Long-Term Vegetation Management Strategies for Roadsides and Roadside Appurtenances (2023)

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54 CHAPTER 5. CONCLUSIONS Controlling vegetation in critical areas has historically involved the use of mechanical methods, such as mowing and trimming, and chemical treatments using herbicides. These can create safety hazards for the traveling public and maintenance personnel. There is a growing body of research and project implementation regarding the use of VMSs. While the industry has emerging products, few studies have been conducted on product performance as it relates to non-herbicide, long-term VMSs. Many DOT implementation and/or demonstration projects using new techniques and/or products are conducted internally and may not be made publicly available through typical website searches. SELECTING A VMS As mentioned previously, VMSs can be categorized into three basic categories: impervious surfaces, pervious surfaces, and select vegetation establishment. Impervious surfaces are designed to cover the designated area, not provide a growth medium for plant materials, and minimize maintenance activities adjacent to the travel lanes. While impervious types of VMSs are very effective, they can be more expensive to install; however, these VMSs require minimal maintenance, have a low life cycle cost, and have high effective longevity. The most used VMSs are some types of concrete and asphalt. Pervious surface VMSs accomplish the same as impervious; however, these VMSs allow for stormwater infiltration. Select vegetation establishment consists of using low-growing, native non-irrigated, and/or ornamental irrigated vegetation. These plant materials are chosen for their ability to out-compete weeds and minimize unwanted vegetation. All VMSs are subject to some sort of maintenance. While impervious and pervious surface VMSs prohibit plant growth through the material, they are subject to windblown soil that provides a medium for weed growth. To provide state DOTs with more detailed information on VMSs and guide the selection of a specific strategy, the research team developed an Interactive Selection Tool. The tool offers guidelines on non-herbicide, long-term VMSs for roadsides and roadside appurtenances. The development of the Interactive Selection Tool was based on the information collected from the literature review, survey of practice, and follow-up interviews with select DOTs. This tool employs a decision algorithm to advise the user of the appropriate VMS treatments for the specific conditions. The tool can be downloaded from https://www.dropbox.com/s/coqfcnab 92gvfx5/NCHRP%20W350_Selection%20Tool.zip?dl=0. A user guide with step-by-step instructions on how to download and use the Interactive Selection Tool is provided in Appendix B (page B-2). The instructions for using the tool are also available in the tool folder when downloaded. The Interactive Selection Tool provides identification and guidelines 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 RDG, MASH testing, 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 described techniques on a proprietary roadside safety hardware device

55 (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 manufacturer’s safety hardware device. 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 of a highway safety appurtenance, then consideration should be given to crash testing the VMS and safety appurtenance together as a system. The various categories of roadside safety appurtenances were discussed regarding the application of VMSs. The use of VMSs around rigid barriers, such as concrete barriers, is acceptable provided the treatment does not extend above the pavement surface and does not alter the height of the barrier. Conversely, the semi-rigid barrier does displace and/or rotate when impacted, and those specific characteristics should not be altered. A VMS applied to a portable semi-rigid concrete barrier should not alter the barrier installation’s ability to displace laterally or rotate upon impact. Metal beam guardrail/guard fence (e.g., W-beam and thrie beam) can be configured as a semi-rigid or flexible longitudinal barrier. For strong post systems (e.g., guardrail installations, including guardrail transitions and guardrail end treatments), the proper lateral displacement and rotation of the posts reduces the chances of rail rupture and is critical to the proper operation of the barrier when impacted. For these systems, the rotation of the strong post should not be impeded or restrained by the VMS. MASH-tested mow strip configurations with properly designed leave-outs around the posts are one form of VMS for these systems. Certain approved backfill materials such as low-strength grout can be used in the leave-outs to control vegetation in those areas. Posts in high-tension cable barrier 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. 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 manufacturer’s safety hardware device. When applying a VMS to a support structure (e.g., sign or luminaire support), 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 or slip plates from slip bases, being in the path of the rotating support after it releases from its foundation or anchor, or significantly altering the impact height of the errant vehicle by altering the effective ground height around the structure or the vehicle approach to the structure. VMS TREATMENT SELECTION USING INTERACTIVE SELECTION TOOL There are numerous considerations in choosing a VMS. Each site has specific needs that include the cost of initial installation, maintenance, and repair. Other considerations are the overall life cycle cost, suitability as new construction or retrofit, and effective longevity. The Interactive

