of Adam Weiser Figure 10. Asphalt Under Cable Barrier System. Stamped and colored asphalt concrete can provide many of the same aesthetic advantages of colored, textured concrete and pavers at a reduced cost of installation and maintenance. Installation involves stamping warm, pliable asphalt with a patterned, woven wire template pressed into the asphalt using standard compaction equipment, as shown in Figure 11. The templates are re-used along the course of the area. The color is in the finishing coat and is topically applied. The RDG (AASHTO 2011b) and MASH (AASHTO 2016) provide leave-out compliance requirements when using stamped asphalt concrete around strong post W-beam guardrail and other appurtenances. This VMS should incorporate edge restraints and pavement reinforcement fabric for additional resistance to cracking and deformation (Caltrans 2017c).
22 Figure 11. Stamped Asphalt Concrete at Roundabout (Pattern Paving Products 2016). Advantages for using asphalt as a VMS include lower relative initial costs, commonly used installation equipment, and worker knowledge of installation techniques. Limitations include degradation over time due to the lack of compaction with regular traffic use, which may lead to weed growth. Although initial costs are low, minor concrete is more cost-effective in terms of life cycle costs. Stamped asphalts are subject to surface damage, and asphalt base may bleed through surface color in hot climatic conditions. This VMS has limited slope applications, and the durability of stamped asphalt is moderate. Asphalt Composite Asphalt composite for vegetation control is a cold, spray-applied asphalt emulsion reinforced with fiberglass strands. This technique provides a solid, seamless impervious surface. Removal of the seams helps eliminate the historically problematic areas where unwanted vegetation typically grows. Figure 12 shows a guardrail system using asphalt composite as a VMS. Figure 12. Asphalt Composite Treatment at Guardrail System (Caltrans 2017d). Asphalt composite adheres to asphalt, concrete, wood, and metal. A dilute solution applied at the leading edge adheres to the adjacent loose soil and provides erosion control. One key advantage of using an asphalt composite VMS is the retrofit capabilities under existing guardrails, signs, and other appurtenances. Asphalt composite as a VMS can be simply and quickly installed at existing structures, can be easily repaired, and allows a minimal lane closure period. Asphalt composite can be used in situations where rigid materials such as minor concrete treatment are not practicable. This material is durable enough to withstand machine traffic and flexible enough
23 to move during a guardrail impact. In addition, asphalt composite has a low life cycle cost. One limitation of asphalt composite is that installation requires the temperature to be above 50° F. However, once cured, the VMS can withstand freezing temperatures (Caltrans 2017d). Modular Paving Units (May Be Pervious Depending on Application) Modular paving units consist of concrete or brick pavers used as a VMS (see Figure 13). Modular paving units are durable, come in a variety of shapes and colors, and can be a cost- effective treatment. Pavers can sustain both pedestrian and vehicle traffic depending on the specifications of the pavers. This VMS is most used in gores, raised medians, and pedestrian use locations. As with other textured VMSs, soil and debris can accumulate in the paver spaces and provide a medium for unwanted vegetation. Therefore, this VMS may require spot herbicide treatment or other maintenance actions. Figure 13. Modular Paving Units in Median (TxDOT 2017). The pavers can be classified as either pervious or impervious depending on the base material used. These modular units can be placed on compacted soils, with or without a geosynthetic fabric (herbicide or non-herbicide treated) using a graded sand fill between pavers or placed on a concrete or other impervious base and filled with a mortar/grout material. One advantage of using a modular unit VMS is the ability to access subsurface utilities and the general ease of repair because pavers can be removed and replaced much more easily than a poured-in-place VMS. Unfortunately, their flexibility is also a disadvantage. The pavers are susceptible to deformation from vehicular traffic, especially heavy trucks. Therefore, the paving foundation needs to be adequate to meet the traffic loads and meet safety requirements for pedestrians and vehicles (TxDOT 2017). If set on an impervious surface using mortar material, leave-outs may be required for some applications due to the rigidity of the material. The RDG (AASHTO 2011b) and MASH (AASHTO 2016) provide leave-out compliance requirements. Rock Blanket Rock blanket as a VMS consists of a ground surface covered with rock cobbles at the areas where vegetation control is required. The rock blanket is installed into mortar or concrete and may be used with or without a concrete base. It is a multi-purpose treatment that can be used for aesthetic reasons, erosion control, and weed suppression. Figure 14 shows applications of a rock blanket along the roadside. There are numerous applications for this VMS. Examples include embankment paving, slopes under bridges, medians, and roundabout designs.
