CHAPTER 2

Overview of Previous Research, Standards, and State of the Practice

The National Institute on Disability, Independent Living, and Rehabilitation Research (NIDILRR) commissioned an extensive report on international and national understanding and use of TWSIs (Bentzen et al. 2021). That report served as a foundation for this project, with supplemental literature compiled alongside ongoing and complementary research. Existing standards and current practices were gathered, particularly in relation to use of TDIs in transit facilities in the United States. This was achieved by conducting a narrow search of the literature and doing outreach to local agencies in the United States to identify and interview those who were installing different TWSIs. This chapter is an overview of what is known about TWSIs, what standards exist, and what the state of the practice is relative to their use primarily in the United States. For more in-depth information and a list of research needs identified through this process, see the research roadmap in Appendix A.

“Tactile walking surface indicators” (TWSIs) is an internationally recognized generic term that includes any tactile walking surface being used intentionally to provide warning or guiding information to people with vision disabilities. They have been used internationally since the 1960s, with some standards on their specifications emerging in the 1980s. Currently, DWSs are specified by the 2010 ADA Standards for Accessible Design to be used along transit platform edges (U.S. DOJ 2010), while the ADA Standards for Transportation Facilities require them to also be used at curb ramps, blended transitions, and other at-grade pedestrian crossing locations and boarding and alighting areas at sidewalk or street level (U.S. DOT 2006). There are currently no standards for the use of tactile direction indicators (TDIs) or tactile warning delineators (TWDs) in the United States. The standard dimensions and practices for the use of TDIs and TWDs vary internationally.

Detectability of TWSIs

Indented or grooved geometric designs on walking surfaces are not sufficiently detectable by people with vision impairments (Bentzen et al. 2000), so TWSIs rely on designs that employ patterns of elements raised from the base surface. Detectability is largely influenced by the spacing between the raised elements (e.g., the truncated dome or the bar) compared to the top width of the raised element, the height of the raised element, and the overall area of pavement coverage of the TWSI. Raised elements spaced closer together are less detectable than those farther apart, but even detectable surfaces may be missed by people with low or no vision when approaching a TWSI perpendicular to its length if the width of the TWSI (i.e., the dimension in the approaching direction of travel) is such that people inadvertently step over it. Research consistently shows that TWSIs need to be approximately 24 in. (60.96 cm) deep in the direction of travel across the surface to be detected. This is particularly important where, in the case of a DWS, for example, it is critical for visually impaired pedestrians to detect and identify it so they can assess the potential for a hazard on the other side before crossing over the DWS. People who are blind are more

likely to step over or miss shorter depths of TWSI surfaces due to the average natural gait and stride length of pedestrians (Peck and Bentzen 1987; Mitchell 1988; Tijerina et al. 1994; Hughes 1995; O’Leary et al. 1996; Bentzen and Myers 1997; Fujinami et al. 2005).

For the raised elements of a TWSI to be highly detectable, their height must be at least 0.2 in. (5 mm). This height has been shown to be detectable and discriminable by people with vision impairments while not impeding people with mobility impairments (Bentzen et al. 1994; NITE 1998; NITE 2000; Sawai et al. 1998), and indeed it is the height specified by most standards internationally and in the United States for DWSs and TDIs. Height is particularly important when TWSIs are installed on a rough surface, as the imperfections in the surrounding pavement may make it more difficult for people with vision impairments to distinguish the raised elements of the TWSI.

Previous research focused on the center-to-center spacing between raised elements in relation to the top widths or diameters of the raised elements and found that proportions of top widths or diameters between 0.7 and 1.4 in. (18 and 35 mm) and center-to-center spacing between 2.4 and 2.8 in. (60 and 70 mm) for domes or 3.0 to 3.4 in. (75 to 86 mm) for bars are detectable (NITE 1998; Sawai et al. 1998). This focus on center-to-center spacing results in a broad range of gap spacing (the space between the edges of the tops of each raised element) currently permitted by the 2010 ADA standards for DWSs, which have not all been tested and demonstrated to be reliably detectable.

While TWSIs are primarily intended to be detected based on tactile information conveyed through the geometry of their surface patterns, most people who are legally blind are not fully blind. It is therefore the consensus in U.S. and international standards and guidelines that TWSIs should have high visual contrast with surrounding surfaces. Based on the results of their controlled research, Jenness and Singer (2006) recommended that the choice of color for DWSs be determined by luminance contrast with the adjoining surface (light on dark or dark on light), and that combinations where the reflectance of the lighter color was less than 10% should not be used. They recommended that any standard express both a minimum luminance contrast and a minimum reflectance value for the lighter of the two surfaces. Federal yellow was recommended where the desire was to have a single uniform color for DWSs because of its high conspicuity rating across different levels of luminance contrast. Yellow is especially effective in association with dark sidewalks. Where sidewalks are light, a good choice for both detection and conspicuity is a dark brick red (red-orange) (Jenness and Singer 2006). Several Japanese studies also support the need for not only color contrast between the installation surface and TWSIs but improved ambient lighting to increase luminance contrast, and found that yellow had the highest detection rate at low light levels (Mitani, Yoshida, et al. 2007; Mitani et al. 2009; Mitani et al. 2011).

