The City Council by resolution may designate any street or portion of a street as a through street or one-way street where necessary to preserve the free flow of traffic or to prevent accidents. No trunk highway shall be so designated unless the consent of the Commissioner of Transportation to the designation is first secured. The city shall cause appropriate signs to be posted at the entrance to designated streets. It shall be unlawful for any person to fail to obey the markings or signs posted under this section.