The Automatic Warning System (AWS) was introduced in the 1950s in the United Kingdom to provide a train driver with an audible warning and visual reminder that they were approaching a distant signal at caution.Its operation was later extended to give warnings for; * A colour light signal displaying a double yellow (steady or flashing), single yellow or red aspect * A reduction in permissible speed * A temporary or emergency speed restriction * An automatic barrier crossing locally monitored (ABCL), an automatic open crossing locally monitored (AOCL), or an open crossing (OC).