In general, knowledge and experience U.S. practitioners have acquired from installing and maintaining DWSs should apply to any TWSI produced with similar material and applied and maintained in similar ways (Ketola and Chia 1994; Bentzen et al. 2000). TWSIs not maintained or poorly installed can compromise their effectiveness in providing wayfinding. Most research on durability and maintenance of different TWSI materials originates from states where freezing and snow conditions are common, and tested products over time for durability of visual contrast (e.g., color fading) and material degradation. Materials like cast iron or solid steel work better where frequent snow removal occurs, and, indeed, snow removal is needed to ensure the raised elements of TWSIs remain detectable (Landry et al. 2010; Couturier and Ratelle 2010).

Discriminability of TWSIs

When TWSIs are used together as a system, they must be not only detectable as a TWSI, but discriminable from one another. People with vision impairments must be able to identify which type of TWSI they encounter because each type conveys a different meaning and calls for a different response from the traveler. Discriminability is largely influenced by the geometry of the TWSI patterns, including the spacing between the raised elements compared to the top width

of the raised element, the height of the raised element, and the shape of the element (e.g., truncated dome or elongated bar). For example, for people with vision impairments to be able to feel the shapes and patterns underfoot or to identify with a long cane if a TWSI is either domes or bars, the dimensions of the raised elements and the spacing between these elements are key. The majority of research done to test discriminability of different TWSI patterns comes out of Japan, including a seminal systematic study that tested 81 combinations of nine DWS geometries and nine TDI geometries (NITE 1998). Results from this study combined with subsequent research that confirmed or further tested detectability and discriminability show that optimal geometries for TDIs are where the top width of the raised bars is 0.7 to 1.4 in. (17 to 35 mm) with a center-to-center spacing between bars of 3.0 to 3.4 in. (75 to 86 mm). For DWSs, a base diameter of 0.9 in. (22 mm), top diameter of 0.5 in. (12 mm), and center-to-center spacing of 2.2 to 2.4 in. (55 to 60 mm) were detectable and discriminable (NITE 1998; Sawai et al. 1998; NITE 2000; Mitani, Fujisawa, et al. 2007). While the broad range of center-to-center spacing permitted by the 2010 ADA standards allows for DWSs with domes as closely spaced as 1.6 in. (41 mm), research from Japan indicates that domes spaced 1.7 in. (43 mm) apart were not highly detectable and discriminable (NITE 1998; Sawai et al. 1998; NITE 2000).

Detectability and Discriminability of TWDs

Tactile warning delineators (TWDs) are unique from DWSs and TDIs in their meaning and geometry. This particular type of TWSI serves not only to warn and guide pedestrians with vision impairments, should they choose to follow along it, but also deters pedestrians from crossing over it into the vehicular travel way and likewise deters operators of vehicles such as bicyclists from crossing over it into the pedestrian travel way. While TWDs are relatively new with no standards in the United States, they have been under consideration and used elsewhere. In two studies out of the United Kingdom, from 1997 and 2010, researchers tested different geometries and patterns of delineator strips, including single continuous longitudinal trapezoidal, bar, U, dome, and inverted T shapes; variations of TDIs with different orientations of the bars; different DWSs; and variations in height and width of the raised elements. They considered detectability of the different delineators, discriminability from DWSs, and traversability for people with mobility impairments or people riding bicycles (Savill et al.1997; Childs et al. 2010). Findings from these two studies informed research produced by Bentzen, Scott, and Myers (2020) in the United States. Their study assessed six different TWSIs for effectiveness as an edge treatment for sidewalk-level separated bike lanes to delineate the pedestrian area from bicyclists. They measured detectability, identifiability, and following tasks with blind participants; traversability with mobility-impaired participants; and frequency of intrusions into the bicycle lane by long cane or bodies of the blind participants. Participants identified the raised trapezoidal delineator (10.08-in.-wide base, 6.33-in.-wide top, 0.75 in. height) as the TWD significantly more than the DWS, two different surface widths of TDI (flat-topped elongated bars), or two different surface widths of corduroy strips (narrow, more closely spaced bars) were identified—all of which were detectable. Long cane or body intrusions into the bicycle lane were minimized with the 24-in.-wide (0.6 m) surfaces of TWSIs compared to the 12-in.-wide (0.3 m) surfaces. The majority of the vision-impaired participants expressed a preference for the trapezoidal delineator. While a majority of the mobility-impaired participants disliked the trapezoid, it was traversable (Bentzen, Scott, and Myers 2020).