24 Figure 14. Rock Blanket Installations (Diversified Landscape Company 2016, Caltrans 2015). Rock blankets should not be placed near clear recovery zones or other areas subject to errant vehicles. As with other heavily textured VMSs, rock blankets are subject to the accumulation of deposited soils that can facilitate weed growth. Care should be taken in the design to minimize the spacing between the rocks. Context sensitivity can be achieved through the choice of rock and mortar color. Those areas not using a mortar base should use a geosynthetic fabric (herbicide or non-herbicide treated). An important guideline to follow while placing the rock blanket is to provide a physical barrier between the rock blanket and pedestrians (Caltrans 2015). Leave-outs may be required for some applications due to the rigidity of the material, particularly when used with strong post W-beam guardrail and other systems. The RDG (AASHTO 2011b) and MASH (AASHTO 2016) provide leave-out compliance requirements. Rubber Weed Mat The rubber weed mat (Figure 15) is designed to block out sunlight and inhibit plant growth. This VMS is comprised of recycled rubber tires bonded together with a resin and shaped into a mat. Color can be added to reflect the surrounding aesthetic context. As with other textured VMSs, rubber weed mats are subject to the accumulation of deposited soils that can facilitate weed growth. Ground penetration holes can be pre-cut or molded into the product before installation. Rubber weed mats around strong post guardrails and possibly other safety devices whose performance depends on permitting the post or support to rotate in the soil may also require leave-outs to be used in conjunction with their application. As noted in the Arrington et al. (2009) study, one proprietary rubber weed mat failed to permit the desired level of rotation during a dynamic post impact when compared to the resistance offered by a grout-filled leave- out that was successfully crash tested. Installation requires an overlap at seams that are sealed using an asphalt crack sealer or resin adhesive. The tiles are best suited for level areas and are not recommended for large, nonlinear areas (Caltrans 2017e). This VMS is subject to damage from high winds, mowers, snow removal equipment, and ultraviolet (UV) degradation.
25 Figure 15. Rubber Weed Mat (Caltrans 2017e). Pervious VMS Treatments Aggregate Base Aggregate base is a compactable, graded rock placed on a prepared surface and compacted to 90â95 percent. This VMS can be used in all application areas. Aggregate base is a generally low cost and readily available material. It is suitable for new and some retrofit locations. Figure 16 shows examples from FHWAâs Public Roads (Meininger and Stokowski 2011) of natural aggregates used in construction. Using aggregate as a VMS may not be applicable in areas with snow removal equipment and snow storage because the VMS can be displaced by errant vehicles and maintenance equipment. This VMS may require spot herbicide treatment and re-compaction. (a) Natural gravel often used as coarse aggregate in concrete, (b) crushed stone coarse aggregate typically used in asphalt mixtures in paving and concrete, and (c) a compacted crushed stone layer used as granular base material. Figure 16. Aggregate Base (Meininger and Stokowski 2011). Asphalt Millings Asphalt millings used as a VMS are comprised of bituminous material removed during cold planing (the controlled removal of the surface of the existing pavement) operations and are ground or crushed into asphalt milling. The advantage of using this VMS is the easy installation in new construction, repair, and retrofit applications. Limitations include improper compaction of the material, leading to weed problems. Compaction may degrade over time. Environmental concerns are noted if millings erode near watercourses. As with most stockpiled materials, stored millings may accumulate weed seeds.
