Washington State Route 519

SR 519 in the state of Washington is unique in that it marks the furthest west boundary for US Interstate 90 (I-90). 4th Avenue south in the city of Seattle is the entry to I-90, heading east which ends in Boston, MA.
This trip (I-90) puts you on the 3rd longest road in the United States covering 3,101.77 miles. Only US Route 20 and US Route 6 are longer.