56 Selection Tool requires three categories of user input with dropdown menus based on site- specific conditions: • VMS location o Cable barrier o Support posts and poles o Edge of pavement o Gore/median o Guardrail o Mow edge o Slope/embankment • Level of aesthetics o Standard o High • Construction type o New construction o Retrofit VMS Location The VMS locations (i.e., cable barrier, support posts and poles, edge of pavement, gore/median, guardrail, mow edge, and slope/embankment) were chosen for use in the Interactive Selection Tool because they stand out in the literature, survey, and DOT documents as the most common areas where VMSs are used. Other areas needing VMSs are placed into each of these categories because they did not have any significant difference in use. Level of Aesthetics The site location and context may be a factor in choosing a VMS. Considerations include whether the site is urban, suburban, or rural and the adjacent land use such as residential, commercial, industrial, or agricultural. This choice is indicated by the level of aesthetics and has a rating of high or standard. A high rating is generally indicative of an urban or suburban location or other locations where stakeholders expect a greater level of aesthetic treatment. An example of this might be a gore/median or slope in a highly urbanized area or city entrance location. A standard rating may apply to less visible uses of VMSs such as under a median cable barrier in a rural location. The Interactive Selection Tool will select VMSs typically used for the given user criteria. However, this does not mean that a VMS selection with a standard aesthetic rating cannot be used in high-aesthetic locations. From this input, the Interactive Selection Tool will provide VMS choices based on user input. Figure 30 shows a typical result.

57 Figure 30. Selection Tool Input Result Example. Information Sheets From these selections, the Interactive Selection Tool will generate commonly used VMS treatments given the user inputs. If using with a strong post guardrail, refer to the “Strong Post Guardrail Use with Rigid VMS Materials” information sheet for leave-out compliance requirements. This information sheet provides critical information regarding the interaction of rigid materials and safety appurtenances. For more information regarding the specific VMS, the user can select the imbedded link “Information Sheet” or the photo, which will bring up an information sheet (found in Appendix C) regarding specifics about the selected VMS that include: • Relative costs o Installation o Life cycle o Maintenance o Repair • Effective longevity • Level of difficulty o Installation o Repair o Maintenance o Retrofit Relative Initial Cost The Interactive Selection Tool includes information on cost ranges for each VMS. Identifying costs for a specific material used as a VMS by DOTs was difficult because the VMS is typically integral to a larger construction project. Costs for VMSs vary greatly based on quantity, availability, and location. Cost ranges are relative to other VMSs and based on costs associated with the specific VMS per Caltrans and TxDOT pricing shown in Table 2. In the tool, the cost ranges are as follows:

58 • High installation cost is $85 yd2 or greater. • Moderate installation cost is between $50 and $85 yd2. • Low installation cost is up to $50 yd2. Effective Longevity Effective longevity is determined by the anticipated material life cycle and the level of maintenance routinely required of the specific VMS: • High effective longevity is at least 10 years. • Moderate effective longevity is 3 to 10 years. • Low effective longevity is up 3 years. Level of Difficulty The safety of maintenance personnel is directly related to the level of difficulty and/or time requirements for material installation, maintenance, and repair. Workers’ safety is greatly affected by increased exposure to traffic and other roadside hazards. The need for prolonged traffic controls necessary to complete the required tasks is important. Therefore, each VMS receives a level-of-difficulty rating of low, moderate, or high. This is not only indicative of the specific VMS material characteristics; it also includes a relative level of worker safety and exposure during installation, maintenance, and repair. Advantages/Limitations/Common Problems The information sheets also list the advantages, limitations, and common problems of each included VMS treatment. These may be considerations such as installation uses, typical maintenance, equipment/practices, or AASHTO RDG and MASH compliance requirements for leave-outs. Common problems may be displacement by errant vehicles and maintenance equipment. VMSs Included in Tool The Interactive Selection Tool includes VMSs found in use by DOTs from the survey results, literature, and DOT websites and documents. Many of the VMSs were either minimally used or are not being used by DOTs per survey results. However, all the VMSs included in the tool were used by at least three of the survey respondents and/or are found in various DOT documents and websites. The VMSs included in the tool are listed below. Appendix C provides information sheets on each VMS. These information sheets are also included in the Interactive Selection Tool. A downloadable table of the included VMSs can be downloaded using the following link. To open/run the Tool click on the link below and follow the directions in “How to Run the Tool” on page B-2. Impervious Surface VMSs Impervious surface VMSs include the following: • Minor concrete pavement • Crumb Rubber Modified Concrete