26 Glass Cullet Glass cullet consists of recycled glass processed into a mulch material (see Figure 17). The processing removes the sharp edges and produces a material safe to handle for a variety of uses. Glass culletâs aesthetic value comes from a variety of sizes and colors available from the recycled glass. The material does not decompose and works well in relatively flat areas; it is not recommended for slopes. An edge/border material may be necessary to contain the cullet (Malcolm 2009). As a VMS, cullet is typically applied over a geosynthetic fabric (herbicide or non-herbicide treated), like wood mulch. However, Caltrans installed cullet under guardrail without using a barrier and reports that it is still functional after 7 years. Although there is limited experience in using glass cullet as a VMS, numerous DOTs use glass cullet as aggregate, granular base, and fill and in other applications. The Pennsylvania Department of Transportation (PennDOT) issued a PennDOT Recycled Material Brief, Crushed Glass Fact Sheet, in 2013 (PennDOT 2013), explaining PennDOTâs use of glass cullet and providing specifications (see Appendix D). Figure 17. Recycled Glass Cullet Mulch (Malcolm 2006). Gravel Mulch Gravel mulch is applied to the soil surface to reduce weed growth and as an aesthetic treatment (Caltrans 2017f). Gravel mulch consists of placing graded, crushed, or quarried rock over a geosynthetic fabric (herbicide or non-herbicide treated). Figure 18 shows the gravel mulch treatment at the roadside. Gravel mulch is one of the least expensive control treatments that can be achieved with the use of existing equipment. One of the biggest limitations of this treatment is that it can be easily dislodged by errant vehicles and should not be used where disruption from errant vehicles is likely. Some of the advantages of using gravel mulch include low maintenance cost, stormwater runoff infiltration, and wind resistance (Davey Tree Expert Company 2017). One of the limitations of gravel mulch is the accumulation of soil and debris that provides a medium for weed growth.
27 Figure 18. Gravel Mulch Treatment at the Roadside (TxDOT 2017) Rock Slope Protection Rock slope protection (also known as rock riprap, dump rock, and others) differs from gravel in size and texture. Rock slope protection consists of larger rocks that are surface applied with or without a geosynthetic fabric (herbicide or non-herbicide treated). This VMS is easy to install with existing equipment. Typical usage is in rural or transition areas, on slopes, under structures, and in other similar locations (see Figure 19). This VMS is not advised for use in areas where errant vehicles may come in contact with it. Rock slopes can be installed on a slope up to 1V:2H. As with other textured VMSs, soil and debris can accumulate and provide a medium for unwanted vegetation. As such, this VMS may require spot herbicide treatment or other maintenance actions. Figure 19. Rock Slope Protection (Ohio Department of Transportation n.d., Indiana Department of Environmental Management 2007). Organic Mulch Organic mulch generally consists of some type of recycled material or by-product, such as chipped wood (see Figure 20). This material can come from ROW clearing operations at a minimal cost to the DOT. Organic mulch may be used as a VMS, but due to its organic structure, degradation/decomposition occurs over time and requires maintenance and/or replacement every 2 to 5 years. This VMS may be subject to displacement by errant vehicles, maintenance equipment, and storm events. This VMS method should be considered as a temporary vegetation control solution for use in areas subject to near-term disruptions (Caltrans 2017g).
28 Figure 20. Organic Mulch (Pacific Landscape Supply 2019). Weed Control Fiber Mat Weed control fiber mats discussed in the Caltrans Roadside Management Toolbox consist of synthetic polyester fibers made from recycled plastics. This VMS works by blocking out sunlight to inhibit plant growth yet allows for the infiltration of stormwater runoff. These mats need to be placed on a semi-level, compacted surface without any underlying debris (see Figure 21). This VMS has many advantages over other treatment methods because it is easy to install at an existing or new roadside location (Caltrans 2017h). Weed control fiber mat is an affordable, easily repairable, and effective measure. Figure 21. Weed Control Fiber Mat (Caltrans 2017h). However, weed control fiber mat may have limitations in areas where winter maintenance equipment is used. Research conducted for the Washington State Department of Transportation (WSDOT) examined several weed barriers and found a need for annual removal of accumulated organic/inorganic debris. Without this maintenance, the organic buildup starts to grow grass and weeds. At sites where herbicide use is restricted, weed barriers provide a viable option although it may be prohibitively expensive for normal guardrail locations (Willard et al. 2010). Select Vegetation Establishment Irrigated/Ornamental Vegetation Irrigated/ornamental vegetation as a VMS is used to sustain desirable vegetation that can suppress the growth of weeds, annual grasses, and other undesirable vegetation in specified
