The outdoor advertising industry on Monday is rolling out a new campaign meant to capitalize on marketers’ growing fears about whether the digital ads they buy are being viewed by real people.

Brands have been pouring dollars into online marketing, but are worried about ads that can be buried in corners of websites that aren’t viewable, and about computerized “bots” that goose traffic results artificially without any humans seeing ads.

The new campaign from the Outdoor Advertising Association of America, dubbed “Feel the Real,” makes the pitch that out-of-home ad platforms like billboards and transit shelters don’t suffer those problems.

The campaign, launched to coincide with the start of Advertising Week, includes billboards that tout “This ad is real,” rail posters that say, “You are consuming an advertisement. You are real,” and signs on telephone kiosks that ask, “Media planners, do you have a reality problem?” In total, the campaign will feature more than 1,600 outdoor displays.

Some signs close to media agencies will call out the agency’s name or its executives. A billboard outside of Ogilvy & Mather’s New York office, for example, will say, “Hey, Shelly, does this ad feel real to you?” in reference to the agency’s chairman emeritus Shelly Lazarus. The media space for the campaign is worth more than $2 million and was donated by OAAA’s out-of-home media member companies, the organization said.

The OAAA’s new campaign will include a billboard in Times Square. Photo: PNYC

“In a world where digital and its ability to deliver what it promises is under significant scrutiny, out-of-home has a unique and compelling point of view that having one foot grounded in the real world matters,” said Matt Dowshen, president of PNYC, the agency behind the “Feel the Real” campaign.

It’s the first time the industry trade group is addressing issues such as ad viewability and ad fraud head on, said OAAA Chief Executive Nancy Fletcher. The ultimate goal, of course, is to encourage advertisers to ramp up their spending on outdoor displays.

“We really look at out-of-home as being a perfect complement to digital and that together both campaigns are stronger,” Ms. Fletcher said.

While most types of traditional media have struggled with advertising declines, spending on out-of-home ads has generally increased in recent years. For the first six months of 2015, spending on outdoor ads in the U.S. rose 2.7% to $2 billion from the year-ago period, according to data from Kantar Media. Outdoor ad spending in the U.S. totaled $4.1 billion in 2014, Kantar Media’s data shows.

Still, the online ad marketis much bigger – about $50 billion in the U.S. in 2014 – and is growing faster at a projected 17% clip from last year, according to eMarketer.

TV advertising also dwarfs outdoor, with TV ad expenditures totaling about $36.7 billion for the first half of the year, according to Kantar Media.

Much of the outdoor medium’s growth has been fueled by new digital billboards that can be updated dynamically and its ability to reach mass audiences in transit. For a marketer, out-of-home ads are also appealing because they can’t be skipped or fast-forwarded through like TV ads recorded on a DVR. Unlike digital ads, ad fraud isn’t much of a concern when it comes to buying billboards.

The marketing push also aims to encourage media planners to view out-of-home ads as a medium that can accelerate a campaign’s digital, social and mobile elements. Each of the outdoor ads in the “Feel the Real” campaign features a call-out to the website www.feelthereal.org.

To hammer home its point, the OAAA campaign’s website asks visitors, “Did you see one of our out-of-home ads?” followed by four options: “Yes, I saw it in the real world,” “Yes, I saw it online,” “No, I heard about it,” and “No, I’m a robot.”

OAAA and PNYC will collect and share data related to traffic and engagement with the campaign’s site to show the ability of out-of-home to drive digital effectiveness.

“Out of home has a big, bold presence in the real world,” Mr. Dowshen said. “If there is a call to action to go online, it guarantees real eyeballs and clicks.”

Write to Nathalie Tadena at nathalie.tadena@wsj.com