Truncated Domes
Detectable Warning at Platform Edge: 2026 Spec
Cojo
May 7, 2026
6 min read
A detectable warning at a transit platform edge is a 24-inch deep truncated dome surface running the full length of any platform with vehicular drop-off, set 0 to 8 inches from the platform edge. ADA Standards 810.5.2 controls the placement at transit boarding platforms, with ADA 705.1 governing dome geometry and ADA 705.2 governing visual contrast. Bus stops, light-rail stations, and ferry terminal platforms all fall under this requirement.
This guide covers platform-edge placement specifically, where the requirements differ meaningfully from a sidewalk curb-cut application.
ADA Standards for Accessible Design, Section 810.5.2, addresses detectable warnings at boarding platforms used by buses, light-rail, commuter rail, and ferries. The federal text reads "Platform boarding edges not protected by platform screens or guards shall have detectable warnings complying with 705 along the full length of the public use area."
The cross-reference to ADA 705 brings in 705.1 dome geometry and 705.2 visual contrast. ADA 810.5.2 itself adds three platform-specific placement rules.
The dome surface runs the entire length of the platform that pedestrians use to board or disembark. This is different from a curb cut where the dome surface is local to the ramp; on a transit platform the dome surface can run hundreds of feet.
The same 24-inch depth as ADA 705.3, measured perpendicular to the platform edge.
ADA 810.5.2 allows the leading edge of the dome surface to sit 0 to 8 inches from the actual platform edge. The setback exists to protect the dome surface from being struck by transit vehicles approaching the platform. Most transit agencies set back 6 inches; some set back 8.
The standard applies to any boarding platform with vehicular drop-off, including:
| Platform type | Application |
|---|---|
| Bus stop with raised curb | Yes |
| Bus stop at street level | Generally not (no vertical drop) |
| Light-rail platform | Yes |
| Commuter-rail platform | Yes |
| Streetcar platform | Yes |
| Subway platform | Yes |
| Ferry terminal platform | Yes |
| Airport jet bridge transition | Special rules under 810.5 |
| Curbside taxi stand | No |
The dome surface placement at a platform edge is geometrically simpler than at a curb cut because the platform edge is a single straight or curved line rather than a 5-component ramp.
Leading edge of the dome surface is at 0 to 8 inches from the platform edge. Most transit agencies standardize on 6 inches.
24 inches minimum, measured perpendicular to the platform edge.
Full length of the public-use area. This typically corresponds to the area where the train or bus can stop and discharge passengers, which for light-rail is often the full length of the platform and for buses is often a 50 to 100 foot zone.
A curved platform edge requires the dome surface to follow the curve, which means custom-cut panels or a mat that conforms to the curve. The 24-inch depth is measured perpendicular to the local tangent of the curve.
The three transit modes have different platform geometries that affect placement.
Most bus platforms are 30 to 50 feet long with a curb height of 6 to 14 inches. The dome surface runs the full platform length. TriMet, Lane Transit, and Cherriots use either replaceable cast-iron systems or polymer-concrete panels.
Rail platforms are 100 to 400 feet long with a curb height of 0 to 8 inches above the rail. The dome surface runs the full platform length, often in 24-inch by 36-inch or 36-inch by 60-inch panel modules. TriMet MAX platforms use cast-iron systems.
Ferry platforms have a unique consideration: the platform edge is the loading transition to the vessel, which moves with tides and water level. The dome surface goes at the fixed-platform edge with the same 24-inch depth and 0 to 8 inch setback.
These are the placement and product errors Cojo crews see at transit-platform retrofits.
A panel installed 12 inches back from the platform edge fails ADA 810.5.2 because the setback exceeds the 8-inch maximum. Common cause: the platform itself was poured deeper than the original design intended, leaving a flat zone between the platform edge and the panel.
A 50-foot dome strip on a 75-foot bus platform fails ADA 810.5.2 length. The dome surface must extend the full public-use area, which typically corresponds to where the bus or train physically stops to discharge passengers.
A composite plastic panel installed on a busy rail platform (TriMet MAX, Sounder commuter) wears below ADA 705.5 within 5 to 7 years. Cast-iron systems are the durability spec for high-traffic transit platforms.
Long platform-edge runs cross expansion joints in the platform itself. Dome systems must accommodate the joint movement, either with discrete panel modules or with a mat that includes joint-compatible adhesive. Continuous-pour stamped overlays can crack at the joint.
Concrete platform surfaces sometimes weather to LRV 50 to 65, which can drop the differential against a federal-yellow dome below 70. LRV verification on the actual platform surface is required, not just the manufacturer's spec.
This article reflects ADA Standards for Accessible Design as of 2026-05-07 and product spec sheets current at publication. Always verify current dimensions, contrast thresholds, and placement requirements with your local jurisdiction and the U.S. Access Board before issuing a final spec. Federal guidance under 36 CFR Part 1191 controls when state or local rules conflict. Transit-agency-specific Trade Standards may layer additional requirements on top of ADA 810.5.2.
On a Portland TriMet bus platform retrofit in February 2026 we replaced 14 cast-iron dome panels along a 60-foot platform edge after the original 1998 panels reached year-28 wear and lost contrast at the leading edge. The new replaceable cast-iron plates installed in 35 minutes per panel because the original frames remained sound. Total project ran roughly 9 crew-hours including the LRV verification on the new plates against the weathered platform concrete. Differential read 76 LRV new plate to 18 LRV concrete, well above the 70-point threshold. Replaceable systems pay off on multi-decade transit assets.
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