29 locations (Caltrans 2017i). This treatment is generally used in urban and suburban locations where there are greater stakeholder expectations for aesthetics, as shown in Figure 22. This control measure is used in lieu of conventional impervious and pervious surface covers. Vegetation is chosen for specific qualities such as maximum height and ability to out-compete weeds. Vegetation has the benefit of providing erosion control, stormwater runoff infiltration, and landscape enhancement. The limitation is that this type of control requires maintenance over its life cycle. Costs vary according to the design parameters. Figure 22. Irrigated Ornamental Vegetation (TxDOT 2017) Native and Non-irrigated Vegetation Like irrigated ornamental vegetation, native and non-irrigated vegetation can be used to suppress unwanted vegetation and replace it with more desirable vegetation (Caltrans 2017j). The advantage of using native plant species is their ability to out-compete weeds and annual grasses when planted on roadsides. The use of this type of vegetation is generally best suited for non- urban areas. Native plant communities generally require less maintenance and can become self- sustaining (see Figure 23). A major advantage of using native vegetation is that many provide habitat for pollinators. Limitations include a slow establishment period that may require maintenance to eliminate unwanted vegetation. Post-construction soils may require an amendment to provide a suitable growing medium. Figure 23. Native Non-irrigated Vegetation (Arizona DOT 2008)
30 Comparison of Commonly Used VMSs Table 2 shows a comparison of the commonly used VMSs. Approximate costs for each VMS were obtained from Caltrans (2017a) pricing and are presented to show relative cost comparisons. The modular paving unit cost reflects the TxDOT 2018 average low bid. Table 2. Comparison of VMSs (Caltrans 2017a, TxDOT 2019) VMS Benefits Limitations Installation Cost per Square Yard Minor concrete pavement ⢠Effective life cycle cost ⢠Easy installation for new construction ⢠Various colors available ⢠Not cost-effective for retrofit application ⢠Requires leave-out section $65* Standard concrete pavement ⢠High longevity and low life cycle cost ⢠Use on slopes ⢠Various colors and patterns available ⢠Repairs difficult ⢠Requires leave-out section $100â$125* Asphalt concrete pavement ⢠Quick and easy installation ⢠Various colors and patterns available ⢠Requires leave-out section ⢠Subject to surface damage ⢠Limited slope applications $40â$55* Asphalt composite ⢠Low life cycle cost ⢠Easily installed and repaired ⢠Seamless application ⢠Retrofit applications ⢠Installation requires above 50° F $52* Modular paving unit ⢠Allows for subsurface access such as utilities ⢠Various styles and colors ⢠High aesthetic value ⢠Retrofit applications ⢠Limited slope application ⢠Subject to damage from heavy vehicles $75â$90** Rock blanket ⢠Wind resistant ⢠Low maintenance costs ⢠Repairs difficult $85â$120* Rock slope protection ⢠High effective longevity ⢠Easy installation, repairs, and replacement ⢠Slopes up to 2:1 ⢠Low aesthetic appeal $25â$85* Rubber weed mat ⢠New and retrofit construction ⢠Easily repaired/replaced ⢠Colors available ⢠Specialized designs available ⢠Subject to damage from wind, mowers, winter operations, and UV degradation ⢠Joints may become problematic ⢠Texture may accumulate debris $53* Gravel mulch ⢠Uses common equipment ⢠Low maintenance ⢠Easily displaced by errant vehicles $10â$23*
31 VMS Benefits Limitations Installation Cost per Square Yard Organic mulch ⢠Retains soil moisture ⢠Enhances soil structure ⢠Provides erosion control ⢠Temporary solution ⢠Degrades over time ⢠2- to 3-year life cycle $40* Weed control fiber mat ⢠Allows for infiltration ⢠UV stable and fire retardant ⢠Easy repairs/replacement ⢠Retrofit applicable but more expensive ⢠Subject to damage from wind, mowers, and winter maintenance equipment $50* Irrigated/ ornamental vegetation ⢠Slope protection and visual enhancement ⢠Once established, out- competes weeds ⢠Subject to high initial cost and maintenance ⢠Requires establishment period $16â$24* Native and non-irrigated vegetation ⢠Self-sustaining when established ⢠Out-competes weeds ⢠Potential pollinator habitat ⢠3- to 5-year establishment period may require soil amendments and herbicides or other control $0.90â$9* * Installation costs reflect Caltrans (2017a) pricing and are presented to show relative cost comparisons. ** Modular paving unit costs reflect the TxDOT average low bid. Innovative/Alternative VMSs Numerous VMSs have been used across the country for various treatment areas. These VMS are composed of materials typically found in transportation construction. However, new materials have potential for DOT use. Geosynthetic Cementitious Composite Mat Geosynthetic cementitious composite mat (GCCM) is a flexible cement-impregnated fabric that hardens when hydrated to form a thin, durable concrete layer (see Figure 24 and Figure 25). This material has different proprietary names such as Concrete Cloth⢠and Concrete Canvas®. Caltrans used Concrete Canvas® in limited locations. DOT use of this VMS was not found to be widespread and had minimal performance measurement data. This VMS consists of: ⢠Dry concrete mix ⢠Reinforcing fiber matrix ⢠Fabric top surface ⢠Polyvinyl chloride (PVC) bottom coating
