Interstate 91 runs for exactly 58 miles through Connecticut and is the major north-south transportation corridor for the center of the state. It serves as the Interstate link between the larger cities of New Haven, Hartford, and Springfield, MA. As such, it is almost always heavily trafficked (especially during rush hour), and maintains at least three lanes in each direction throughout its distance in Connecticut. The three larger cities also serve as Connecticut's control points along its length of the Interstate. Overhead signs in each direction consistently feature one or two of the cities, although very few (if any) conventional highway mileage signs are placed along the Interstate in Connecticut that indicate the remaining distance between the cities.
The highway begins just east of downtown New Haven at a complicated intersection with Interstate 95 and Connecticut Route 34. At the bottom of the ramp for Exit 5, U.S. Route 5 has its southern terminus and the first of its many interchanges with the freeway. Leaving New Haven, I-91 follows a northeastward trek through New Haven's suburbs of North Haven and Wallingford before entering the city of Meriden. In Meriden, about halfway between Hartford and New Haven, I-91 sees an interchange with its first spur route, Interstate 691. I-691 provides a westward link to Interstate 84 and the city of Waterbury. Leaving Meriden, I-91 travels briefly through Middletown before passing through Cromwell, Rocky Hill, and Wethersfield, eventually entering the Hartford city limits. Just south of Hartford, the highway begins to directly parallel the Connecticut River, and will do so until reaching St. Johnsbury, VT, almost 200 miles north. In Hartford, I-91 interchanges with east-west I-84 before leaving the city limits. I-91 will maintain an urban character throughout its distance to Springfield. Halfway between the two cities, Exit 40 provides direct access (via Connecticut Route 20) to the busy Bradley International Airport. The airport is the second-largest in New England and serves both the Hartford and Springfield metropolitan areas. I-91 will run through Windsor, Windsor Locks, East Windsor, and Enfield (with several exits in each town) before crossing into Massachusetts at milepost 58.
I-91's final exit in Connecticut is Exit 49, which offers northbound travelers access to Longmeadow, MA. This is the highest-numbered exit along I-91's route in the three states. Just after Exit 49, I-91 enters Massachusetts.
Interstate 91 travels through the Pioneer Valley of western Massachusetts along the Connecticut River. In Massachusetts, I-91 serves as the major transportation corridor linking the cities of Springfield, Northampton, and Greenfield, the shire towns and most populous cities of the three Massachusetts counties that I-91 enters. The three cities serve as the control cities listed on guide and mileage signs, along with Brattleboro, VT. Brattleboro is featured as the long distance control city throughout I-91's entire northbound length in Massachusetts, beginning with the first northbound conventional mileage sign in Longmeadow. Hartford, CT is featured as a control city on two of the southbound mileage signs.
I-91 enters Massachusetts at the town of Longmeadow, a wealthy suburb of Springfield, though it does not have an interchange there. To access Longmeadow, one must use Connecticut's exit 49 (if traveling north) or Massachusetts' exit 1 in Springfield (if traveling south). After Longmeadow, I-91 enters Springfield and immediately sees many exits in quick succession before entering Chicopee. From I-91 in Springfield, one can access the Basketball Hall of Fame as well as Interstate 291, a spur route heading east from I-91 to give access to the eastbound Mass Pike (I-90) in eastern Chicopee. North of Springfield, I-91 briefly enters Chicopee itself and sees an interchange with the spur of Interstate 391 before turning due west to cross the Connecticut River into West Springfield. I-391 gives better direct access to the cities of Chicopee and Holyoke. From this point northward, I-91 will run on the western side of the river.
Just after the river crossing, I-91 has a major interchange with the east-west Mass Pike, (I-90) before entering the city of Holyoke. Exit 15 provides access to the Holyoke Mall at Ingleside, one of Massachusetts' largest malls. Just after Exit 16, I-91 reduces from a six-lane freeway to a four-lane freeway, and will remain just 4 lanes wide for the rest of its journey northward. After a short exit-less stretch, I-91 enters Northampton and has a few more interchanges. The towns of Hadley and Amherst, home to the main campus of the University of Massachusetts, are accessible from I-91 exits in Northampton via Massachusetts Route 9.
Continuing north, I-91 enters Hatfield, and sees the beginning of its straightest section; nearly 6 miles without so much as a bend in the road. Several exits provide access to U.S. Route 5 and Massachusetts Route 10 in Hatfield and Whately before entering Deerfield. In Deerfield, I-91 passes the headquarters of the Yankee Candle Company, whose flagship store is one of Massachusetts' most popular attractions. Deerfield is also home to Magic Wings Butterfly Gardens and the houses of Historic Deerfield.
