The E Street Expressway is a spur from I-66 into the Foggy Bottom neighborhood of Washington, D.C. I-66 begins at a directional T interchange with I-81 near Middletown, Virginia.

I-66 continues east and crosses the Blue Ridge at Manassas Gap, paralleling SR 55 (John Marshall Highway) and meeting US 17 at a partial interchange with no access from southbound US 17 to westbound I-66.

It then has a complex interchange with George Washington Parkway and SR 110 (Richmond Highway), providing access to Alexandria and the Pentagon, respectively.

(Westbound tolling begins after exit 73 to US 29, in order to allow traffic crossing the bridge an opportunity to transfer to free roads.)

The highway interchanges with the E Street Expressway spur before passing beneath Virginia and New Hampshire Avenues in a short tunnel, also running on the east side of the Watergate complex.

The purchase would eliminate the need to build grade separation where the railroad crossed I-66 and would provide 1.5 miles (2.4 km) of right-of-way for the highway, saving the state millions.

[20] The abandonment proceedings took more than three years, as customers of the railway and transit advocates fought to keep the railroad open and delayed work on the highway.

[21] During that time, on November 10, 1967, the Washington Metropolitan Area Transit Authority (WMATA) announced that it had come to an agreement with the Highway Department that would give them a two-year option to buy a five-mile (8.0 km) stretch of the right-of-way from Glebe Road to the Beltway, where I-66 was to be built, and run mass transit on the median of it.

[23][24] At the same time, the federal government wanted to pave the right-of-way from Washington Boulevard and Glebe Road to Rosslyn for an experimental busway, which Arlington County opposed, in part because they thought it might delay and add to the cost of I-66.

[27] The EIS proposed an eight-lane limited access expressway from the Capital Beltway to the area near Spout Run Parkway.

On January 4, 1977, Coleman approved federal aid for a much narrower, four-lane limited access highway between the Capital Beltway and the Theodore Roosevelt Bridge.

[27] As part of the deal, Virginia officials agreed to provide more than $100 million (equivalent to $393 million in 2023[16]) in construction work and funds to help build the Metro system, which has tracks down the I-66 median to a station at Vienna in Fairfax County; to build a multiuse trail from Rosslyn to Falls Church; and to limit rush-hour traffic mainly to car pools.

[27][28][29] Three more lawsuits would follow, but work began on August 8, 1977, moments after US District Court Judge Owen R. Lewis denied an injunction sought by highway opponents.

[31] On December 22, 1982, the final section of I-66 opened between the Capital Beltway and US 29 (Lee Highway) in Rosslyn, near the Virginia end of the Theodore Roosevelt Bridge.

[27] On October 9, 1999, Public Law 106-69 transferred from the federal government to the Commonwealth of Virginia the authority for the operation, maintenance, and construction of I-66 between Rosslyn and the Capital Beltway.

[32] Because I-66 is the only Interstate Highway traveling west from Washington, D.C., into Northern Virginia, traffic on the road is often extremely heavy.

[33] They later settled on three planned "spot improvements" meant to ease traffic congestion on westbound I-66 inside the Capital Beltway.

In 2014–2015, US 29 was largely grade-separated in the area, including an interchange at its current intersection with SR 619 (Linton Hall Road).

[40] In 2015, the Virginia Department of Transportation planning board added I-66 HOT lanes to their list of priority projects for the I-66 corridor.

Residents living within the I-66 corridor have set up "Transform 66 Wisely", a website describing local community impacts that the VDOT projects may cause.

[43] The local Stenwood Elementary School would lose its attached field, leaving it with blacktop-only recess space.

The Virginia Commonwealth Transportation Board (CTB) is responsible for overseeing VDOT and allocating highway funding to specific projects.

The board has 18 members appointed by the Governor,[45] includes the Virginia Secretary of Transportation, Aubrey Layne, and is the group that will be making the final decision and allocating funding for VDOT's plans for I-66.

[46] VDOT also announced during 2016 that it would initiate on I-66 a dynamic tolling system in the peak travel directions during rush hours.

On December 4, 2017, VDOT converted 10 miles (16 km) of I-66 between US 29 in Rosslyn and the Capital Beltway to an HOV variable congestion pricing tolling system.

[48] Prices were as high as $47 one-way during the lanes' first weeks of operations, attracting controversy and national media attention.

[53] The project added 22.5 miles (36.2 km) of new dynamically-tolled express lanes alongside I-66 from I-495 to University Boulevard in Gainesville.

It also built new park and ride facilities, interchange improvements and 11 miles (18 km) of expanded multi-use trail.

[54] Construction on widening eastbound I-66 as part of the "Transform 66 Inside the Beltway" improvement project began in June 2018 and was completed in 2020.

The project added a travel lane on eastbound I-66 between the Dulles Access Road and Fairfax Drive (exit 71) in Ballston, and provided a new ramp-to-ramp direct access connection from eastbound I-66 to the West Falls Church station at the SR 7 interchange and provided a new bridge for the W&OD Trail over US 29.