What is the proper use of the word "within"?

I’m not sure of this specific situation, but I have long learned that if you’re a little bit adept with language, you’ll find that many signs contain linguistic errors – it wouldn’t be surprising at all.

It is used just as you surmised, those enclosed by the boundaries of the crosswalk.

“Within” and “in” would be interchangeable in this instance. This strikes me as a matter of tone more than anything else. Official pronouncements often have more gravitas when they use the less obvious word choice. Even the use of “individuals” in this sign is an example of the same thing. It’s not the word you’d tend to use in casual speech, so it has a tone of authority.

Being a writer, I like these kind of questions.

Agree with @thorninmud completely. And this context, I feel that “within” defines the space a better more emphatically than “in.”

While ‘in’ would be considered correct, ‘within’ is a subset of ‘in’ and has a more specific meaning: “Inside the boundaries or limits.”

I agree with @Pied_Pfeffer In this instance “within” is far more specific, “in the crosswalk” just doesn’t sound right to me.

Oops, on second read I see I agree with @Pachy as well, sorry about that Pachy.

“Within” is more limited than “in” in the given context. “In the crosswalk” could arguably apply to a person with one foot in and one out. “Within” implies both feet are in.