At the intersection of two or more streets, or a street and any driveway, no vegetation, sign, fence or wall or any other obstruction to vision (other than sign posts) which is higher than 30 inches above curb level shall be permitted in any sight triangle. Such sight triangle shall be defined as the area between a twenty-foot setback from the road or driveway yielding to the main road at the subject intersection, and a distance of 10 feet along the center line of the main road, measured from its intersection with the center line of the intersecting road or driveway, for every one mile per hour of the posted speed limit on said main road.