Interstate 66

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, 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.

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 tolled HOT lanes from US 29 in Gainesville to the Capital Beltway.

The section of I-66 in Virginia east of the Capital Beltway is named the Custis Memorial Parkway, a toll road with variable tolls 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.