No person shall ride, drive, propel or cause to be propelled any vehicle or animal across or upon any improved sidewalk, except over permanently constructed driveways and except when it is necessary for any temporary purpose to drive a loaded vehicle across a sidewalk; provided, that such sidewalk area is substantially protected by wooden planks two inches thick, and written permission is previously obtained from the City Traffic Engineer. Such wooden planks shall not be permitted to remain upon such sidewalk area during the hours from 6:00 p.m. to 6:00 a.m.