an easy way to buy tickets on the move by using their smartphones.

The transit agency said Monday it has signed a contract with

, to offer paperless ticketing for buses, MAX and WES commuter rail starting next spring. Once it's launched, the system will save riders the frustration of wrestling with ticket machines or diving into their wallets for exact change.

"It introduces enormous customer convenience," said Nat Parker, chief executive of

. "It also reduces the overall cost of fare collection."

The company is not charging TriMet development fees. Instead, the agency will pay GlobeSherpa a commission. Mary Fetsch, TriMet spokeswoman, said the commission will be based on ticket sales, with the agency paying a maximum of 11 percent.

"That's the ceiling, but it's likely to be less than that," Fetsch said.

Here's how the system will work: Riders will download the free app onto an iPhone or Android smartphone, register a debit or credit card and buy a ticket. The app will allow commuters to choose among adult, youth or honored citizen fares and buy everything from a two-hour ticket to a pass for a day, five days, month or year. The ticket will appear on the screen to show to drivers or fare inspectors.

The app will also connect to TriMet's mobile website, allowing riders to plan trips, keep tabs on bus and train arrivals and receive alerts about service disruptions.

"Wherever you are, you can buy a ticket," Fetsch said. "And you can activate it when you need it."

And if you don't have a smartphone but ride TriMet, don't worry: The app won't replace vending machines or over-the-counter sales.

The company and TriMet will conduct an alpha test of the system through December, when TriMet will recruit riders to participate in beta testing starting in January. If all goes according to plan, the app will be launched to the public next spring.

Parker said the app is secure, using encrypted data for all sensitive information. "It's very similar to when you purchase something on Amazon or use Starbucks' mobile payments," he said.

TriMet General Manager Neil McFarlane said in a news release that the system will be faster and more convenient for riders, does not involve any initial financial outlay by the agency and supports a local company.

GlobeSherpa was started in Northwest Portland two years ago by two Portland State University grads who met while completing their MBAs. The company's focus is mobile ticketing.

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