I-91 finally reaches the control city of Greenfield, and has two exits there. At Exit 26, a pleasant rest area/visitor information center for Franklin County is available, as well as several restaurants and hotels. I-91's final exit in Massachusetts is Exit 28 in Bernardston. Passing Exit 28, I-91 continues for about 5 more miles before crossing into Vermont.
Massachusetts' portion of I-91 is the only portion to feature motorist-aid call boxes, which are becoming largely obsolete with the growing use of cellular phones. The call boxes are still important, as a good part of I-91 in Massachusetts is rural, unlike many other freeways in the state. There are some places in MA where I-91 runs for several miles between exits, just like I-91 in VT. In addition, Massachusetts is the only state traversed by Interstate 91 in which another numbered highway is co-signed onto the Interstate (in this case, U.S. Route 5, for a half-mile near the Springfield-Longmeadow town line).
Interstate 91 runs along the eastern border of Vermont and serves as a major transportation corridor for both Vermont and western New Hampshire, though it nevers enters New Hampshire directly. Many exits along Vermont's length of I-91 feature New Hampshire towns on the guide signs (for example, Exit 3, which lists Brattleboro and Keene, NH, as the points of access). I-91 will run for 177 miles in Vermont, nearly two-thirds of the Interstate's total length, yet only carries 29 interchanges along that length. The highway's rural character and long distances between exits in Vermont are in stark contrast to that of its length in Connecticut and greater Springfield, MA, where exits are more frequent and the road carries four lanes of traffic in each direction at some points. The major control "cities" are Brattleboro, White River Junction, St. Johnsbury, and Newport. Of these, only Newport is actually chartered as a city, although the rest are fairly sizable towns.
I-91 enters Vermont in the town of Guilford and straddles the town line between Guilford and Vernon, though neither town has a direct interchange. They can be accessed at Exit 1, just after the highway enters the town of Brattleboro. I-91 runs through the geographic center of town and has three exits in Brattleboro. Exit 3 is considered to be one of the busiest exits along I-91 in Vermont. Brattleboro's busy suburban-like retail center is located here, including one of Vermont's few Home Depots, a supermarket, and a multi-screen movie theater. There are also several hotels and motels as well as a few fast-food outlets. Following Vermont Route 9 eastward, one can reach Keene in 15 miles. After Exit 3, I-91 heads out of Brattleboro to pass through the towns of Dummerston, Putney, Westminster, Rockingham, Springfield, Weathersfield, Windsor, Hartland, and White River Junction. White River Junction, listed as a control city on mileage signs as far south as Greenfield, MA, is where I-91 and I-89 interchange and provide access to many points in Vermont and New Hampshire. Motorist services like hotels and restaurants, as well as many stores, are available in both states near the intersection of the Interstate highways.
North of the interchange with I-89, I-91 continues towards St. Johnsbury and passes through the small towns of Norwich, Thetford, Fairlee, Bradford, Wells River, and Barnet before coming to its next major intersection. Towns in New Hampshire on the other side of the river can also be easily accessed in this stretch. At Exit 19, travelers reach the northern terminus of Interstate 93, one of the major highways in New Hampshire. Taking I-93 south, they can pass through the White Mountains on a trip towards the larger cities in southern NH. Just after the I-93 interchange are three exits for the town of St. Johnsbury, including a major intersection with U.S. Route 2. Along westbound U.S. Route 2, the capital city of Montpelier is eventually reached from I-91, although I-89 provides Montpelier with immediate Interstate access.
Interstate 91 continues northward, passing through the center of Vermont's Northeast Kingdom and the towns of Lyndon (home of Lyndon State College), Sheffield, Barton, and Orleans before entering Newport, just a few miles south of the Canadian border. Two exits serve Newport and its neighboring town of Derby. I-91's final exit in Vermont, just after mile marker 177, is Exit 29 in Derby Line. Beyond the exit ramp, northbound motorists enter Canadian customs. To reverse direction without crossing the border, motorists must use Exit 29 to re-enter I-91 southbound. The road passing over the Interstate actually straddles the international border; westbound drivers are driving in Canada while eastbound drivers are driving in Vermont. This road, if followed westward for a mile, eventually meets U.S. Route 5 at its northern terminus.
The highest point on the road is just north of mile marker 150 in Vermont, 1856 feet.
As with Connecticut and Massachusetts, U.S. Route 5 closely parallels Interstate 91 for their entire lengths in Vermont. Indeed, most of the large exit signs on I-91 list U.S. 5 as one of the roads (if not the only road) that can be accessed from the exit, as seen in the exit list below. While paralleling I-91 in Vermont, U.S. 5 is never co-signed onto the freeway, but remains its own two-lane road.