I-90 terminated at Interstate 93 in Boston until it was connected through to the Ted Williams Tunnel in January 2003 as part of the Big Dig, extending I-90 to Boston's Logan International Airport, an additional 1.3 miles beyond the 3111.52 miles already paved.
(The following information was accurate until 1995) In Montana right by the Idaho border, I-90 is not a divided highway for a few stretches.
Until 1999, the speed limit in Montana was "reasonable and prudent"; it is now 75 mph.
Not only is the I-90 section of the NYS Thruway marked backwards, with mile numbers going down as you go east, but there are two metric only signs when you drive westbound. They are around Syracuse, which is nowhere near Canada.
The NYS Thruway administration decided to test metric signage (which I am told included [briefly] an 88 km/h speed limit sign) on the Thruway. Now the legend part: When one of the metric pushes was on, the state representative from that district was deadset against it. The Thruway chose his district (Liverpool, next to the former GE plant) as the test site.
I-990 is the highest number given to an Interstate.
To add to the quirks about the longest interstate, it also has the longest distance twinned with another interstate (I-80 in Ohio and Indiana, I-94 in Illinois and Wisconsin) in the most states 4 (Ohio, Indiana, Illinois, and Wisconsin.) It also intersects the same interstate six times [(I-94) near Michigan City IN, Chicago (southside), Chicago (northside), Madison WI, Tomah WI, and Billings MT.]
It has been said that technically I-90 is not a complete "Interstate" as the Chicago Skyway did not meet the current interstate standards, making I-80 the longest "true/no interrupted" interstate without any gaps between the endpoints.