Riders of public transportation are used to ads by now. Indeed, if you're a regular bus or subway user, chances are pretty good you'll see an advertisement at your stop, on the exterior of your vehicle and once again on its interior -- all before even grabbing a seat. Sometimes, it may seem like the only way to escape it all is by closing your eyes. But in some places, that might not even be enough anymore because the latest trend in transit advertising is audio commercials.

Those commercials -- not to mention a bevy of other forms of advertising -- are increasingly viewed as a way to supplement revenue in this tight fiscal climate. "It's revenue we need to help us provide great service and keep our fares low," says Jan Kijowski, marketing director at the Champaign-Urbana Mass Transit District in Illinois, which has earned about $150,000 since late 2009 for audio advertising on its buses.

While that's not a huge sum, transit experts say every penny counts these days. "You've got to prove to the public you're doing everything you can," says Joel Volinski, director of the National Center for Transit Research at the University of South Florida and a former head of the Broward County, Fla., transit agency.

Indeed, a report published in 2011 by the American Public Transit Association found that more than half of transit agencies cut service or raised fares in the wake of the recession. That's made the prospect of finding advertising dollars in unconventional places appealing.

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In Toledo, Ohio, buses began carrying the audio advertisements in 2010 in response to falling revenue, says Steve Atkinson, marketing director at the Toledo Area Regional Transit Authority, which is funded largely by property tax revenue. "We're always looking for unique ways of cutting costs and being efficient," he says.

Audio advertisements typically are short -- about 10 or 15 seconds -- and can take advantage of something that traditional print advertisements can't: location. Buses are equipped with federally-mandated technology that triggers announcements identifying the upcoming stop. That same technology can now be used to trigger location-specific commercials.

In Champaign-Urbana, for example, a McDonald's commercial plays shortly before a bus stop near the restaurant. Advertisements can be triggered to run at stops known to be busy or full of riders of a certain demographic. They can also be triggered to run at certain times of day. Stautzenberger College, a small school near Toledo, runs it advertisements on bus routes that stop in front of high schools in hopes of reaching prospective students. "I liked being able to try something new and specifically go after exactly who I wanted," says director of admissions Amanda Boyd.

Commuter Advertising, the leading contractor that provides audio advertising to transit agencies, has inked deals with transit agencies in the Chicago suburbs; Cincinnati; Dayton, Ohio; Jacksonville, Fla.; Kansas City, Mo.; Pittsburgh; and Rockland County, N.Y., in addition to Toledo and Champagne-Urbana, all since 2009.

And it's not just audio advertisements that are gaining momentum. Volinski of the University of South Florida says transit agencies are working to find new twists to old advertising methods. Based on survey data he's collected from transit agencies across the nation, there seems to be no limit to just how far transit agencies will go when it comes to finding new ways to run ads. Advertising is already on transit agency websites, mobile apps, bus schedules, fare cards and the sides of park-and-ride garages. But now, a growing number of bus systems are exploring the use of LED advertising on the sides of their vehicles. That style -- already popular on roadside billboards -- is more flexible than traditional advertising because it can be triggered by time and location too, and it's visible at night.

Transit agencies are also looking to put advertising on the ceilings and floors of buses.

Even the names of stations themselves are up for grabs. In recent years, AT&T bought the naming rights to a stop in Philadelphia, and Barclays bought the naming rights to a Brooklyn station. In Cleveland, two hospitals teamed up to rename an entire line. Today, that bus rapid transit route once known as the Silver Line is now the HealthLine, thanks to a deal that pays the transit agency $250,000 annually for 25 years. All total, the Greater Cleveland Regional Transit Authority gets $1 million of its approximately $250 million annual budget from advertising. "With transit advertising, it's not a big moneymaker," Joe Calabrese, general manager of the system, told Governing last year. "but it's an important moneymaker."

Meanwhile, some agencies that have historically shied away from traditional advertising are now embracing it. The transit agency in Charlotte, N.C., for example, once had advertisements on the exterior of its buses, but eliminated them in 2000 when its adverting contract ended. Ten years later, it started them up again as a direct response to a 20 percent decline in local sales tax revenue. This year, the agency will generate at least $1 million in advertising revenue and is considering adding advertising at its light rail stations and selling naming rights. When asked what, exactly, the agency might sell naming rights to, Olaf Kinard, the system's director of marketing and communications, simply responded: "To anything."

But some question whether the barrage of advertising is too much. Yonah Freemark, editor of The Transport Politic wrote, "The whole situation raises the frightening prospect in the near future that, instead of riding the Broad Street Subway from City Hall to Pattison, people will take the Coca-Cola Trolley from Pizza Hut to AT&T."

Volinski, for his part, acknowledges that advertising may not necessarily be pleasant for riders. But if it can help protect service or prevent higher fares, he's in favor of it -- and he bets passengers would be too. "In a perfect world none of us would like to be bombarded by this from every angle ... but we don't live in a perfect world."

Still, not everyone has embraced all forms of transit advertising. Some cities have ordinances that limit the extent that advertising is allowed on publicly owned property, including bus shelters and vehicle exteriors. The Boston transit system opted against running audio advertisements, for now, after getting some negative press when it considered a deal in 2011. Famously, the agency scrapped T-Radio, which pumped news, music and advertisements into stations, after just two weeks because it faced a flurry of complaints.

In Toledo, Atkinson says the on-board commercials aren't the end of its advertising plans. The agency is exploring the possibility of printing advertisements on the half-million bus schedules it prints each year, as well as its mobile website and the exterior of its bus shelters. And so far, he says, passengers don't seem too annoyed by those audio ads. "I haven't gotten any complaints," Atkinson says. "The only complaints come from the bus operators. It's the repetition."