The historic Route 66 covered a total of 2,448 miles, and while that sounds like a blast with the top down in your convertible Ford Mustang, it's not the longest highway in the US. Spanning 3,085.27 miles across the country, the I-90; which connects Seattle, Washington to Boston, Massachusetts, is the longest interstate in America. The total length of the US Interstate system is said to be 48,756 miles, however - a fun fact to remember!