It shall be the duty of the board of county commissioners of each county in this state to cause to be placed conspicuously and permanently at the forks of all public and main-traveled roads in their respective counties, and at all such roads crossing or leading away from such public and main-traveled road, metal index boards attached to metal posts; such metal index boards shall have printed thereon in letters of sufficient size to be easily read by travelers, with directions plainly marked thereon designating the most noted places to which each of such roads leads.