No person shall ride, drive, propel or cause to be propelled any vehicle or animal across or upon any sidewalk excepting over permanently constructed driveways and excepting when it is necessary for any temporary purpose to drive a loaded vehicle across a sidewalk; provided further, that such sidewalk area be substantially protected by wooden planks two inches thick or other approved protection, and written permission be previously obtained from the city traffic engineer. Such protection shall not be permitted to remain upon such sidewalk area during the hours from six p.m. to six a.m. (Ord. 858 NCS §1; prior code §15.8.3.)