The maximum speed limit of all vehicles on state park and preserve drives, roads, and highways shall be thirty-five miles per hour. All driving shall be confined to designated roadways. Whenever the commission determines that a thirty-five mile-per-hour speed limit is greater than is reasonable or safe under the conditions found to exist at any place of congestion or upon any part of the park roads, drives, or highways, the commission shall determine and declare a reasonable and safe speed limit, which shall be effective when appropriate signs giving notice of the changed speed limit are erected at the places of congestion or other parts of the park roads, drives, or highways.