Interstate 81 (I-81) is a part of the Interstate Highway System that runs from Interstate 40 at Dandridge, Tennessee, to the Thousand Islands International Bridge at Wellesley Island in New York, beyond which the short (2.7 mile) Ontario Highway 137 links it to Ontario Highway 401. In the U.S. state of New York, I-81 extends from the Pennsylvania state line southeast of Binghamton to the Canadian border at Wellesley Island northwest of Alexandria Bay. The freeway runs north–south through Central New York, serving the cities of Binghamton, Syracuse, and Watertown. It passes through the Thousand Islands in its final miles and crosses two bridges, both part of the series of bridges known as the Thousand Islands Bridge.