Sept/Oct 2004
Vol. 68 · No. 2

Testing Truncated Domes

by Mark Chandler

Accessibility guidelines for the disabled require detectable warnings on all curb ramps. Here's how some States are getting the job done right.

Intersections that permit vehicles and pedestrians to interact are complex environments, especially for people who are blind or visually impaired. For safe and independent travel, persons with limited or no vision depend on environmental cues such as curbs, texture changes underfoot, ambient sounds, and physical elements that can be sensed by a cane. People with low vision also rely on color contrast as a navigational aid.

The Americans with Disabilities Act (ADA), passed in 1990, required the establishment of design criteria for building and altering commercial and public facilities, including sidewalks and curb ramps. In 1991, the U.S. Department of Transportation (USDOT) and the U.S. Department of Justice (USDOJ) developed a set of regulations for new construction and alterations. The regulations include standards that reference the ADA Accessibility Guidelines for Buildings and Facilities (ADAAG), developed by the U.S. Access Board-an independent Federal agency devoted to accessibility for people with disabilities. The guidelines require the installation of detectable warnings on sidewalks, street crossings and curb ramps, hazardous vehicular ways, and transit platform edges.

Members of the Public Rights-of-Way Access Advisory Committee evaluate a retrofit installation of truncated domes on a curb ramp in Portland, OR.

The guidelines define a detectable warning as "a standardized surface feature built in or applied to walking surfaces or other elements to warn visually impaired people of hazards on a circulation path." Detectable warnings are texturally unique and standardized features, intended to function much like stop signs. The warning alerts visually impaired pedestrians to the presence of hazards in the line of travel, indicating that they should stop and determine the nature of the hazard before proceeding further.

Since 1991, truncated domes have been the standard design requirement for detectable warnings on curb ramps and at flush transitions from sidewalks to street crossings. The U.S. Access Board temporarily suspended the standard in 1994 but allowed the suspension to expire in 2001, reestablishing the mandate.

The small, flattened domes provide a surface that is distinguishable underfoot and by cane, and they are closely spaced so that pedestrians can maintain stability. In addition, the color of the domes contrasts with the surrounding pedestrian ramp to provide a cue for low-vision persons that a transition from the pedestrian area to the vehicular area is forthcoming.

The ADAAG requires that municipalities and States install truncated dome surfaces on all new curb ramps and on any projects involving alterations to existing ramps. "The lack of curb ramps and noncompliance with design standards for these facilities constitute the greatest number of ADA complaints in the pedestrian environment," says Associate Administrator for Civil Rights Frederick Isler of the Federal Highway Administration (FHWA). "There continues to be a misconception that detectable warnings are not a requirement, but they are." (See "Truncated Warning Domes and the Americans with Disabilities Act" on this page.)

Defining Truncated Domes

According to the U.S. Access Board, research conducted in the 1980s indicated that the truncated dome is the most effective system for providing a distinctive pattern detectable by cane and underfoot. The research showed that other designs, such as grooves, striations, and exposed aggregate, are not detectable in the sidewalk and roadway environment because of similarities to other surface textures and defects.

Warnings should adjoin or abut the hazard to signal the impending change and extend beyond the average stride length so a person can detect, understand, and react to the warning before encountering the hazard.

Truncated warning domes typically fall into one of three categories: inset, glued, or stamped, depending in part on whether the project involves new construction or a retrofit. Inset products are those that are pressed into fresh concrete or recessed into the cutout portion of an existing sidewalk, including ceramic or concrete tiles and pavers (landscaping bricks). Glued-on products are those that involve applying flexible mats of domes onto an existing sidewalk using an adhesive. Stamped concrete systems involve imparting the dome texture on a fresh concrete surface using either rigid or flexible stamping tools, typically made of rubber or polyurethane.

In terms of compliance, Peter Kemp, with the Technology Advancement Unit at the Wisconsin Department of Transportation (WisDOT), stresses the importance of differentiating between the 1991 regulation standards, ADAAG, and the subsequent draft guidelines for public rights-of-way. "The draft guidelines now are underway for a new rulemaking on right-of-way access," he says, "but it may be several years before the draft guidelines become regulations. In the interim, the U.S. Access Board has given individual State departments of transportation guidance on how to implement the draft guidelines and meet the 1991 standard."

