On a sign, Handicap Accessible refers to, say, a parking space. As it functions, I guess the word, Handicap, would have to be considered an adverb. It modifies or clarifies Accessible, which itself describes the parking space, making it an adjective. But something about the construction - using the noun/verb form of the word handicap seems, at least unusual, and even feels sort of wrong. I feel it should read “handicapped accessible” to indicate that it’s accessible to handicapped people. It would feel similarly incorrect if it said - in p.c. terms - disable accessible. In any case, is this an unusual use of the noun/verb form functioning as an adjective? Or can I just not think of any offhand?
“Handicap Accessible” is signage, not a sentence and English speakers tend to give signage a lot of leeway.
The function of signs is to give the greatest amount of instantly understandable information in the least space. The rules of formal English don’t apply. Ellipsis, the omission from a sentence or other construction of one or more words that would complete or clarify the construction, is the primary format.
Handicap Accessible can be interpreted as a full, formal sentence in a number of ways. “Parking is both legal and convenient here for those with permits indicating a form of handicapped status” is one. Handicap therefore is a noun encapsulating “those with permits indicating a form of handicapped status.”
Signese produces word choices and phrases that would normally be deemed incorrect in another setting. Context is everything.