E-ZPass & E-ZPass Flex
Carpools with enough occupants to satisfy the HOV requirement (either two or three occupants, depending on the location) can use the Express lanes toll-free with an E-ZPass Flex set to HOV mode.
When HOV restrictions are in effect, all users of the Express Lanes need an E-ZPass®. There are two kinds of E-ZPass: standard and E-ZPass® FlexSM. Both allow drivers to pay tolls electronically, without slowing down. The E-ZPass Flex allows vehicles with at least three people in them to use the Express Lanes without paying a toll.
E-Z Pass devices are available at The Commuter Store.
ExpressLanes.com -- Learn how to safely use the I-495 and I-95 Express Lanes, pay the toll, use E-Z Pass, and more.
66ExpressLanes.org -- Information about I-66 Express Lanes inside the Capital Beltway.
VDOT's E-Z Pass page -- Information about E-Z Pass and E-Z Pass Flex.