32 Figure 24. Concrete Canvas® GCCM Section (Concrete Canvas 2020). Figure 25. Concrete Cloth⢠(Srinivas and Ravinder 2012). GCCM products have the following benefits: ⢠Rapid and easy installation ⢠Product comes on a roll (needing no concrete mixing) ⢠Waterproof ⢠Fireproof ⢠Flexible enough to allow material to conform to a surface ⢠Unset material can be cut as necessary for a given situation TRANSPORTATION AGENCY RESEARCH Transportation agencies are trying to find cost-effective, low-maintenance solutions for maintaining a vegetation-free zone in critical areas. However, these VMSs must work in conjunction with the safety parameters for guardrail and other fixed objects on the roadside. Delaware Department of Transportation The Delaware Department of Transportation (DelDOT) funded a study to explore various methods of treating vegetation under guardrail (Barton and Budischak 2013). This study
33 examined three herbicide formulations, four weed control barriers, and competitive vegetation. The four VMSs investigated included: ⢠Permeable recycled fiber mat ⢠Permeable recycled fiber mat customized to fit the edge of the roadway and variances in guardrail post width ⢠Semi-rigid, interlocking panels made of 100 percent recycled plastic (this product is no longer available) ⢠Rubber matting with three punched guardrail cutouts to provide flexibility for installation Figure 26 shows the four VMSs used in this study. Permeable recycled fiber mat, standard installation Permeable recycled fiber mat, custom installation Semi-rigid, interlocking plastic panels Rubber matting with punched guardrail cutouts Figure 26. Weed Control Barriers Evaluated in DelDOT Study (Barton and Budischak, 2013).
34 Barton and Budischak (2013) found that weed control barriers are difficult to install in retrofit applications. As with other VMSs, random weed growth is somewhat problematic. The caulk areas were the weakest point in the systems and allowed for breakthrough vegetation growth. Another key point of the research was that new products and systems are subject to installation errors. Table 3 shows a cost comparison for the weed barriers tested. According to Barton and Budischak (2013), âWhen you consider amortization over a 10-year life span, weed control barriers are still the most expensive vegetation control option under guardrail. They may be warranted in highly sensitive areas where herbicide use is unacceptable or other conditions warrant the complete lack of vegetation under guardrail.â Table 3. DelDOT Weed Barrier and Competitive Vegetation Cost Comparison (Barton and Budischak 2013). Treatment Installation Cost (per 100 Linear Feet of Guardrail) Yearly Maintenance Cost (per 100 Linear Feet of Guardrail) Installation Cost Amortized over 10 Years (per 100 Linear Feet of Guardrail) Total Yearly Cost Including Amortized Installation Cost (per 100 Linear Feet of Guardrail) Permeable recycled fiber mat, standard installation $1789.52 $24.00 1 $178.95 $202.95 Permeable recycled fiber mat, custom installation $2197.54 $8.00 2 $219.75 $227.75 Semi-rigid, interlocking plastic panels $2607.00 $0.00 $260.70 $260.70 Rubber matting with punched guardrail cutouts $2537.17 $24.00 1 $253.72 $277.72 Low fescue $444.33 $47.023 $44.43 $75.34 Hand trimming $0.00 $24.001 $0.00 $24.00 Zoysia sod $1,582.28 $0.00 $158.23 $158.23 Zoysia seed $45.28 $16.004 $34.53 $50.53 FlightTurf $541.93 TBD $54.19 $54.19 1 Includes 1.5 hand trims/100 linear feet, no herbicide treatment 2 Includes 0.5 hand trims/100 linear feet, no herbicide treatment 3 Includes 1.5 hand trims/100 linear feet and 1.25 herbicide treatments/100 linear feet (the herbicide treatment for low fescue is assumed to be required for 3 years until the low fescue stand becomes thick enough to out-compete other vegetation) 4 Includes one hand trim/100 linear feet (this is based on only one year of data and assumed to be at least 1.5 times in future years) In addition to cost, the DelDOT study included an evaluation of effectiveness in controlling weeds and suitability for use with guardrails. Table 4 presents these results using ratings on a Likert scale of 1 to 5. For suitability, a 5 indicates the guardrail was uncompromised; for weed control, a 5 indicates no weeds present.
