Truncated domes are the raised tactile bumps installed on walking surfaces to alert people with visual impairments that they are approaching a hazardous change in grade — typically the transition from a sidewalk to a street. They are formally called "detectable warning surfaces" and are required by both the ADA and the California Building Code (CBC Chapter 11B) at specific locations. Getting the placement, size, and color right is a frequent plan check item.

Where Truncated Domes Are Required

  • Curb ramps at street crossings — the most common application. Required at every curb ramp where a pedestrian route crosses a vehicular way.
  • Transit platforms — at the edge of rail platforms and bus boarding areas where the platform borders the vehicle travel lane.
  • Hazardous vehicular areas — where a sidewalk or accessible route crosses a vehicular route that is not separated by a curb, railing, or other physical barrier. Examples: shared-use areas in parking lots, drive aisles without curbs, and areas where vehicles back out across a pedestrian path.
  • Reflecting pools and fountains — at the edge of pools and decorative water features adjacent to walking surfaces (less common in site civil work but required by code).

Dimensions and Layout

Per ADA Standards Section 705 and CBC 11B-705:

  • Dome diameter: 0.9 inches nominal (0.82 to 1.0 inch range)
  • Dome height: 0.2 inches nominal (0.15 to 0.25 inch range)
  • Center-to-center spacing: 1.67 to 2.35 inches
  • Base-to-base spacing: 0.65 inches minimum
  • Dome arrangement: Square grid or radial pattern, aligned with the direction of travel

Field Dimensions at Curb Ramps

  • Depth (in direction of travel): 24 inches. This is the dimension from the back edge of the dome field to the front edge, measured in the direction a pedestrian would walk across the ramp.
  • Width: The full width of the curb ramp. The dome field extends from one side of the ramp to the other. If the ramp is 48 inches wide, the dome field is 48 inches wide.
  • Location: The front edge of the dome field must be at the bottom of the ramp, at the transition to the street surface. The back edge is 24 inches up the ramp from the front edge.

Color and Contrast

The detectable warning surface must provide visual contrast with the adjacent walking surface. The most common color is bright yellow on gray concrete. In California, CBC 11B-705.1.1.2 specifies that the detectable warning must have a "visually contrasting color" — which is generally interpreted as requiring a color that has at least 70 percent contrast with the surrounding surface.

Standard color combinations:

  • Yellow domes on gray concrete (the most common and universally accepted)
  • Red or brick-red domes on gray or light-colored concrete
  • Dark gray domes on light-colored concrete (less common, some inspectors question the contrast)

Avoid matching the dome color to the surrounding surface. A gray dome on gray concrete provides tactile detection but fails the visual contrast requirement.

Material Options

  • Cast-in-place — the domes are formed directly into the wet concrete surface using a stamping mat. This is the lowest-cost option but requires skilled concrete finishers and careful timing. If the concrete sets before the stamp is applied, the domes will be poorly formed.
  • Surface-applied tiles — prefabricated polymer, composite, or stainless steel tiles that are glued, bolted, or set into the concrete surface. These provide a consistent, factory-controlled product but cost more and can delaminate if the adhesive fails.
  • Replaceable insert — a metal or polymer frame is cast into the concrete, and the detectable warning panel snaps or screws into the frame. This allows replacement if the panel is damaged. This is the preferred option for transit agencies and high-traffic locations.

Common Mistakes

  • Wrong depth. The dome field must be 24 inches deep, not 36 inches or the full length of the ramp. Oversizing the dome field is not a code violation, but undersizing it is.
  • Dome field does not extend the full width of the ramp. Gaps between the edge of the dome field and the side of the ramp leave an unprotected path that a visually impaired person could follow off the ramp without warning.
  • Installed at the wrong location. The dome field goes at the bottom of the ramp where it meets the street, not at the top of the ramp. Placing it at the top warns pedestrians that they are approaching a ramp, which is not the intended warning — the warning is for the vehicular surface at the bottom.
  • Not installed at all. On projects with many curb ramps (campus sites, large commercial developments), it is common for one or two ramps to be poured without the truncated dome installation. The building inspector catches it during final inspection, and the correction requires saw-cutting the ramp surface and installing a surface-applied panel — a much more expensive fix than installing it correctly during the initial pour.
  • Installed at locations where they are not required. Truncated domes are not required at every curb or every ramp — only at locations where the pedestrian route crosses a vehicular route. A curb ramp that leads to a landscaped median (not a vehicle travel lane) does not require truncated domes. Over-installation creates a "boy who cried wolf" effect where the warning becomes meaningless because it appears everywhere.

Maintenance

Truncated domes wear over time, especially in high-traffic locations. Worn domes lose their tactile detectability and visual contrast. Damaged or missing domes must be replaced promptly because the absence of a detectable warning at a required location is an ADA violation. Surface-applied panels can be replaced individually. Cast-in-place domes require more extensive repair (typically saw-cutting and patching or applying a surface panel over the worn area).