The governing body of any county, city, or town operating its own system of roads may by ordinance authorize the city or town manager or some other local officer to designate intersections, other than intersections at which one or more of the intersecting streets have been designated as a part of the primary state highway system in a town which has a population of less than 3,500, at which vehicles shall come to a full stop or yield the right-of-way. No such ordinance shall be violated if, at the time of the alleged violation the sign or marker placed in conformity with this section is missing or is defaced so that an ordinarily observant person under the same circumstances would not be aware of the existence of the regulation.