35 Table 4. DelDOT VMS Suitability and Weed Control Rating (Barton and Budischak 2013). Treatment DelDOT Suitability Weed Control 2011 2012 2011 2012 Permeable recycled fiber mat, standard installation 3.72 3.25 2.95 2.80 Permeable recycled fiber mat, custom installation 5.00 0.87 4.83 4.58 Rubber matting with punched guardrail cutouts 4.58 4.20 4.43 3.83 Low fescue 3.37 4.20 0.93 3.44 Zoysia sod â 4.93 â 4.47 Zoysia seed â 4.00 â 2.27 Hand trim 3.82 3.77 3.09 2.87 Georgia Department of Transportation The Georgia Department of Transportation (GDOT) authorized tests to be performed on guardrail installed in accordance with GDOT Standard Detail S-4-2002, which was used in Georgia prior to 2017 and included an asphalt mow strip with a nearby curb. In March 2017, GDOT directed that all new guardrail construction projects on Georgia roadways use asphalt layers that are paved up to the face of the post, leaving the post itself and the area behind unrestrained (Scott et al. 2018). Iowa Department of Transportation In 2002 the Iowa DOT examined vegetation control mats made from tire crumb rubber bound together with a urethane resin for use around roadside delineator and guardrail posts. The study also weighed the cost of purchasing and placing the mats versus hand trimming costs. Iowa DOT was looking for a product that would reduce the need for hand trimming and, therefore, maintenance worker exposure in high-traffic areas. Results determined that the mats performed adequately and could have a significant impact on maintenance worker safety (Dunn 2002). Texas Department of Transportation TxDOT also conducted a demonstration study using the same tire crumb rubber mats. As with the Iowa DOT study, the areas of concern were around signposts and guardrail posts. This research effort found the following advantages of this VMS: ⢠Minimizes labor and material costs ⢠Achieves aesthetic goals ⢠Uses recycled material (scrap tire crumb rubber) ⢠Reduces environmental damage from emissions from string-trimming and herbicide use (TxDOT 2012). Figure 27 shows the recycled crumb rubber VMSs used for the project. The installation process for the demonstration project is in Appendix A.
36 Figure 27. TxDOT Demonstration Project Using Crumb Rubber VMS (TxDOT 2012). TxDOTâs Cable Median Barrier Maintenance Manual (TxDOT 2008) suggests several different mow strip designs (concrete, asphalt, and rip rap) with typical installation cost per mile ranging from $45,000 to $55,000 for concrete and $75,000 to $90,000 for asphalt. TxDOT encourages the use of mow strips to reduce hand mowing or herbicide operations due to the cable barrier system installation. Additionally, a mow strip provides additional resistance to the movement of socket foundations (see Figure 28). Designers should consider mower widths when determining the appropriate distance between the edge of a travel lane and the cable barrier (TxDOT 2008). Figure 28. Cable Barrier and Roadside Vegetation (TxDOT 2008). Washington State Department of Transportation WSDOT conducted a study in 2005 to investigate various vegetative control methods (Willard et al. 2010). Among these were weed barrier mats placed under guardrail. The study showed a maintenance requirement for all products that include the removal of organic and inorganic debris accumulation at the product edge and at seams. The use of pavement or other types of
37 impervious VMSs is best under guardrail, especially in environmentally sensitive areas that prohibit the use of herbicide control treatments. The research looked at eastern and western Washington climates to determine the effects of climate on the product installation and performance. Table 5 shows the products tested and results. Table 5. WSDOT VMS Results (Willard et al. 2010). VMS Location Product Description Maintenance Costs Findings/Recommendations Western Washington Site 1 Permeable recycled fiber mat, standard installation Under guardrail Woven fiber permeable mat $22/mile/year This is the oldest WSDOT installation of this product (2002), and this location requires annual removal of accumulated debris. Site 2 Permeable recycled fiber mat, standard installation Under guardrail, behind an asphalt curb, with limited overhanging tree canopy Woven fiber permeable mat $22/mile/year This installation performed much better than the other installations of the same product. Site 3 Permeable recycled fiber mat, standard installation Under guardrail, extensive existing weeds and brush Woven fiber permeable mat $195/mile/year Weed growth through the material at the edges and joints. Recycled tire material and adhesive blown by hose Under guardrail Ground-up tire mulch, placed over weed fabric, sealed with polyurethane coating $37/mile/year Within 2 years after installation, weeds and grass began to seed in over the top. This product is no longer marketed for roadside use. Semi-rigid, interlocking plastic panels Under guardrail Interlocking molded plastic tiles $22/mile/year The product as installed was brittle and subject to cracking. This was a prototype product, and the manufacturer has since improved the design and durability. Rubber matting with punched guardrail cutouts Under guardrail Interlocking rubber tiles made of recycled materials $22/mile/year The most expensive material of the weed barriers but easiest to install. It is durable and functional in preventing vegetative growth.
