The wheelchair symbol—a stick-person sitting in a stick-wheelchair (well, on a stick-wheel at least)—is a common, internationally-recognised symbol.
I suppose it originally meant something like “mainly for people using wheelchairs”. For example, in cases where a building's main entrance involves steps, but there's a secondary entrance that uses a ramp instead, this symbol is often used with an arrow to point towards that ramp. But it's a bit of a dodgy symbol for this purpose.
Perhaps the main intention behind the ramp is to allow access for people who use wheelchairs; but others are quite likely to use it as well—people with pushchairs, for example; or on bikes (if it's an outdoor ramp). Maybe you're a small dog (or with one) and don't fancy climbing up steps half your height. Maybe you can walk, but can't easily lift your feet to the height of a step. Maybe you're a skater.
I suspect that many ramps that use the wheelchair symbol were only installed in the first place in order to comply with anti-discrimination legislation, which (in addition to having a poetically rhythmic name) has come into force in the United Kingdom recently. In this frame of mind, someone thought, “OK, chaps. We're putting this ramp in for the disabled people, so we're gonna put up a big sign saying ‘This is for disabled people.' That makes sense.”.
And it sort-of does, but they're thinking about it far too hard. They should use a symbol of a ramp instead—something like the UK road sign for an incline, but without any gradient. This would show what's there, factually and simply, without making assumptions about who's going to be using it. Analogously, a simple pictorial of some stairs is often used to represent a staircase, and quite sensibly.
That example of the wheelchair symbol's use is relatively innocuous—the ramp is intended mainly for wheelchair users. But there are other cases of its use where it's wholly inappropriate, mainly on the web.
A lot of websites use simple tests to distinguish real (presumably) human users from automated spamming machines. These tests usually involve reading a picture of a string of letters and numbers, that's been made intentionally difficult for a computer to read. This also makes them virtually impossible for people with poor sight to read. In order to provide access for these people, most websites that use such tests provide an alternative one that involves recognising sound instead.
(Both of these are useless for deafblind people. There have been attempts at devising more sensible tests that don't assume that real people can either see or hear, such as Eric Meyer's WP-Gatekeeper.)
These audio tests are often indicated by the wheelchair symbol. —which makes no sense whatsoever.
The rationale behind this is presumably that the wheelchair symbol has become a general symbol for disability. That's a shame, as it lumps everyone with anything that's considered a disability into one category. And using a wheelchair needn't be a disability in every situation, and certainly isn't on the web (which is incongruous, because that's where its image is being used as a symbol for disability).
There is a symbol for blindness, which would be more appropriate for this purpose than the wheelchair symbol; it's a person walking with a cane to the ground in front of them. Unfortunately, it assumes that all people walking with canes in front of them are blind, which is reasonably fair; and that all blind people can walk, which is not.
A better symbol for blindness would be an eye with a slash through it; the UK's Royal National Institute of Blind People uses such a symbol as their site's icon. Analogously, the symbol for deafness is an ear with a slash through it. (This is a very sensible symbol.) The symbol for deafblindness could then, logically, be an eye and an ear, with a slash through each (so, the deafness and blindness symbols combined).
But even the blindness symbol would only be as appropriate for indicating an audio-based test as the wheelchair symbol is for indicating ramps. It's a sound-based test; it should be represented by a symbol for sound; a speaker with “sound waves” emanating from it would be perfect.
There are some uses of the wheelchair symbol that are a bit more awkward. The wheelchair symbol is often used to mark disabled people's parking spaces*—those reserved for drivers and passengers who are “registered disabled”, with extra room and in the most convenient positions. “Registered disabled” means that the person in question uses a wheelchair, uses crutches, has poor or no sight, or has another condition that makes a better parking space a practical necessity. (I'm not sure whether having poor or no hearing gets you a disabled-badge**, but I don't see why it would mean you'd need a space nearer to the building.)
* (I say “disabled people's parking spaces” rather than “disabled parking spaces” because the latter seems to imply that the parking spaces themselves are disabled.)
**: (Similarly, “disabled badge” seems to imply that the badge itself is disabled.)
The wheelchair symbol isn't really appropriate here. Like with the ramp, these parking spaces aren't only for people who use wheelchairs; they're also specifically for people who fall into other groups. Unlike the ramp, this is an artificial distinction: the spaces are explicitly for certain groups of people and—more importantly—specifically not for everyone else. The ramp was just “probably less convenient” for people who can use steps.
In this case, I can't think of a better symbol to use. I think it would be counter-productive to invent a new symbol to generally represent “disability”, because using any symbol like this arbitrarily lumps a lot of disparate groups of people together. However, it might remove the implication that, in any case where a certain biological or medical condition would cause problems, wheelchair users are always “disabled”—or literally, incapable.
