By Carlo Versano

Clear Channel Outdoor Holdings ($CCO), now officially an independent company following its spin out from iHeartMedia, is making a bet that the oldest form of advertising in the world still has legs in a digital era.

"We're the fastest growing traditional medium," said Clear Channel Outdoor CEO William Eccleshare, speaking to Cheddar fresh off ringing the opening bell at the NYSE Thursday. Out-of-home (OOH) advertising ー that's billboards, bus stops, subway signage ー is indeed showing strength even as more advertising shifts online and is gobbled up by Facebook, Google, and now Amazon.

There's a fairly simple explanation for the ability of outdoor ads to weather the digital disruption, according to Eccleshare. Unlike nearly every other kind of ad, OOH doesn't interrupt an experience.

"We reach people when they're very receptive to advertising," he said.

Think about it. When you're waiting for a bus, or riding the train, or sitting in traffic, you've got the time and inclination to read about a new product or show. It doesn't feel like a bombardment, the way a radio or TV commercial or an online banner ad often does. That's the secret sauce to OOH, Eccleshare said, and it stands the test of time.

"We're the oldest form of advertising of them all," he said. "Cavemen used to draw pictures on the wall ー that was an outdoor advert."

Still, there are ways to innovate. The Achilles' heel of OOH is that it has historically been very difficult, if not impossible, to measure an ad's success. But Eccleshare said companies like Clear Channel are now finding ways to use anonymized mobile data to "track the consumer journey", a somewhat dystopian way of saying that they can see whether an ad made someone who saw it go buy something.

OOH advertisers are also experimenting with dynamic and programmatic advertising on digital billboards, surfacing different ads for different times of the day or different audiences (say, a train station showing one ad for when a stadium empties out after a ball game; another for the morning work commute). Some day, one might imagine billboards that change to target individuals walking by (a la Minority Report).

Eccleshare said Clear Channel decided to separate itself from iHeart because the synergies just weren't there, and the company can now focus exclusively on out-of-home.

"Outdoor is a very precise and discreet purchase decision," he said. There aren't a lot of brands that want to bundle their billboard ads with radio or digital spots. OOH its own marketing strategy.

And Eccleshare said as more billboards get converted to dynamic screens, there were be more opportunities to innovate and disrupt a business that's thousands of years old.

"There's huge upside still for digital," he said.

For full interview click here.