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 the 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 are not permitted to remain upon such sidewalk area during the hours from six p.m. to six a.m. (Ord. 432 § 2 (part), 1982).