The U.S. Transportation Research Board reported in 2012 that nearly $1 billion is spent annually on advertising in public transit facilities and vehicles, including buses, trains and taxis. Technology makes transit ads available in a variety of places and forms, with versions moving beyond print to deploy audio, video and global positioning system-triggered pitches. The ubiquity of transit ads in urban settings ensures they'll reach lots of eyeballs, but the jury is out on their effectiveness relative to TV, radio and Internet ads.

Ad Venues

Transit advertising is part of a much larger category known as out-of-home advertising, which rose 4.2 percent in 2012 to reach $6.7 billion, according to the Outdoor Advertising Association of America. Consulting firm PQ Media points to popular transit-specific examples, including video ad networks found in taxis, buses, airports and gas stations; digital message billboards seen in train stations and truck stops; and ambient ads consisting of "full wraps" applied on buses, subway cars and trucks .

Ad Technology

Advertisers seeking to reach captive audiences are working with public transit systems, which are often looking for new revenue, to get location-specific advertising onto trains and buses. For instance, ads are played over bus speakers just before stop announcements, and global positioning system, or GPS, technologies now allow ads for specific businesses to be triggered when transit vehicles reach a stop close by. Meanwhile, advertisers and transit systems are seeking ways to avoid alienating riders, including limiting the frequency, length and audio volume of announcements to minimize intrusiveness.

Who's Spending

According to the Outdoor Advertising Association, the top growth among out-of-home advertising spenders in 2012 came from government and political organizations, accounting for 30 percent of the growth thanks to an election year. Advertising by retailers, entertainment services, schools, camps and seminars also grew significantly, though mostly by single-digit percentages. Seven of the top 25 out-of-home advertisers were media companies, such as Warner Bros. Pictures, NBC, Comcast and Sony Pictures.

Measuring Results

The outdoor ad association's data show the cost-per-thousand to reach audiences is generally much lower for ads on transit shelters, posters and bulletin boards than for spot ads running on local TV news or prime-time TV or for daily newspaper ads, among other media. However, the transportation research board notes no credible method exists for measuring the audience for most transit ads in the way that viewers are tracked in other media. The transit industry is working with consultants to devise formulas for tracking metrics, including who sees the ads -- by age, sex and other demographics -- as well as how frequently ads are seen in various settings.