Yellow is the colour code recommended by the American Public Works Association for pipes containing natural gas, oil, steam, petroleum, and other gaseous substances. It is the Utility Location and Coordination Council and American National Standards Institute standard utilised by the majority of states for designating subsurface utilities.
Near utility runs, the majority of states mandate warning and color-coded flagging of any excavation sites. Red is required for electrical, orange for communications, blue for potable water, green for sewers and drains, and purple for reclaimed water and irrigation. Typically, corporations require a 48-hour notice before to digging in order to properly survey and mark the prospective dig site.