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Suggested Citation:"Chapter 1 - Background." National Academies of Sciences, Engineering, and Medicine. 2010. Recommended Procedures for Testing and Evaluating Detectable Warning Systems. Washington, DC: The National Academies Press. doi: 10.17226/22937.
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Suggested Citation:"Chapter 1 - Background." National Academies of Sciences, Engineering, and Medicine. 2010. Recommended Procedures for Testing and Evaluating Detectable Warning Systems. Washington, DC: The National Academies Press. doi: 10.17226/22937.
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Page 5
Suggested Citation:"Chapter 1 - Background." National Academies of Sciences, Engineering, and Medicine. 2010. Recommended Procedures for Testing and Evaluating Detectable Warning Systems. Washington, DC: The National Academies Press. doi: 10.17226/22937.
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Page 5

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3Detectable warning systems are installed at curbs throughout the United States in response to requirements of the Americans with Disabilities Act (ADA). Detectable warning surfaces warn visually impaired persons that they are approaching a street or crosswalk. Increasing wheelchair accessibility by replacing curbs with ramps at crosswalks has had the unintended effect of eliminating the tactile cue (curb) that visually impaired persons relied upon to detect the end of the sidewalk. To address this problem, the ADA has required that detectable warning systems that are also suitable for wheelchair and foot traffic be placed at curb ramps. The ADA Accessibility Guidelines (ADAAG) specify a tactile detectable warning system in the form of truncated domes with base diameter of 0.9–1.4 inches (23–36 mm), top diameter of 50–65% of the base diameter, nominal 0.2-inch (5.1 mm) height, 1.6–2.4-inch (41–61 mm) center-to-center spacing, and a nominal base-to-base spacing of 0.65 inch (17 mm). The detectable warning system is required to have a visual contrast with adjoining surfaces (US Access Board, 2005). Contrasting colors (light on dark or dark on light) provide a visual clue to visually impaired persons, while the tactile surface of truncated domes provides a warning to blind persons using a cane for navigational aid, as well as a warning that can be felt underfoot. Detectable warning systems are commercially available in a wide variety of materials and are anchored to the sidewalk by several different attachment mechanisms. Some products are intended to be used only in new construction, while others are intended to be used to retrofit existing sidewalk surfaces. The materials used in detectable warning systems include flexible polymer mats, metal, precast concrete and brick pavers, stamped concrete, rigid fiber-reinforced polymer composite panels, and others. Products for use in new con- struction are generally intended by the manufacturer to be embedded in freshly placed (plastic) concrete and may have a supplementary anchorage system, although they may alter- natively be set in a sand bed or affixed with thin-set mortar. Retrofit products are usually adhered to the concrete surface with an adhesive and may also include supplementary mechan- ical anchorage. The exact shape and texture of the truncated domes varies from product to product. There is no description in the ADAAG recommending material type or identifying durability requirements for detectable warning systems. To fill this need, National Coop- erative Highway Research Program (NCHRP) Project 4-33, “Recommended Procedures for Testing and Evaluating Detectable Warning Systems,” was initiated, and the goals of this project were the development of testing methodologies and guidance for determining which of the many materials systems available may be durable for a given set of environmental conditions. The findings of this project will assist state and municipal Departments of Transportation (DOTs) by provid- ing laboratory testing methods than can be used to generate objective data to aid in the selection of durable detectable warning systems. The many variations of materials, textures and attachment mechanisms, as well as the different environ- mental and traffic conditions throughout the United States make evaluating durability of detectable warning systems challenging. Detectable Warning Systems Performance and Deterioration Mechanisms Prior to identifying tests suitable for evaluating durability of detectable warning systems, the desirable properties for these systems were identified. These are the properties that allow the detectable warning systems to perform their warning function and to serve as a safe travel surface for pedestrians and people in wheelchairs. These properties are required in the initial system and need to be maintained for the detectable warning system to assure functionality. The desirable properties are: • Color contrast • Slip resistance C H A P T E R 1 Background

