The following standards apply to directional and official signs and notices located within six hundred and sixty (660) feet of the right-of-way of the Interstate and Federal-aid primary systems and to those located beyond six hundred and sixty (660) feet of the right-of-way of such systems, outside of urban areas, visible from the main traveled way of such systems and erected with the purpose of their message being read from such main traveled way. These standards do not apply to directional and official signs erected on the highway right-of-way.