Public exigency is a sudden and unexpected happening. It is an unforeseen occurrence or condition; a perplexing contingency or complication of circumstances; or a sudden or unexpected occasion for action. [Good Roads Machinery Co. v. United States, 19 F. Supp. 652 (D. Mass. 1937)].
The following is an example of a state law (Ohio) defining public exigency:
Public exigency means an injury or obstruction that occurs in any public works and that materially impairs its immediate use or places in jeopardy property adjacent to it. It is an immediate danger of such an injury or obstruction; or an injury or obstruction, or an immediate danger of an injury or obstruction, that occurs during the process of construction of any public works and that materially impairs its immediate use or places in jeopardy property adjacent to it. [ORC Ann. 123.15].