First, it’s beside the only elevator in a busy, 4-story university building. You can imagine how often this sign is ignored.
The second thing worth noting is the normative wording, the plea to “be considerate.” This frames the underlying injunction as a request to think about the needs of others — in particular, the needs of people with disabilities.
Third, note that of all the categories of people who might really need to use an elevator — the elderly, the pregnant, etc. — the sign-maker has chosen to single out people with disabilities.
Fourth, note that although the sign starts with a plea to “be considerate,” it next goes on to say something more categorical: “This elevator is reserved for people with disabilities.” [emphasis added]
Finally, an anecdote. When I asked at the building’s Help Desk how to get to a particular room, which happened to be in the basement, I was told to “take the elevator down one floor” (and no mention was made of a moderately obscure set of stairs that would also get be there.)