Well, technically speaking, it ended on June 27, 1985. It exists no longer. U.S. Highway 66 was decommissioned on that date.
In its heyday, it ran from Chicago, Illinois to Santa Monica, California. It once ended in Los Angeles (as noted in the song "(Get Your Kicks On) Route 66"), but the route was extended to Santa Monica in 1939. Over the years, especially in the 1970s, the route was progressively truncated until it existed no longer.
However, many sections of the road do still exist, and are signed as "Historic U.S. 66." It's a fun ride, if I do say so myself.
Most states have a State Route 66, and there is also an Interstate 66, currently serving northern Virginia and the District of Columbia.
Good sites about U.S. Highway 66 and its current "Historic" form:
· 1 decade ago