“Please offer me a seat. Not all disabilities are visible,” reads a new sticker that’s available to buy online, intended for disabled people who struggle to find seating on public transit.

The idea is the brainchild of Catherine Callahan, who volunteers with Bay Area Rapid Transit, the transit system that serves San Francisco and the Bay Area, and product designer Chris Arvin.

Since birth, Callahan has lived with spina bifida, a condition where the spine doesn’t fully form. While she doesn’t always use a wheelchair, she does every time she takes public transport, because she’s not confident she would get a seat. “If I didn’t get a seat right away on public transit, I wouldn’t be able to stand,” she says. “I’d be on my butt, immediately.”

As part of a BART task force that helps raise accessibility concerns and generate solutions, she leads a “travel training” program, where disabled and elderly passengers train others with similar conditions, on best accessibility practices, so that public transit doesn’t become a barrier for them.

The sticker idea was inspired by a similar initiative in London, where disabled riders often wear buttons or carry cards that read “Please offer me a seat. Not all disabilities and conditions are visible.”

She eventually connected with Arvin, a product designer, and transit enthusiast. After working on a design together, the duo launched the sticker on July 17, via Twitter.

The stickers are priced at $0.35 per piece and can be affixed to a BART transit card, or $0.95 for one with a lanyard. There is also a free PDF version that’s printable from home, to ensure that everyone who needs a sticker can get one.

As Callahan explains, the stickers are not only a tool to help disabled people but are also designed to gradually grow cultural awareness among the larger community. “Once they know about [the problem],” she says, “they’ll hopefully be more on the lookout for someone who might need their seat.”