outcastVideos ads are multiplying at the nation’s gas stations. You can expect to see a lot more of them thanks to a joint venture being announced today by the two largest companies in the business.

Outcast is teaming up with AdtekMedia, owner of PumpTop TV, to create a network of gas pump video displays. The two companies will have a combined monthly audience of 20 million viewers a month. That’s roughly equal to a top five primetime TV show.

Gas station videos are taking off because the viewers are captive. You just can’t do much else while you’re fueling your car. And you can’t skip through the ads like you do at home with Tivo.

Matthew Stoudt, chief executive at Santa Monica, Calif.-based Outcast (formerly Fuelcast), said these ads are more effective because they’re much closer to the point of purchase at a gas station checkout counter.

Together, the two companies will have more than 12,600 high-definition screens in 15 of the top 20 markets. They’re part of a growing market for digital out-of-home advertising displays, such as those made by Ripple and Danoo, which are on display in stores with big lines such as Starbucks.

With a single network, advertisers don’t have to worry about taking too much time to target ads at a fragmented network, said Douglas Woo, executive vice president at AdtekMedia in Irvine, Calif. The ads can be updated wirelessly, either through satellite, cell phone, or Internet-based options. Hence, every location can have a different kind of ad, depending on what is most effective in that market. This is called dynamic targeting, and it can take into account such things as  demographics, geography, and real-time weather and traffic conditions. A new campaign can be activated within a day.

Under the joint venture, Outcast  will run the ad sales and marketing of the network while AdtekMedia will drive product and technology innovation. AdtekMedia has a partnership with display maker Westinghouse Digital.  Advertisers who are already using the companies to pitch ads at the pump include McDonald’s, MasterCard, Audi, Rolaids, Jack in the Box, Best Buy and Honda.

Outcast was founded in 2004. Its investors include  Daktronics, K Investments, Ansera Ventures and principals of the Endeavor Talent Agency.