Deuteronomy 19:3

Thou shalt prepare thee a way Make a plain road to them, keep it in good repair, and distinguish it by evident marks, to prevent delays and mistakes, that the manslayer might meet with no difficulty in escaping to the nearest city. And divide the coasts of thy land Thy possessions on the west of Jordan into three equal parts, and in the central part of each open a place of refuge, which being nearly at an equal distance with respect to the inhabitants of that district, all might have the same benefit by it.