Currently, the draft guidelines for public rights-of-way, published in June 2002, describe detectable warnings as a surface of truncated domes arranged in a square grid pattern. The domes need to have a base diameter of 23 to 36 millimeters (0.9 to 1.4 inches), a top diameter of 50 to 65 percent of the base diameter, and a height of 5 millimeters (0.2 inch). Dome center-to-center spacing is allowed in the range of 41 to 61 millimeters (1.6 and 2.4 inches) and a base-to-base spacing of at least 16 millimeters (0.65 inch), measured between the most adjacent domes on the square grid. Detectable warnings also need to contrast visually with adjoining surfaces, either light-on- dark or dark-on-light. The surface of the detectable warnings must be 61 centimeters (24 inches) by the width of the curb ramp.

Since 2001, a number of municipalities, States, and other organizations have initiated product trials to evaluate the truncated warning dome systems available on the market. Highlights from research in Wisconsin, New Hampshire, and Texas offer insights on selecting the most effective products.

Wisconsin Tests Domes For Winter Wear

In 2002, responding to the reenacted regulation requiring truncated domes, WisDOT partnered with FHWA and the city of Madison to conduct a study of products on the market.

One worker uses a rubber mallet to sink an inset tile product into a new curb ramp in Madison, WI, while another smoothes the fresh concrete.

"We wanted to ensure that the methods and materials we chose would comply with the ADA rule and provide the lasting performance we wanted to see on our projects," says WisDOT's Kemp. "When we went looking for information about our options, we noticed a lack of good baseline data on what works as far as aesthetics, durability, color retention, and slip resistance. So we chose to do a limited study to identify products and improve our confidence level."

WisDOT selected the products to represent a cross section of the systems currently available. Installations were either cast in place for new sidewalks or retrofits to existing facilities. Retrofitted products included materials that were glued on, either in sheet form or applied individually to the surface of the sidewalk. The department evaluated eight products representing six manufacturers.

Staff from WisDOT and Madison's engineering division installed truncated warning domes on 44 ramps at 11 sites throughout the city. The sites were selected based on sidewalk condition, ramp configuration, and the possibility of incorporating installations into existing contracts for upgrading sidewalks. The installations began in fall 2002, and the team evaluated the performance of each product through the winter and following spring. In November 2003, WisDOT published its final report, Truncated Warning Dome Systems for Handicap Access Ramps (WI-04-03).

According to Kemp, the team has approved inset systems only, based on criteria such as ease of construction, consistency in quality, aesthetic quality, durability, and color retention. "Insets take minimal additional labor to install," he says, "You don't have to grind the sidewalk, so you have a more consistent look. Some of the glued-on products left adhesive along the edges. One product was installed up to 0.5 inch [1.27 centimeters] above the existing sidewalk."

An additional aspect of the product trials in Wisconsin was a study of color contrasts. In 2000, the U.S. Access Board published Detectable Warnings: Synthesis of U.S. and International Practice, a report indicating that "safety yellow is a color that is standardized for use as a warning in the pedestrian-highway environment." A retired technical advisor to Madison's engineering division, Duane Sippola, who has more than 30 years experience working with tactile cues for curb ramps, suggested bringing in staff members from the Wisconsin Council of the Blind to evaluate the color of the products. The objective was to determine which colors were best for visually impaired persons and at what distance they start to pick up the contrast of a dome-patterned ramp.

"Designers and public officials often are drawn to a variety of colors for aesthetic reasons, but we need to remember that our goal is to protect pedestrians," Sippola says. "In our tests, we found that yellow stands out really well for partially sighted pedestrians. Yellow and white could be seen at the farthest distances, in most cases, the width of a residential street, 32 to 45 feet [9 to 14 meters]. It seems that low-vision persons do notice the yellow color on signposts and curbs. Since all crosswalk lines are painted white, it seems to make sense that domed ramps should be yellow to provide the necessary contrast."

