Whenever the condition of any city street is such that for any reason its use or continued use by vehicles or by any class of vehicles will greatly damage such city street, or will be dangerous to traffic thereon; or when the same is being constructed, altered, repaired, improved or maintained in such a manner as to require that such city street or a portion thereof be closed to travel by all vehicles or by any class of vehicles for any period of time, the mayor, after finding that an emergency exists, may then order all or portions of such city street temporarily closed by posting notices at each end of the closed portion thereof and at all intersecting streets, which order shall be effective immediately. (Ord. 382 § 1, 1972; Ord. 282 §§ 91, 117, 1968).