Whenever any street, or portion of street, in the city is being improved, and the sign "Street Closed" is placed on the street, or portion of street, it is unlawful for any person to drive or lead any beast of burden, or propel any vehicle over or across the street or portion of the street; provided, however, that the provision of this section shall not apply to the contractor, or his employees, actually engaged in the work of the improvements. (Ord. 46 CS §18; prior code §21.16.)