The 495 Express lanes (left lanes here) provide a faster passage, but at a variable price. (Michael S. Williamson/The Washington Post)

Use of the E-ZPass electronic payment system has grown exponentially across the Washington region in recent years as the network of toll lanes has expanded in the Interstate 95 corridor with new facilities from Stafford County to just north of Baltimore.

Both Maryland and Virginia are actively promoting E-ZPass, hoping to reduce cash payment at toll plazas — where the option exists — and to encourage the seamless use of all-electronic toll facilities such as the 495 Express Lanes and 66 Express Lanes in Virginia and the Intercounty Connector in Maryland.

In that spirit, Maryland announced last week that it is making E-ZPass transponders free, eliminating the $7.50 sign-up fee and betting on a greater number of drivers going cashless at the state’s eight toll facilities, including the Chesapeake Bay Bridge.The change, however, did not apply to specialized transponders such as the E-ZPass Flex, which is in high demand among drivers who carpool regularly and use Northern Virginia’s High Occupancy Toll (HOT) lanes such as the 495 and 95 Express Lanes.

Maryland doesn’t have HOT lanes, which explains why the state isn’t pushing E-ZPass Flex sales, nor is it reducing the cost for the transponder. However, John Sales, a spokesman for the Maryland Transportation Authority, said the state makes the Flex transponders available to E-ZPass Maryland customers who wish to use them in Northern Virginia — for $16.50.

That means Maryland residents who regularly commute across state lines might still choose to get their Flex transponder in Virginia, where both the regular and Flex transponders are free. Virginia also has made it easy for nonresidents to obtain a Virginia E-ZPass by partnering with AAA stores and other retailers to make the devices available outside the state.

“The same rules apply to Virginia and non-Virginia residents,” said Jenni McCord, a spokeswoman with the Virginia Department of Transportation. “There are no additional costs or processing fees.”

While Maryland sees expanding E-ZPass usage as a way to ease into a cashless toll system, Virginia is on a race to grow its E-ZPass program as it expands its network to more than 90 miles of HOT lanes by 2022. The state plans to add lanes on Interstate 395, I-66 outside the Beltway and extend the 95 Express Lanes to Fredericksburg all within the next four years.

And, in a region where there are more and more toll facilities, the need for a transponder is almost inevitable. There was a big rush to get the devices in the days before and just after the start of tolling on I-66 inside the Beltway, with some retailers running out. AAA’s store in the District, for example, sold more than 250 transponders in December alone and ran out of the devices twice.

And after Maryland Gov. Larry Hogan (R) announced the state would make its regular transponder free, the MDTA had a surge in phone calls from residents requesting the devices, Sales said.

“The reality is that E-ZPass is the most cost effective way to pay the tolls,” he said.

Here are the basics on how the systems work whether you’re in Virginia or Maryland

The E-ZPass transponder, mounted on a windshield or license plate, allows tolls to be collected electronically. When a vehicle with a transponder travels through an E-ZPass toll facility, an antenna at the toll plaza reads the information on the transponder and the toll is automatically debited from the pass holder’s account.

Transponders are required to use toll facilities that don’t have cash toll booths such as the Intercounty Connector in Maryland and the 495 Express Lanes in Virginia.

E-ZPass transponders can be used on all toll roads with specially marked “E-ZPass” lanes. Other states that participate in the program include Delaware, Illinois, Indiana, Maine, Massachusetts, New Hampshire, New Jersey, New York, Ohio, Pennsylvania, Rhode Island, West Virginia, North Carolina and Kentucky.

What’s the difference between E-ZPass and E-ZPass Flex?

EZ-Pass Flex allows drivers to flip a switch to indicate when there are multiple passengers in the vehicle. This allows them to travel free or at a discount in Virginia’s HOT lanes (or in HOT lanes in other E-ZPass states). You must have an E-ZPass Flex to use this option.

One thing to keep in mind when getting a Flex transponder is that if it’s not used for six months, drivers may be required to exchange it for a regular E-ZPass or be subject to a one-time “transponder functionality charge” of $10. (This applies only to transponders issued after Oct. 1, 2014.)

Maryland residents can buy the E-ZPass Flex for $16.50, and still enjoy the carpool benefit in Northern Virginia.  It is unclear whether the six-month usage requirement applies to Maryland Flex holders.

The good news for Maryland residents is that using E-ZPass Maryland saves them 25 percent or more on the state’s tolls.  At the Bay Bridge, for example, the toll is $2.50 for an E-ZPass customer and $4 for someone paying with cash.

Is the E-ZPass free? 

Technically, yes. But opening an account requires some money.  In Virginia, you can sign up for the E-ZPass with a $35 deposit that goes toward tolls; Maryland requires a $25 deposit that also goes toward toll payments.

Speaking of payment, Virginia and Maryland allows users to open an account using cash, check, or credit card.

So how many people are enrolled in E-ZPass?

In Virginia, there are about 1.5 million active accounts with more than 2.2 million transponders, according to VDOT.  In Maryland, 1.1 million people have an E-ZPass and nearly 79 percent of the toll transactions are made with an E-ZPass transponder, station transportation officials said.

Who should get E-ZPass? 

If you ever travel by car across the region, it may be a good idea to have an E-ZPass account. You can save on tolls and time by paying electronically at the toll booth. They come in handy on trips to the Eastern Shore beaches or up and down the East Coast where toll facilities are popular and unavoidable. Keep in mind that some roads such as I-66 have an HOV2 requirement while others like the 495 Express Lanes require three passengers to get the free ride.

How do I get an E-ZPass account?

In Virginia, visit ezpassva.com for details about how to enroll online. Transponders are also available at select branches of the Virginia Department of Motor Vehicles, AAA stores in the region, and at some Wegmans and Giant supermarkets. Here is a full list.

In Maryland, visit ezpassmd.com. Applications for a new transponder can be made online and via mail.