The City may mark lanes for traffic on street pavements at such places as traffic conditions require, consistent with this Traffic Code. 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 of Iowa, Sec. 372.13[4] and 321.255)