Michigan Compiled Laws, Chapter 691. Judiciary § 691.1403

Sec. 3. No governmental agency is liable for injuries or damages caused by defective highways unless the governmental agency knew, or in the exercise of reasonable diligence should have known, of the existence of the defect and had a reasonable time to repair the defect before the injury took place. Knowledge of the defect and time to repair the same shall be conclusively presumed when the defect existed so as to be readily apparent to an ordinarily observant person for a period of 30 days or longer before the injury took place.