The portion of Interstate 87 from Brewster, N.Y. to White Plains, N.Y. crossing the northwest corner of Greenwich, Connecticut was numbered that way for only two years; today it is called I-684.
The segment opened on Oct. 29, 1968, and created a numbering conflict with state route 87; no two highways in the state can share the same number. The Connecticut DOT made plans in 1969 to change CT 87's number to 287.
However, in 1969 New York decided to move the I-87 designation westward, and call the Brewster to White Plains segment I-684. Connecticut put its renumbering plan on hold; in 1970, when the New York plan was made final, Connecticut reassured towns along CT 87 that their highway would keep its number.
The route had been planned as I-87 since its alignment through Greenwich was under discussion in 1963. What's interesting is a 1965 article calling the proposed route as part of "an Interstate 84 extension."