Metrobus riders say the transit service’s NextBus Web tool is unreliable. Metro says improvements are on the way. (Linda Davidson/The Washington Post)

Six years after Metro launched a service to deliver up-to-the minute bus arrival information to mobile devices, the tool continues to frustrate many who try to use it.

Regular riders say the NextBus service, which uses Global Positioning System tracking and historical data on traffic patterns to predict when a bus will arrive, is seldom accurate.

It “simply doesn’t work,” said Allison Ransom of Arlington County, who said she has been riding Metrobuses throughout the region since 1993. And, she added, it has gotten worse.

“It’s horrible,” she said as she waited one recent morning at the Ballston station for the 1A bus to Vienna. “To call it a service is a joke. It’s not convenient for getting information, and it’s supposed to be.”

According to her device, the bus was supposed to arrive by 10:05 a.m. At 10:15 it still wasn’t there, and there was no updated information on its arrival.

Metro says its predictions on bus arrival are correct roughly 90 percent of the time.

NextBus, not to be confused with the similarly named NextBus DC mobile application that was recently relaunched under a new name, was started as a pilot program in 2007. Almost immediately there were problems with its accuracy. The transit agency halted the pilot to make technological upgrades and relaunched it in 2009.

Metro’s NextBus technology is operated under contract by NextBus Inc. of Emeryville, Calif. The transit agency has invested $3 million to upgrade the system and says it has other improvements on the way in the coming year.

In addition, Metro has spent $39 million over the past three years upgrading technology on its 1,500 buses and at its control center to create a unified system that can better track the location of buses and real-time traffic, according to Metrobus officials.

“We are overtaxing the system, and we need to get it on an even keel,” said Jack Requa, Metro’s assistant general manager for bus services.

The NextBus DC app has had its own hiccups. This winter, about 7,000 D.C. area commuters complained after the app stopped working amid a business dispute between the founder of the company that created the bus tracking technology and NextBus Inc. The developers of the NextBus DC app said they have remade it, and they recently launched a new version of the app, under the name iCommute DC.

As part of Metro’s bus upgrades, which are expected to be done in the next six months, Metro said NextBus arrival times will be updated every 30 seconds instead of every two minutes — a move that Metro says will give customers more accurate predictions.

In the absence of the NextBus DC app, Metro’s NextBus system drew 801,723 page views on its Web site in January, a 48 percent jump compared with the same period in 2012.

Still, riders complain that the NextBus area of Metro’s Web site is hard to navigate on a mobile device. Others complain about “ghost” buses — ones that are supposed to come according to the online updates but never show up.

Metro said there can be several reasons for such problems.

Old equipment on the bus that helps track where it is and matches it with the schedules can be broken. Bus operators sometimes fail to log in to the computerized fare box when they take on a route, making it difficult to track the bus. And there are the unpredictable traffic delays.

Metro said it is working to install signs at hundreds of Metrobus shelters throughout the region over the next four years that will give NextBus information and other data.

But the improvements Metro is promising are not coming fast enough for those who use the NextBus service daily.

Lucy Cunningham of Northwest waited recently at the Friendship Heights station for the E2 bus to take her home to Brightwood. On her device, it showed eight minutes until the next bus. She waited, and waited, and waited.

After an hour, she gave up and took a cab home.

“It can be very frustrating,” she said. “I call them, I e-mail them, I tweet them if something is terribly wrong. You just want them to know.”

She said she chatted with other riders who were also waiting.

“We kept thinking, ‘Oh, it is going to come,’ ” she said. “It gets to the point where you’re thinking, ‘Gosh, I am never going to get home at this rate.’ ”