The city’s Metropolitan Transportation Authority has been trying to provide a better sense of predictability in recent years by adding displays in stations that state when the next train is expected. Now, a Web development firm called Densebrain says that it can do the same thing at practically no cost, by analyzing how people lose phone service when they head underground.

Urban planners, technology companies and officials from local governments see potential in projects like these that mine data collected from phones to provide better public services.

Boston is developing a system called Street Bump that uses a smartphone’s accelerometer and GPS system to detect when a driver hits a pothole and then sends that information to city officials.

Techniques like this may help cities collect data that until recently would have required expensive network sensors.

“It is unlikely that we are going to be able to invest in that sensor system. But what we’ve recognized is that many, many constituents have already invested in a sensor platform,” said Chris Osgood, co-chairman of the Mayor’s Office of New Urban Mechanics in Boston, which is responsible for establishing Street Bump.

Densebrain’s project works by taking note of which cellphone tower a phone is communicating with. It then looks for disruptions in service followed by significant changes in location. If a phone located near Times Square suddenly loses service and reconnects at Prince Street and Broadway 15 minutes later, then it has almost certainly traveled there using the N or R trains.

This type of data, when taken from large numbers of phones and analyzed algorithmically, could give an accurate look at the performance of the entire subway system in real time.

Or so Alex Morgan Bell hopes. Mr. Bell began designing the system last year, when he was studying electric engineering at Columbia. After trying to get the idea going by himself and luring only several hundred people as users, Mr. Bell joined Densebrain, a Web development company that makes NYCMate, a transit map app (and is perhaps best known for SitorSquat, an app that maps public restrooms).

Users of the free transit app, who number about 600,000, according to the company, will be asked to activate the feature starting on Monday. Mr. Bell believes that the system needs 10,000 users to give a reliable view of the trains in Manhattan.

There are still questions to work out. In its pilot stage, the NextTrain app will work only for trains underground. The system will also include an experiment that uses phones’ microphones to sense when riders are on buses, but Mr. Bell believes that some sort of hardware would probably have to be installed for the system to work above ground.

There are other ways to track mass transit. NextBus, a technology company based in California, works with about 90 transit systems nationwide to analyze data drawn from GPS devices to provide real-time updates on the movements of buses and trains. Los Angeles began using NextBus for its entire bus system in May, the largest transit agency to do so.

Mr. Bell said the information appearing on the authority’s subway arrival clocks did not help riders who were still above ground. The authority said that though it would like to provide that information to developers eventually, it had no specific plans to do so.

“You can stay in the Starbucks instead of leaving, because you’ll know when to say, O.K., now I’m going down into the hot sweaty disgustingness,” Mr. Bell said.

The authority says that NextTrain could be a useful service for riders as a supplement to its own projects, and an engineer at the authority said that Densebrain’s data might prove useful for its own planning.

Data automatically collected from large groups of cellphones is a new frontier for planners and local governments, said Frank Hebbert, director of civic works for Open Plans, a nonprofit technology and planning association.

“It’s a completely different source of data,” said Mr. Hebbert. “The idea that you suddenly have data sets coming to you in which you haven’t had to go and physically put in infrastructure is pretty amazing.”

Another smartphone app, Waze, combines data on how fast users’ cars are moving with other data sources to determine traffic patterns. It then suggests alternate routes.

Waze, which says it has about four million active users, said it was in talks with several city governments to provide insight into traffic patterns near large construction projects. The company says that its benchmark for critical mass is to have 0.25 percent of drivers in a metropolitan area as users. It has not reached that goal in any American city.

Apple and Google have been collecting traffic data from iPhone and Android phones for similar purposes. Mr. Hebbert said he would eventually like to see phone companies provide a database of anonymous location information that planners and developers could use to build applications relevant to civic projects.

This could be a challenge, as it is clear that many people are uncomfortable with technology companies or government agencies tracking their every move.

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