• Mechanical integrity – Adhesion/attachment to substrate – Material strength • Dimensional stability of truncated domes These properties have a direct impact on the performance of a detectable warning system on a sidewalk. A contrasting color guideline is included in the ADAAG, and any fading or other changes in color as a result of exposure can affect func- tionality. A minimum level of slip resistance is required on all walking surfaces, and changes in slip resistance (coefficient of friction) can lead to an unsafe walking surface. Both the mechanical attachment to the sidewalk and the inherent strength of the detectable warning system to withstand var- ious kinds of in-service loading are important to maintain functionality. A detectable warning system that comes par- tially debonded from the sidewalk can be a tripping hazard, while a system that becomes completely debonded can lead to both a tripping hazard and the loss of functionality of the system as a detectable warning. Cracking and warping of the detectable warning system can lead to decreased detectabil- ity, creation of a tripping hazard, and reduced capability to withstand additional environmental exposure. Finally, changes in the dimensions of the truncated domes may affect detectability. For all these reasons, a system that is not durable cannot be relied on to perform its function over time and is not desirable. While determining what constitutes detectability and loss in detectability (e.g., how changes in the dimensions of domes affect detectability) is outside the scope of this research, changes in the properties of the detectable warning systems from the original values, or the results of testing performed after laboratory exposure, can be used to assess performance and durability. Durability testing is needed because environmental exposure may have a deleterious effect on the performance of a detectable warning system. Examples of deteriorated detectable warning systems can be found throughout the United States (Figure 1). 4 Figure 1. Photographs showing deteriorated detectable warning systems.

The following deterioration mechanisms have been identified as significant: • Freezing and thawing • Exposure to ultraviolet radiation • Snow removal • Abrasion • Exposure to moisture • Thermal cycling and exposure to high temperatures • Impact • Vehicle traffic • Corrosion • Exposure to moisture and alkalinity • Salt scaling and corrosion of metallic systems • Settling of pavers Each of these deterioration mechanisms is caused by natural or man-made environmental exposure. In some geo- graphic areas, detectable warning systems may be vulnerable to all of the listed deterioration mechanisms, while in other areas, some mechanisms may be negligible. Further discussion of these deterioration mechanisms is provided in Chapter 2. The ability of detectable warning systems to resist each of these mechanisms has been considered during the develop- ment of the evaluation protocol that is the objective of this research project. Objectives and Scope of Completed Research The objectives of this research were to (1) recommend test methods for evaluating durability of detectable warning systems and (2) provide guidance on the use of these methods for identifying detectable warning systems likely to be durable in different conditions. The completed research consists of a set of draft testing methods suitable for evaluating durability of detectable warn- ing systems. The methods have been developed with the goal of developing tests and evaluation criteria that are independent of the type of system; the methods are applicable to any type of detectable warning system. This project was focused on the development of these test methods, rather than the evaluation of detectable warning systems. While commercially available products were used in the test method development, no comparisons of durability between products were made. Organization of Project Documents This report is organized into the main project report and two supplementary documents: 1. The main report provides introduction and background to the durability of detectable warnings, discusses the research approach, presents the findings and applications, and gives conclusions and recommendations for future work. 2. The attachment consists of the test protocol separated into a master test method, describing the overall testing protocol, and individual test methods, describing each test in detail. These methods are presented in AASHTO Standard Method of Test format and were written in a form suitable for review and possible adoption by AASHTO. 3. The appendix (available on the NCHRP Report 670 sum- mary web page: www.trb.org/Main/Blurbs/163989.aspx) consists of narrative documents describing the process by which each test method was developed, as well as reviewing the specific need answered by each method and the objec- tives during the development process. A section providing guidance on the interpretation of the test results is also provided. 5

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 670: Recommended Procedures for Testing and Evaluating Detectable Warning Systems explores a set of recommended test methods for evaluating the durability of detectable warning systems. These methods address exposure regimes, test procedures, and evaluation criteria to help select detectable warning systems that provide long-term performance and durability while meeting the requirements of the Americans with Disabilities Act Accessibility Guidelines.

The appendix contained in the research agency’s final report provides further elaboration on the work performed in this project. This appendix titled Research Leading to the Development of Methodology for Durability Assessment of Detectable Warning Systems is available online.

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