During the study, Sippola borrowed from the Wisconsin Council of the Blind a set of low-vision goggles simulating 20/200 vision. He then took digital photos through the goggles at distances ranging from 12.5 meters (41 feet) to 0.75 meter (2.5 feet) to illustrate the importance of color and contrast in recognizing ramp treatments. "It's hard for sighted people to imagine what visually impaired folks experience," Sippola says. "This experiment really points out the effectiveness of the yellow color."

To comply with ADA guidelines, WisDOT selected yellow and white as the standard colors for domed ramps. The agency implemented the new standard for truncated warning domes starting in July 2003. Further, WisDOT developed detailed drawings, construction notes, specifications, and an approved product list, and will continue evaluating new products, including stamped concrete and precast masonry panels.

For more information, contact Peter Kemp at 608-246-7953 or peter.kemp@dot.state.wi.us.

To illustrate the importance of color and contrast in ramp treatments, researchers from WisDOT and the city of Madison took digital photographs through low-vision goggles at distances of (left) approximately 9.8 meters (32 feet) and (right) 1.5 meters (5 feet). Photos: Duane Sippola.

New Hampshire Builds Test Sidewalk

In December 2002, the New Hampshire DOT initiated a study to document the ease of installation and durability of eight truncated warning dome systems under winter maintenance and weather conditions, including plowing and surface deicing treatments.

"We had very little information on what products would work best in our region," says Assistant Research Engineer Denis Boisvert, with the New Hampshire DOT. "Manufacturers typically don't provide data on the performance of their products in the winter or under the plow. They test durability through wear resistance, using 60-grit sandpaper under a 1-kilogram [2.2-pound] load, which might simulate pedestrian traffic well, but it's not appropriate for assessing the wear from a plow."

Along Hazen Drive in Concord, NH, the department constructed a 70-meter (229-foot)-long sidewalk consisting of individual test sections to accommodate each dome system. Five installations required cutting recessed surfaces, one involved stamping the domes directly onto a fresh concrete surface, and the other three featured typical sidewalks for surface-applied retrofits. The shop-fabricated test sections were transported to the site by flatbed truck and trailer, and then lifted into place by a truck-mounted crane in February 2003.

The city of Concord plowed and treated the test sections as part of its regular maintenance routine for municipal sidewalks, using a 1.5- meter (5-foot)-wide, four-wheel drive vehicle with a hydraulically angled plow blade.

Staff from the New Hampshire DOT documented the installation and evaluated the performance of the test sections during the first winter through 20 plowing cycles. The first two cycles involved natural snowfall, but since the test sections were installed in late February, the researchers were concerned whether enough storms would occur to constitute a satisfactory number of plowing cycles. Therefore, the department planned to generate as many artificial snowfall cycles as possible in 1 day, piling snow on the test sections using a front-end loader, followed by repeated removal by the city's plow.

Like Wisconsin, New Hampshire found inset systems to hold the most promise. "We identified two inset products that we will apply to upcoming construction projects," Boisvert says. "They are the most durable of the compliant products we tested. We are monitoring the sidewalk for a second season for long-term performance."

The stamped product was the least attractive due to deformities of the domes and background mat that resulted in dimensional noncompliance (in terms of height or diameter) with the guidelines. "The domes showed substantial damage and wear after the initial testing of 20 plow passes," Boisvert says.

Boisvert and his colleagues note that testing revealed two types of failures. "The domes themselves wear quickly, or the entire system is torn off or peels off, particularly surface-applied products," he says. "The plow catches the edge and tears the product or rolls it off the surface. The domes that receive the worst wear are the first row. Once the plow is on top of the dome matrix, the wear is less. But after a few seasons, they too may be in tough shape. Even with the better performers, this is an area for further research to extend product life. If the products were recessed even more, so the domes were flush with the top of the sidewalk, we might not see as much damage."

The department published a final report, Durability of Truncated Dome Systems (FHWA-NH-RDMPS2002- 2), in April 2003. For more information, contact Denis M. Boisvert at 603-271-3151 or dboisvert@dot.state.nh.us.

NHDOT researchers used this plow to evaluate the durability of various truncated warning dome systems during snow removal. Damaged domes are visible in the foreground.

