Drivers who used Maryland toll roads in the past year but haven’t received an invoice for the trip shouldn’t expect to avoid paying the toll.

The Maryland Transportation Authority is billing motorists for millions of toll trips made after invoices were paused during the coronavirus pandemic. The backlog has ballooned to more than 23 million transactions awaiting processing as the agency pleads for patience amid a torrent of customer complaints.

The MDTA, which manages the state’s eight toll roads and bridges, said it already has mailed bills for about 5 million transactions for video tolls worth $27 million. The agency in recent days has started processing notices for more video tolls — in which motorists without an E-ZPass transponder pay by mail — that were incurred back to August.

The backlog is catching many motorists by surprise: Some people — not expecting a bill for a trip made months ago — aren’t seeing or are ignoring notices as they arrive in the mail. Meanwhile, the agency’s customer call centers are overwhelmed with inquiries from people who are opening the invoices.

The average toll for transactions billed so far has been $5.40, a pace that would leave the state seeking to recover well over $100 million in unpaid tolls. State officials say they don’t have an estimate of the outstanding amount.

Officials are urging people who drove on a toll road without an E-ZPass transponder not to wait for a paper bill to arrive. Tolls can be paid at before a $25 late fee kicks in. The fee is applied 30 days after the notice is mailed, but some users have reported receiving notices too late or never getting them at all.

In a letter this month to toll road drivers, MDTA Executive Director James F. Ports Jr. warned that delays in the U.S. Postal Service are “hampering both MDTA efforts for timely delivery of tolling notices and customer efforts to pay notices on time.”

“We ask for your patience and understanding. We know you are trying to reach us, and to say that call volumes are extremely high is an understatement,” Ports wrote. “Rest assured that contested mail delivery delays and the time it takes for our team to research your request will NOT be held against you.”

Callers who typically would wait less than five minutes to connect to an MDTA representative are facing wait times of up to an hour, officials said. To increase capacity at its call centers, the MDTA recently expanded hours on Thursdays and Saturdays and is increasing staffing.

Maryland Del. Alfred C. Carr Jr. (D-Montgomery), who has been critical of the MDTA’s handling of overdue fees and called for lower late-fee fines, said he has heard from residents forced to pay late fees because they didn’t receive toll notices on time. Recent Postal Service delays have only compounded the problems, he said.

“Holding a bill for months and months can be problematic,” said Carr, noting that many people aren’t expecting a bill or late fee. “A lot of people are struggling. The punitive approach, the fine approach, is going to be hard on people.”

The MDTA last year reduced the video toll late fee from $50 to $25 after pressure from state legislators and advocates who have lobbied against fines.

Carr said he also has heard from residents who drove on a toll road with an E-ZPass but the toll was not billed to their account, creating another layer of problems with late payments.

“The [MDTA] is supposed to charge you through the account, not through the mail. And that is not happening” in some cases, Carr said, referring specifically to E-ZPass account holders.

Problems with an E-ZPass transponder or a lack of funds in an account can trigger a mail bill.

Other toll road users have expressed confusion over the authority’s July decision to pause the posting of E-ZPass transactions for users of the Intercounty Connector, which connects Gaithersburg in Montgomery County to Laurel in Prince George’s County. That includes another 14 million unpaid toll road transactions in the state.

MDTA spokesman John Sales said the pause ensured that ICC toll trips were being recorded accurately before the launch of the agency’s “next-generation tolling system” this year. He said the MDTA is resuming posting those transactions to E-ZPass accounts.

“The MDTA will be metering the postings to control the volume of trips hitting an account at one time,” Sales said.

He said E-ZPass users should maintain a positive balance to account for future toll road trips but also for backlogged trips that will post in the coming weeks. Otherwise, a toll invoice could be sent in the mail.

The transition to a new tolling system statewide, which will include a new call center this spring, will add capacity to respond to customer inquiries, officials said.

About a year ago, the authority eliminated a cash option for paying tolls at the Bay Bridge and other tolling facilities to limit interactions between drivers and toll collectors during the pandemic. In August, the state declared the implementation of all-electronic tolling an “operational success,” saying it was ready to make the change permanent.

Maryland started the transition to all-electronic tolling in 2019, but the full conversion was not expected for years. That quicker move to cashless tolling led to the state assessing significantly more video tolls.

The authority’s pause on mailing toll notices for transactions incurred between March and October 2020 was intended to provide financial relief during the pandemic, officials said.

“However, motorists who used MDTA facilities remained responsible for paying tolls,” Sales said.

The MDTA resumed sending bills for unpaid trips in mid-October but is far from catching up. Still to be mailed are bills for video toll transactions from August to now, the agency said.

It could be several months before it works through the backlog as motorists continue to rack up tolls from new trips.

Sales said the agency understands the pandemic has affected Maryland families and is working with customers having difficulty paying bills. He said the agency gave users a break last year when it charged vehicles without an E-ZPass transponder the cash rate via mail, instead of the more expensive video tolling rate.

The MDTA resumed standard video tolling rates on Jan. 1.