59 • Standard concrete pavement • Patterned concrete pavement • Asphalt concrete pavement • Stamped asphalt pavement • Asphalt composite • Rock blanket • Rubber weed mat Pervious Surface VMSs Pervious surface VMSs include the following: • Modular paving units (may be pervious depending on application) • Weed control fiber mat • Glass cullet • Gravel mulch • Rock slope protection • Organic mulch • Recycled asphalt millings • Aggregate base Selected Vegetation Establishment VMSs Selected vegetation establishment VMSs include the following: • Irrigated ornamental vegetation • Native and non-irrigated vegetation Alternative and Innovative VMSs A search of the literature and DOT documents and websites found little in the way of innovative practices or new materials. Many products and materials are of the typical VMS types but with different product names. One VMS noted as tried by Caltrans is a GCCM. This mat is a flexible, concrete-filled geotextile that hardens on hydration to form a thin, durable, waterproof, and low- carbon concrete layer—essentially concrete on a roll. This material may require leave-outs to maintain safety performance of roadside appurtenances. GCCM products show potential for use as VMSs, but there are no weed control performance data for this use; Caltrans has used one of these products in limited locations. Although experience with this VMS is very limited, an information sheet on GCCMs is included in Appendix C. FURTHER ADDITIONAL UPDATES TO AASHTO GUIDELINES FOR VEGETATION MANAGEMENT There is an opportunity for future research to advance transportation agencies’ knowledge regarding non-herbicide, long-term VMSs. This project also identified several knowledge gaps and needed updates/additions to the AASHTO Guidelines for Vegetation Management.

60 Resilience Transportation resilience is the ability of a transportation system to function at an acceptable rate in the event of extreme weather events, major crashes, and equipment or infrastructure failures. Quick recovery of a system is critical to avoid long-term effects. State DOTs need information on VMSs that can increase resilience of transportation facilities and the transportation system. Pollinators In June 2014, the White House issued the PM, “Creating a Federal Strategy to Promote the Health of Honeybees and Other Pollinators.” The PM directs federal agencies to take additional steps to improve habitat for pollinators, including honeybees, native bees, birds, bats, and butterflies—critical contributors to our nation’s economy, food system, and environmental health. With millions of acres of highway roadsides, state and local transportation agencies own or control land with the ability to conserve and/or create important habitat corridors that link otherwise fragmented pollinator habitat. State DOTs need information on how to update their vegetation management policies, practices, and standards to align with the PM on pollinators. Herbicides Herbicide-resistant weeds are becoming problematic for roadside vegetation managers. Herbicide manufacturers are changing the basic chemistry to combat resistance. This is becoming more and more difficult because some chemicals are being deleted from the roadside maintenance arsenal. The U.S. Department of Agriculture has taken over all aspects of herbicide control (i.e., licensing, training, etc.). State DOTs need information on how to respond to herbicide resistance and the current regulatory environment. Managed Succession Many DOTs are implementing more non-mow or reduced-mow areas within their ROW due to the cost, safety, and environmental benefits of managed succession of roadside vegetation outside the safety clear zone. Many of the benefits fall under ESs. These ES benefits include ecosystem diversity, stormwater quantity and quality management, carbon sequestration, erosion control, pollinator corridor development, wildlife habitat, and aesthetics. State DOTs need information on how managed succession can be used to advance environmental benefits, deliver ES, and better manage roadside vegetation.