The City Engineer is authorized to determine and designate those heavily traveled streets upon which shall be prohibited the use of the roadway by motor-driven cycles, bicycles, horse-drawn vehicles, or other nonmotorized traffic and shall erect appropriate signs giving notice thereof. When signs are so erected giving notice thereof, no person shall disobey the restrictions stated on such signs. [Code 1970 § 10.64.030; Code 1954 § 8-38.12.]