The City Engineer is authorized to mark traffic lanes upon the roadway of any street or highway where a regular alignment of traffic is necessary. Where such traffic lanes have been marked, it is unlawful for the operator of any vehicle to fail or refuse to keep such vehicle within the boundaries of any such lane except when lawfully passing another vehicle or preparatory to making a lawful turning movement. [Code 1970 § 10.20.090; Code 1954 § 8-12.36.]