Because I-66 is the only major highway running west from Washington, D.C., into Northern Virginia, traffic on the road is often extremely heavy. For decades, there has been talk of widening I-66 from 2 to 3 lanes each way inside the Capital Beltway (Interstate 495) through Arlington, Virginia, although many Arlington residents are adamantly opposed to this plan. The Virginia Department of Transportation (VDOT) is studying the prospect of implementing this one-lane extension on westbound I-66 within the Beltway (thus alleviating congestion for people commuting away from DC).
Due to heavy commuter traffic, I-66 features high-occupancy vehicle (HOV) lanes in varying degrees. Between the Prince William Parkway (State Route 234) in Gainesville, Virginia and the Beltway, the left lane on eastbound I-66 is reserved for HOV-2 during morning rush hour (5:30 a.m. to 9:30 a.m.), and the left lane on westbound I-66 is reserved for HOV-2 during evening rush hour (3:00 p.m. to 7:00 p.m.). Also during those rush hours, between Route 50 and the Beltway, the right-hand shoulder is available for use as a travel lane. Within the Beltway (between the Beltway and the Theodore Roosevelt Bridge), the entire eastbound (inbound) roadway is reserved for HOV-2 and Washington Dulles International Airport traffic during morning rush hour (6:30 a.m. to 9:00 a.m.), and the entire westbound (outbound) roadway is reserved for HOV-2 and Dulles Airport traffic during evening rush hour (4:00 p.m. to 6:30 p.m.). This is not readily and easily enforced, since single-passenger vehicles are allowed to come onto the highway within the beltway in the direction of rush-hour traffic when they are intending to use the Dulles toll road at exit 67.
Between Vienna and western Arlington County, the Orange Line of the Washington Metro runs along the median of the interstate. Four stations (Vienna/Fairfax-GMU, Dunn Loring-Merrifield, West Falls Church, and East Falls Church) are located along this section.
I-66 east has two exits, one from each side of the carriageway, to the Inner Loop of I-495. One is a right exit, while one is a left exit; the latter is retained to make it easier for traffic to join the Inner Loop without having to weave across the entire carriageway. Also, there is no access from the Outer Loop of I-495 to I-66 east; traffic wishing to make this movement must use State Route 267 east.
I-66 is named the "Custis Memorial Parkway" east of the Capital Beltway in Virginia. The 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.