It is the duty of every person cutting or making any excavation in or upon any public street or sidewalk to take reasonable precautions to place and maintain at each end of such excavation and along the line thereof barriers or flags or warning lights or other equipment as may be prescribed by the administrative authority. When dictated by field conditions, warning sign shall be placed far enough in advance of the construction operation to alert traffic properly within a street. (Ord. 119 § 1(1010), 1969).