All such sign or guide boards throughout the entire state shall be of uniform size, construction, color, and erection. Said sign or guide boards shall show thereon the next city, town, or village to be reached by following each fork of said road, and the distance to be traveled along such road to reach said city, town, or village. All such sign or guide boards as may be erected at forks or crossroads on the main or trunk roads within such county shall indicate direction along said road and distance to the county seat of the county or the principal city, town, or village within said county, and the principal city, town, or village within next adjoining county located on such main or trunk roads.