I-91 is 290 miles long and travels north and south: 58 miles in Connecticut, 55 miles in Massachusetts, and 177 miles in Vermont. I-91 parallels U.S. Route 5 (US 5) for all of its length, and many of the exits along I-91 provide direct or indirect access to the older route. Much of the route of I-91 follows the Connecticut River, traveling from Hartford, Connecticut, northward to St. Johnsbury, Vermont
I-91 extends 55 miles through the Pioneer Valley of western Massachusetts paralleling the Connecticut River. I-91 serves as the major transportation corridor through three Massachusetts counties, linking the cities of Springfield, Northampton, and Greenfield. These three cities serve as the control cities listed on guide and mileage signs, along with Brattleboro, Vermont beginning with the first northbound conventional mileage sign in Longmeadow.
I-91 has two exits in Greenfield. At exit 26, the southern end of its overlap with Route 2, there is a rest area and visitor information center for Franklin County. At exit 27, also in Greenfield, is the northern end of its overlap with Route 2 where access to that road is provided via a directional T-interchange, and exit and entry ramps on the left side of southbound I-91. Exit 28 in Bernardston is the last exit in Massachusetts. Beyond exit 28, I-91 continues for about five miles more before crossing into Vermont. Massachusetts is the only state traversed by I-91 where another numbered highway is concurrent with the Interstate (in this case, US 5, for a one-half mile spur near the Springfield-Longmeadow town line and Route 2, for approximately three miles in Greenfield).
The above should not be relied upon as a travel guide as I-91 exit numbers have changed in 2021.