The eventual completion of Autoroute 35 will lead to a nonstop limited-access highway route between Boston and Montreal, following I-93 south from I-89's terminus.

I-89 is one of three main Interstate highways whose route is located entirely within New England, along with I-91 and I-93 (both of which also have their northernmost pavement in Vermont).

I-89 connects smaller cities and rural areas within New Hampshire and Vermont, and maintains two lanes of traffic in each direction throughout the route.

The two major population centers along I-89's length in New Hampshire are Concord, at its southern terminus, and Lebanon, on the Vermont state line.

Just after this interchange, the highway crosses the Connecticut River and enters Vermont, where it remains for the rest of its run northwest to the Canadian border.

Williston, which has become Burlington's big-box retail center (and one of the fastest-growing towns in the state) over the past decade, also has an interchange along I-89.

The Interstate passes through the towns of Sharon, Royalton, Bethel, Randolph, Brookfield, and Williamstown before reaching the "twin cities" of Barre and Montpelier in the middle of Vermont.

After exit 11 in Richmond, I-89 leaves the Green Mountains to enter the Champlain Valley, and a notable shift in the landscape is visible.

A full cloverleaf interchange at this exit provides access to downtown Burlington, the University of Vermont, and the retail-heavy Dorset Street, via US 2.

Heading north from Burlington, the landscape quickly fades from suburban development into rolling hills more characteristic of northern New England, providing a vista overlooking Lake Champlain.

Although the divided highway continues about five miles (8.0 km) into Philipsburg, Quebec, as Route 133, this changes back to a two-lane road, through to Autoroute 35's current southern terminus in Saint-Sébastien, some 9 miles (14 km) north of the United States border, and continues to Saint-Jean-sur-Richelieu and Chambly before it meets Autoroute 10, which goes to Montreal.

There are plans to complete the extension of Autoroute 35 from Saint-Sébastien to the border crossing at I-89's northern terminus by 2025, creating a freeway-to-freeway connection.

[6] I-89 was commissioned as part of the Federal-Aid Highway Act of 1956, meant to connect Norwalk, Connecticut, to the Canada–United States border via the US 7 corridor, which is the current northern terminus of I-89.

[7] I-89 was originally supposed to be a directly north–south route from I-95 in Norwalk, Connecticut, to its current northern terminus at the Canada–United States border.

The route shifted after opposition came from residents and local lawmakers in interior New England who did not want an Interstate running through their countryside and towns.

One major problem that was a big part in sinking the project was the fact that the highway would have to go through the Green Mountain National Forest in Vermont.

[8] Parts of the Interstate were built in Connecticut, between Norwalk and Wilton and from Brookfield to Danbury, a short bypass around Lenox, Massachusetts, and in southern Vermont between Bennington and Manchester and are currently designated as US 7.

The current route of I-89 is the main artery between Boston and Montreal, two large metropolitan areas in the US and Canada, respectively.

The route between the two cities is not complete, however, as Autoroute 35 in Quebec still needs to be extended south of its current terminus to connect to I-89 at the Canada–United States border.

In 2019, it was announced that the highway would be complete by 2023,[10] As of July 2022[update], grading of the right of way between the existing terminus at exit 15 to Chemin Champlain is in progress.

Vermont and New Hampshire are working together to reconstruct the Vietnam Veterans Memorial Bridge over the Connecticut River.