When it is deemed necessary for safety, at the end of runways used for coasting, as provided for by this article, there shall be placed for a distance of approximately thirty feet sufficient ashes, cinders or other substances to enable those coasting on sleds to stop in safety; and ashes, cinders or other similar substances may be placed for a distance of thirty feet each way from the runway on any street or alley which crosses the coasting runway, wherever deemed advisable.