Texas Domes Weather the Heat

While durability in cold weather and under snow removal is a primary concern in the north, in the warmer States, like Texas, the effects of the sun's heat represent the biggest challenge to durability. Since 2002, the Texas DOT has used brick pavers with truncated domes on several hundred projects, with considerable success.

After 20 plow passes, this recessed inset dome system was among the most durable of those tested by NHDOT.

Three plow passes destroyed this surface-applied product.

"We were fortunate to benefit from lessons learned by the city of Austin as they experimented with various products in the late 1990s," says Elizabeth Hilton, director of plan development with the Texas DOT Design Division. "Some engineers tried using stamped concrete to create the truncated domes, but these were difficult to construct properly and tended to break off easily. The only problem we have had with the brick pavers is achieving a flat surface on the curb ramp. The city advised us that when they placed the pavers on a sand bed, as is typical with landscaping bricks, the sand washed out due to the slope of the curb ramp, resulting in an irregular surface. Therefore, our placement detail requires that contractors place pavers on a mortar bed."

To expand the options, the department recently initiated an informal study to identify additional products for use in both new installations and retrofits. The purpose, according to Hilton, is to identify an array of acceptable products that the department can choose from, ultimately enhancing competitive bidding among manufacturers.

"We offered vendors the chance to install their products at our Riverside complex in Austin," she says. "We are evaluating how well the domes stay on the surface and how the products weather, particularly in the Texas heat. Our main concern is with adhesive products coming unglued in the heat. Durability of the color contrast, or the light reflective value, also is a concern."

Although the evaluation is ongoing, Hilton says that, so far, most of the products have yielded acceptable results. In fact, the department has begun allowing use of the products on the State highway system. For more information, contact Elizabeth Hilton at 512-416-2689 or ehilton@dot.state.tx.us.

Meeting the Challenge

Landscape pavers with truncated domes, like those shown here on a sidewalk in Austin, TX, have been used on many projects in Austin and elsewhere in Texas.

According to the U.S. Access Board's report, complex traffic operations, including actuated signals and right turns on red, have made it increasingly difficult for visually impaired persons to analyze the roadway environment using vehicular sound. High traffic volumes and ambient noise often mask the sounds of vehicles starting and stopping. In addition, the trend toward more aggressive driving has reduced the likelihood that drivers will stop for pedestrians in crosswalks at unsignalized intersections. Now, more than ever, efforts like truncated warning domes are essential to ensuring safety and access for visually impaired persons.

"Education is critical for engineers to know why the domes are needed so they can locate them in the correct place," says Hilton from the Texas DOT. "Our solution is to train engineers in the department on curb ramp design, including the appropriate use of truncated domes."

In addition to educating in-house staff, Dennis Cannon of the U.S. Access Board notes that government staff members need to communicate the requirement for truncated warning domes to the contractors who install curb ramps. "Every curb ramp eventually will need to be replaced," Cannon adds, "whether because of wear and tear, installation of utility lines beneath the sidewalk, or a widening project. The key is to survey all curb ramps, locate those that are in bad shape, and then rank them from worst to best to prioritize scheduling replacements. It's often best to start with the downtown or the areas that are most traveled."

Ultimately, it is up to State and local agencies to determine which compliant products will work best in their environments and when to install them, but resources exist to help engineers, designers, and decisionmakers make the most effective choice. The challenge is to make everyone's accessible route a safe one.

Mark Chandler, PE, CMfgE, is the technology transfer and quality engineer with the FHWA Wisconsin Division. Chandler has an undergraduate degree in geoengineering and a master's in manufacturing systems engineering, with a concentration on quality. He is a candidate in the Indiana State University distance Ph.D. program in technology management, with a specialization in quality systems.

The American Association of State Highway and Transportation Officials funded a project through the National Cooperative Highway Research Program (NCHRP) that will gather and compile research and evaluation data available from State DOTs on maintaining detectable warnings. The Texas Transportation Institute is performing the research, and a final report is expected in 2005. For more information, the NCHRP contact is Dr. Amir N. Hanna at ahanna@nas.edu.

For more information on truncated warning domes, contact Mark Chandler at 608-829-7514 or mark.chandler@fhwa.dot.gov.