Interstate 66 (I-66) is a 76.32 mile east–west interstate highway in the eastern United States. The highway runs from an interchange with I-81 near Middletown, Virginia, on its western end to an interchange with U.S. Route 29 (US 29) in Washington, D.C., at the eastern terminus. The route parallels State Route 55 (SR 55) from its western terminus at I-81 to Gainesville, Virginia, and US 29 from Gainesville to its eastern terminus in Washington. I-66 is unrelated to US 66, which was located in the Midwest-West region of the United States.
The E Street Expressway is a spur from I-66 into the Foggy Bottom neighborhood of Washington, D.C.
Planned expansions of I-66, including an auxiliary route, I-266, were ended by community opposition in the 1970s. I-266 would have begun in Arlington, Virginia, and terminated in Washington at K Street NW.
Route description
|-
|VA
|74.8
|120.54
|-
|DC
|1.6
|2.57
|-
|Total
|76.4
|123.11
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Virginia
Interstate 81 to Dunn Loring
thumb|Western terminus of I-66 at the interchange with [[Interstate 81 in Virginia|I-81 in Middletown, Virginia]]
thumb|I-66 in [[Oakton, Virginia|Oakton, with a Washington Metro train using the tracks in the median]]
I-66 begins at a directional T interchange with I-81 near Middletown, Virginia. It heads east as a four-lane freeway and meets US 522/US 340 at a partial cloverleaf interchange. The two routes head south to Front Royal and north to Lake Frederick. 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. SR 55 also merges onto the freeway at this interchange, forming a three-way concurrency that ends near Marshall, with SR 55 leaving along with U.S. Route 17 Business (US 17 Bus.) and US 17 leaving at the next exit. The freeway then passes through Bull Run Mountain at Thoroughfare Gap.
Expanding to six lanes, and continuing to parallel SR 55, I-66 enters the towns of Haymarket and Gainesville, reaching interchanges with US 15 (James Madison Highway) and US 29 (Lee Highway) in each town, respectively. The highway then expands to ten lanes and heads to the south of Manassas National Battlefield Park and to the north of Bull Run Regional Park. The highway reaches another interchange with US 29 and passes to the north of Centreville and meets SR 28 (Sully Road) at an interchange with cloverleaf and stack elements to it. SR 28 heads north to Dulles International Airport and south to Manassas.
The freeway then meets SR 286 (Fairfax County Parkway), US 50 (Lee Jackson Memorial Highway), and SR 123 (Chain Bridge Road) at a series of interchanges providing access to D.C. suburbs. The Orange Line and Silver Line of the Washington Metro begin to operate in the median here, as the highway reaches a large interchange with the I-495 (Capital Beltway).
I-66 has two variably tolled HOT lanes from US 29 in Gainesville to the Capital Beltway, meaning the price depends on the traffic. It is often called the I-66 Express Lanes Outside the Beltway.
Dunn Loring to Theodore Roosevelt Bridge
The section of I-66 in Virginia east of I-495, often called I-66 Inside the Beltway, is officially named the Custis Memorial Parkway. It is a toll road during peak hours. The road narrows to four lanes as it heads through Arlington County. The parkway meets SR 7 (Leesburg Pike) at a full interchange. SR 267 (Dulles Toll Road) meets the parkway with an eastbound entrance and westbound exit. Continuing through neighborhoods, the route yet again meets US 29 at an incomplete interchange and continues east into Arlington County, meeting SR 120 (Glebe Road) and continuing to Arlington County. It meets Spout Run Parkway and enters Rosslyn. The freeway turns southeast and runs in between US 29 as it approaches the Theodore Roosevelt Bridge, reaching another eastbound entrance and westbound exit as US 29 continues north on the Key Bridge. It then has a complex interchange with George Washington Parkway and SR 110 (Richmond Highway), providing access to Alexandria and the Pentagon, respectively. US 50 (Arlington Boulevard) merges onto the highway with a westbound exit and eastbound entrance and the two traverse the bridge.
The "Custis Memorial Parkway" name commemorates the Custis family, several of whose members (including Martha Dandridge Custis Washington, George Washington Parke Custis, Eleanor "Nelly" Parke Custis Lewis and Mary Anna Randolph Custis Lee) played prominent roles in Northern Virginia's history. Because of its terminus in the Shenandoah Valley, some early planning documents refer to I-66 as the "Shenandoah Freeway," although the name did not enter common use.
Between the Capital Beltway and the Theodore Roosevelt Bridge, the eastbound (inbound) roadway is a high-occupancy toll (HOT) road from 5:30 to 9:30 am, and the westbound (outbound) roadway is an HOT road from 3:00 to 7:00 pm. (Westbound tolling begins after exit 73 to US 29, in order to allow traffic crossing the bridge an opportunity to transfer to free roads.) E-ZPass is required for all vehicles except motorcycles, including Dulles Airport users. I-66 is free during those times for HOV-3+ drivers with an E-ZPass Flex and for motorcycles. Other drivers must pay a variable toll depending on traffic levels. Outside of these hours, I-66 is free for all drivers to use.
Washington, D.C.
In Washington, D.C., the route quickly turns north, separating from US 50. 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. After an indirect interchange with the Rock Creek and Potomac Parkway (via 27th Street), the highway terminates at a pair of ramps leading to the Whitehurst Freeway (US 29) and L Street. The portion of Interstate 66 within Washington, D.C., is known as the Potomac River Freeway.
E Street Expressway
thumb|The E Street Expressway just east of I-66
The E Street Expressway is a 480 meter long spur of I-66 that begins at an interchange with the interstate just north of the Roosevelt Bridge. It proceeds east, has an interchange with Virginia Avenue Northwest, and terminates at 20th Street Northwest. From there, traffic continues along E Street Northwest to 17th Street Northwest near the White House, the Old Executive Office Building, George Washington University, and the Corcoran Gallery of Art. Westbound traffic from 17th Street takes a one-block segment of New York Avenue to the expressway entrance at 20th and E streets northwest. The expressway and the connecting portions of E Street and New York Avenue are part of the National Highway System.
In 1963, the construction of the E Street Expressway caused the demolition of multiple buildings of the Old Naval Observatory.
;Exit list
The entire route is in the Foggy Bottom neighborhood of Washington, D.C. All exits are unnumbered.