38 VMS Location Product Description Maintenance Costs Findings/Recommendations Pavement Under guardrail Pavement under guardrail $895/mile/year Of all the under-guardrail barriers, this is the least expensive to install and most durable. Maintenance costs would be comparable to those above ($22/mile) if cleaned annually. * Eastern Washington Rubber matting with punched guardrail cutouts Under guardrail Interlocking rubber tiles made of recycled materials $22/mile/year Relatively easy to retrofit to an existing guardrail, this product performed very well with little maintenance costs. Extremely expensive installation cost precludes this from most sites. Semi-rigid, interlocking plastic panels Under guardrail Interlocking molded plastic tiles $22/mile/year This product was relatively difficult to install and suffered significant damage throughout the test. This was a prototype product, and the manufacturer has since improved the design and durability. Permeable recycled fiber mat, standard installation Under guardrail Woven fiber permeable mat $22/mile/year Installed in an area with significant snowfall and snow and ice operations. The product performed very well with minimal maintenance costs. Extremely expensive installation cost precludes this from most sites. * Initial installation cost: $18,480/mile. Installation costs estimated based on current average bid prices for asphaltic pavement on WSDOT projects, assuming guardrail can be installed through the pavement at the same cost as it would off the edge of pavement. Ongoing maintenance cost: $895/mile/year. Cleaning paved shoulder under guardrail once every 7 years: $6,265/mile. SUMMARY One difficulty the research team encountered over the course of this project was in identifying specific VMSs used by DOTs. Many of the VMSs placed under guardrail and cable barriers and in other locations are not identified in the DOT literature, manuals, and/or specifications as VMSs. They are simply part of a larger construction component. While there is a growing body of research and project implementation regarding the use of standard-type VMSs (i.e., concrete and asphalt) and emerging products within the industry, few studies have been conducted on product longevity and performance. For many DOTs, implementation and/or demonstration projects using new techniques and/or products are the most assured method of gathering the necessary data to determine whether a VMS will become a DOT standard. Therefore, data regarding their use, cost, effectiveness, and other related issues are limited unless specifically tracked by the DOT or as part of a research project. The relationship between rigid VMS materials and roadside appurtenances is a critical component of roadside safety. Each rigid material VMS presented reacts with the roadside appurtenance in a specific manner. In application areas such as guardrail, leave-outs may be
39 required to maintain compliance with the RDG and MASH as well as requirements specific to the respective DOT. Many barrier systems are undergoing retesting for compliance. Further, proprietary systems have specific requirements. Therefore, it is imperative for the user to check with current requirements for the specific appurtenance and VMS selected for use. Cost is a concern for DOTs. Material choice needs to consider a broad range of issues. As found in the Iowa DOT study (Dunn 2002), DOTs need to weigh the cost of materials and installation versus maintenance worker exposure in high-traffic areas. Some VMSs may have a low initial cost but are labor intensive to install and maintain. Another consideration is the environmental impact of various VMSs. In environmentally sensitive areas where herbicide use is unacceptable, highly effective VMSs are a key decision factor over material and installation costs. Finally, worker safety is a key issue for state DOTs and was a major impetus for this research. The use of VMSs can reduce the need for routine vegetation management activities on the roadside, thus reducing worker exposure to traffic. Worker exposure is also an important consideration when evaluating installation requirements for a specific VMS. Selecting a VMS requires consideration of many advantages and disadvantages including potential interactions with roadside safety appurtenances, weed control performance, worker exposure, maintenance requirements, installation requirements, cost, and availability.
