Traffic seen in a recent view at rush-hour along Interstate 66 in Vienna. On Dec. 4, the Virginia Department of Transportation started collecting tolls from solo drivers who use the roadway during rush hour. Tolls, based on the amount of traffic on the road, went as high as $34.50. (Marvin Joseph/The Washington Post)

One byproduct of the shift to tolling during rush hour on Interstate 66? A sudden rush to buy E-ZPass transponders.

Retail outlets that sell the transponders, which allow tolls to be collected electronically, reported a significant uptick in sales in the days before the start of tolling on I-66, with some retailers running out.

Jennifer McCord, a spokeswoman for the Northern Virginia office of the Virginia Department of Transportation said outlets that have run out of the devices can reorder and that replacement stock should arrive within two days.

If you want to drive on I-66 during rush hour, you need an E-ZPass. If you’re driving solo, the system will automatically deduct a toll. If you have two or more people in your vehicle and set your transponder to “carpool” mode, you can ride free. Motorcyclists are the only drivers not required to have an E-ZPass — and they too ride free. There are no toll booths, and cash payments are not accepted.

On Monday, the first day of rush-hour tolling, about 86 percent of users traveled with an E-ZPass or an E-ZPass Flex, McCord said. Of those, about 37 percent had multiple occupants in their vehicle, which meant they didn’t have to pay a toll; about 14 percent, however, did not have a transponder and could face a fine.

If you’re new to the system, here’s how it works. First, there are two kinds of transponders: E-ZPass and E-ZPass Flex.

E-ZPass Flex allows you to flip a switch to indicate when there are multiple passengers in your vehicle, allowing you to travel free or at a discount in Virginia’s High Occupancy Toll (HOT) lanes. If you want to take advantage of the free carpool option when traveling on I-66, you’ll need to get an E-ZPass Flex. Keep in mind however, if you don’t use your Flex transponder for six months, you 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.)

You can buy an E-ZPass On-the-Go kit for $35 at select retail stores. Of that amount, $15 is instantly usable for tolls; the remaining $20 is made available once you register your account. The transponder can be attached to your windshield or license plate. As you pass through an E-ZPass facility, the toll will be automatically deducted from your account.

You can go online here to open an E-ZPass account or call 1-877-762-7824.

E-ZPass transponders are sold at select branches of the Virginia Department of Motor Vehicles, AAA stores in the region, as well as some Wegmans and Giant supermarkets. Click here for a full list.

If you have an E-ZPass but want to exchange it for an E-ZPass Flex, you can request an exchange by logging into your online account or by calling the service center at 1-877-762-7824 and requesting an exchange. You can also visit a service center and make the exchange in person.

When you receive your Flex transponder, be sure to return your old one to: E-ZPass Service Center, PO Box 1234, Clifton Forge, VA 24422-1234 — or you can make the exchange at a service center. If you lose your transponder, it will cost you a $10 fee.

There are more than 1.3 million active E-ZPass users in Virginia.

In addition to Virginia, the transponders can be used in states with toll roads that have specially marked E-ZPass lanes including: Maryland, Delaware, New York, New Jersey, Massachusetts, Maine, New Hampshire, Ohio, Pennsylvania, Rhode Island, West Virginia, North Carolina, Kentucky, Illinois and Indiana.

And, if you’ve traveled I-66 without a transponder, you can visit 66expresslanes.org, select “Missed a Toll,” and enter your license plate to pay online and avoid receiving a violation notice. VDOT officials said the transaction should be posted within three days of your trip and can be paid within six days to avoid receiving a violation notice by mail. Questions? Call the E-ZPass Customer Service Center at 1-877-762-7824.