If the Maine Turnpike is looking toward a future without cash, why is it building traditional staffed tollbooths in South Portland, at one of its busiest exits?

The answer is complicated, according to the Maine Turnpike Authority.

Yes, all-electronic tolling is likely coming, but it isn’t here yet. And with the turnpike’s unique mix of users, switching to an electronic system haphazardly could cost the agency millions of dollars a year.

“When the day comes that we decide we don’t need cash collectors out there, it will happen all at once,” Turnpike Authority Executive Director Peter Mills said. “Until we foresee that day, any new construction will continue to provide for the collection of cash.”

At present, new construction means the rebuilt Exit 45 adjacent to the Maine Mall in South Portland. The $27.7 million project is part of a bigger, yearslong endeavor to add a travel lane on each side of the four-lane highway through Portland.

Part of that price tag is $900,000 for two tollbooth buildings. But those structures are more than places for toll cashiers to stand, Mills said. They also house electronics, power backups, digital memory storage, servers and network cables necessary to operate tolling systems.

A building that could house such electronics without space for a cash collector would cost less “but not so much less as to justify abandoning cash collection,” Mills said.

Earlier this year, the Turnpike Authority said it intended to experiment with electronic cash collection with an eye to eliminating cash transactions in the future.

But that future might be far off and there is no plan yet. Until it develops, cash transactions are here to stay.

Cash collection amounts to about 15 percent of the Turnpike Authority’s annual toll revenue, roughly $21 million last year. During the summer, the proportion is higher, as U.S. and Canadian tourists without electronic transponders pay cash to cruise southern Maine.

That makes the Turnpike different from toll agencies reliant on commuters for a regular revenue stream, Mills said. And getting money back from tourists in a cashless system can be challenging. States such as Massachusetts that have implemented all-electronic tolling employ an army of technicians to send bills to drivers who lack transponders but are identified by camera images of their license plates.

The Maine Turnpike doesn’t have the staff to manage such a huge operation. Even if it did, some Canadian provinces will not hand over driver information to allow their residents to be charged, Mills said.

Having a cash option also means the Maine agency experiences less toll delinquency than other places, since nearly all drivers are paying one of two ways and very few speed through without paying at all.

“Our violation rate is really low because our mechanisms are in place to enforce payment. If you eliminate cash as an option, that goes away,” Turnpike Authority spokesperson Erin Courtney said.

Right now, all turnpike lanes take electronic payment through the E-Z Pass system. People slow down anyway on entrance and exit ramps, so it won’t make a difference if cash booths are manned or not, Courtney said.

While the Maine Turnpike comes up with a plan, more agencies across the U.S. are making their tolls all-electronic. This month, the George Washington Bridge between New Jersey and New York City went cashless. The Massachusetts Turnpike implemented all-electronic tolling in 2016.

Nearly six out of every 10 U.S. toll facilities – points where drivers are charged for using the highway – are operated without a cash option, according to the International Bridge, Tunnel and Turnpike Association. Nearly every new toll facility built in the last 12 years was cashless, as are 55 percent of all U.S. tolled roads, the association said.

That makes the Maine Turnpike an outlier. At some point, the cost of keeping nearly 100 toll takers on staff will begin to outweigh the benefit – or novelty – of accepting paper bills and coins.

“My board is very conservative about this,” Mills said. “Until staff can come up with a good plan that makes it worth doing, we’ll stick with the cash.”