In planning and in adopting the secondary road program or project by the board of supervisors, the board and the county engineer shall give due and careful consideration, to the location of primary roads, and of roads previously improved as county roads, to the market centers and main roads leading thereto, and to rural mail and school bus routes. It is the intent of this chapter that the secondary road program or project will, when finally executed, afford the highest possible systematic, intracounty and intercounty connections of all roads of the county.