Considerations for People with Mobility Impairments

Sometimes, the geometric design and dimensions that make a TWSI pattern highly detectable and identifiable to a vision-impaired person are the same characteristics that cause discomfort or loss of stability or take more effort to traverse by people with mobility impairments. Several sources verify that a maximum height of 0.2 in. (5.1 mm) for DWS or TDI patterns have minimal

effects on effort, slippage, stability, and wheel or tip entrapment even when placed on ramps with a 1:12 slope (Peck and Bentzen 1987; Bentzen et al. 1994; Hauger et al. 1996; Hughes 1995). Gap spacings in relation to the top widths of raised elements may cause more or less vibration for people using wheeled mobility aids, with wider gaps and narrower top widths being more adverse (ISO 2019). The orientation of the TDI bars can also more or less adversely impact mobility-impaired travelers; the bars are more easily negotiable when they are parallel to the person’s direction of travel (Bentzen, Scott, Emerson, and Barlow 2020).

Orientation of TDIs for Different Applications

While TDIs are typically thought of as guiding surfaces indicating a direction or path of travel to follow along with the bars parallel to that direction of travel, additional research has investigated their applications in other wayfinding tasks, such as to identify or mark a crossing location like a midblock crosswalk or a crosswalk at a roundabout or channelized turn lane where other environmental or landscaping cues are not present, or to indicate an alignment heading where other cues are missing or misleading, like at a skewed intersection where the curb ramps may not slope in the direction of travel on the associated crosswalk. Langevin et al. (2014) found that people with vision impairments could successfully approach a strip of TDI installed across the width of the sidewalk with the bars perpendicular to their direction of travel along the sidewalk, detect it, and then turn and follow the TDI (now oriented parallel to their direction of travel) to the crosswalk and subsequent DWS. Bentzen et al. (2017) confirmed the effectiveness of using a TDI installed across the width of the sidewalk to mark crossing locations at midblock and roundabout crossings and further found that by installing the TDI with the bars parallel to pedestrians’ initial direction of travel, not only did they detect the TDI, but once they turned and oriented the bars perpendicular under their feet and followed it to the crosswalk, they could establish a correct alignment or heading to cross. Orienting the TDI so the bars were perpendicular to the direction of travel at the crosswalk resulted in pedestrians with vision impairments better aligning to cross than when the bars were parallel to the direction of travel at the crosswalk (Takeda et al. 2006; Scott et al. 2011a; Scott et al. 2011b; Bentzen et al. 2017).

Standards and Practices

Everywhere except in the United States, countries use domes and raised bars as a TWSI system, where the domes serve as attention fields for warning of hazards and to indicate turns, intersecting paths, and key points of interest like bus stops, elevators, tactile maps, or other waypoints (Bentzen et al. 2021). The ranges in patterns and dimensions of the raised elements are based on the research about detectability and discriminability, with the understanding that smaller elements can be closer together while larger elements need more gap spacing to maintain detectability. The range of standard dimensions for the size and spacing of the raised elements from six different countries, including the United States, and the more broadly used International Organization for Standardization (ISO), are provided in Appendix A, Tables A-1 and A-2. General takeaways from reviewing practices inside and outside the United States revealed the following consistencies in applications:

• DWSs are typically installed at curb ramps, curb cuts of medians and pedestrian refuge islands, and transit platforms. Depending on the country, they may also be installed at the tops of stairs or reflecting pools. When used as an attention field to indicate a hazard, they run the full length of the hazard they demarcate.
• DWSs are typically 22 to 25.6 in. (56 to 65 cm) deep in the direction of pedestrian travel to reduce the likelihood that people with vision impairments will step over and miss the TWSI.

• In most countries other than the United States, DWSs are also used to call attention to a certain location, such as at turns, intersections, or decision points along TDI paths.
• Most countries use raised bars as the geometric design for their TDIs. Where they are installed to mark a path to follow, the bars are oriented parallel to the direction of travel, and the TDI path is typically 9.8 to 11.8 in. (250 to 300 mm) wide.

There is broad variation in the application of TDIs internationally. Some countries install TDIs along sidewalks only where other cues in the built environment are missing or insufficient to provide wayfinding to people with vision impairments, while other countries may install them to define a continuous path of travel. While there are no standards for TDIs currently in the United States, there are other best practice recommendations to consider:

• U.S. DOT Accessible Shared Streets discusses the use of TWSIs as detectable edges (Elliot et al. 2017).
• National Association of City Transportation Officials (NACTO) Urban Street Design Guide recommends tactile strips along shared use path entrances (NACTO 2013).
• American Society of Landscape Architects (ASLA) Universal Design contains recommendations for “perpendicular tactile paving” to indicate hazards (ASLA n.d.).
• American Public Transportation Association (APTA) Transit Universal Design Guidelines emphasizes that tactile paths should be “distinct from detectable warning surfaces to preserve the communication of essential safety concerns” (APTA 2020).
• The next edition of the AASHTO Guide for the Development of Bicycle Facilities is currently under review and recommends using TDIs to separate the pedestrian and bicycle ways when they are on the same level (Schulthiess and Chrzan 2019).

Meanwhile, in 2019, 14 transit and municipal agencies were identified in the United States as using some form of TWSI or wayfinding surface in addition using DWSs at the edge of crossing locations and transit platforms. Ten of these agencies were interviewed to learn more about their use of different guiding surfaces or their applications. The information gleaned from these interviews is presented in Appendix B.