(CNN) — When you go shopping in a mall, you create an invisible path as you head from one store to another. For the manager of a mall, it would be useful to see the paths made by you and hundreds of other shoppers over time. Now, there’s a tool for detecting those paths: the cell phone.

Mobile phones have become ubiquitous in advanced economies, to the point where in some countries cellular subscriptions outnumber people, and worldwide they number well over 3 billion.

Handsets frequently send tiny communications — basically saying “here I am” — to cell towers and other receivers, regardless of whether you’re using them.

With triangulation, it’s possible to determine the handset’s (and thus your) approximate location. Over a period of time, your path through a mall or other space can be tracked.

If those tiny communications can be kept anonymous — so that a specific phone’s movements can be tracked but not connected to an individual — then they and the patterns that they reveal can be put to commercial or civil use without infringing on privacy rights.

The potential applications are powerful, says Roger Dennis, an associate with innovation consultancy Innovaro, because cell phones “become de-facto tracking devices when applied to populations.”

“Essentially what you are doing is watching rivers of people,” he says.

“Anywhere where there are flows, there will be applications. Mass transit operations will be a prime user of this type of technology to monitor flows of commuters at hubs and interchanges.”

Path Intelligence, a startup in the UK, says it’s in talks with airports, museums and amusement parks, among others. They’re interested in its FootPath system, which lets them track cell phones (and anonymous customers) through their public places.

FootPath, which went on sale last summer, is already being used by mall operators. A key feature is that it lets them measure how long customers stay in the mall. From there, they can experiment to see what makes them spend more time — and, by correlation, more money — before leaving.

Creating a more complementary mix of retailers also becomes easier. One mall manager, for instance, was disappointed by the performance of a café but hesitated to replace it, fearing that it strongly drew customers to surrounding stores.

With FootPath the manager determined this wasn’t the case, and today a better-performing store stands in the café’s place. The software can also help mall operators during rent negotiations.

In one case a retailer sought a rent reduction because of the economic downturn. That request was dropped after the mall manager showed data from FootPath indicating there had been no corresponding drop in foot traffic.

Tracking, then tackling, traffic

There are, of course, other ways to track customers. They could be asked to carry around RFID tags, or such tags could be embedded in shopping carts.

Cell phones, though, are being carried around already, and they are usually kept close to the body. Customers could also be tracked by cameras, but that would be inefficient.

Other startups are focused on measuring road traffic via mobiles. People still carry (and unfortunately chat on) their cell phones when they’re driving, after all.

Cellint, an Israeli startup, offers a system called TrafficSense that delivers travel times, traffic speeds and incident alerts in real-time by tracking anonymous cell phones.

On a highway in Atlanta, the Georgia Department of Transportation determined that TrafficSense compared favorably against more traditional road sensor systems.

The latter are much more expensive and tend to be used only on major metropolitan highways. The cellular-based systems, at a fraction of the cost, can more feasibly cover entire road networks by measuring secondary roads as well.

“The world is crying out for cost-effective solutions for traffic monitoring and real-time traffic data,” says Michal Eshkol, a Cellint representative.

Among those needing it, he says, are departments of transportation, navigation companies, traffic data disseminators, and commuters themselves.

Traffic information is just the tip of the iceberg, says Cy Smith, CEO of Atlanta-based startup AirSage. His company’s TrafficWise suite, released last year, includes a variety of traffic applications based on anonymous cellular signals.

Smith believes anonymous movement patterns will help significantly with a wide range of applications, including real-estate development, marketing, advertising, and emergency management, plus urban, transportation and evacuation planning.

Another way to track road traffic would be GPS navigation systems. That, however, might require extra wireless bandwidth, whereas cell phone signals are already occurring anyway. GPS phones are another possibility — Nokia has done traffic-monitoring experiments with them in California — but relatively few phones have that feature.

Blue-sky thinking

Cell phones, it appears, might be ideal for tracking movement patterns in certain situations, but what if they can detect environmental conditions?

It might sound far-fetched, but the likes of Intel and Nokia are serious about cell phones being equipped with sensors to unobtrusively measure things like UV levels, pollen count, radioactivity, temperature or air pollution.

Thousands of sensor-enabled cell phones in a particular area would give scientists, governments and citizens a new weapon against public dangers. The World Health Organization estimates air pollution causes more than 2 million premature deaths every year, so it would be information that matters.

Even highly localized dangers, like an illegal incinerator affecting a few blocks, might be more readily detected and tracked down.

Intel researcher Eric Paulos envisions a “participatory urbanism” emerging that sees, among other things, citizens collecting and sharing environmental data on their sensor-enabled mobile devices.

A potential hurdle to this idea: who pays?

An extra sensor for detecting conditions, however efficient, would likely consume battery power and other resources on the phone. And transmitting the collected data, however simple, would require network bandwidth (if cell phone owners aren’t on a flat-rate plan, it might add to their bill).

Perhaps users of such phones could get a discount of some sort, suggests Daniel Scuka, founder of Wireless Watch Japan and a long-time industry observer.

“Why should you or I pay for data time or eat into our monthly megabyte allowance just so that Weather.com can know the temperature in an area more accurately?”

However that issue gets addressed, Scuka agrees that these sorts of unattended, anonymous tracking usages for mobiles are the start of a trend. And, he notes, “this couldn’t have happened before — the density of cell phones is a key factor in any such pattern tracking.”

Aloysius Choong, an analyst with research firm IDC, draws a comparison with nature to visualize how cell phones work in this new capacity: “If you think about communications over mobile phones today, it’s a bit like ants communicating using their feelers. With the confluence of mobility, location and the Internet, each mobile phone can become the eyes and ears connected to a central nervous system.”

So humans with cell phones can reasonably be compared to ants with feelers. And there are now billions of us cell phone users roaming the planet. Let the sensing begin.