Anyone who's tried to find a cab in midtown Manhattan knows the challenge of getting a taxi driver's attention. Well, now taxis are going to be working harder to get your attention, even if they won't be getting any easier to hail.

Clear Channel Communications is among a handful of companies that are trying to make taxi tops a more effective place for marketing. The company is making the familiar wedges that sit atop most hacks more dynamic. Called Smart Tops, the devices sport three LCD screens per side, and feature flash-animation graphics, much like what you'd see in a banner ad on the Web.

Advertisers can pre-program ads or transmit real-time graphics through a cellular data network. At press time, Clear Channel was offering the service only in Boston and New Orleans, but COO Charlie Ditoro says he hopes to be up and running in six cities by the end of this year.

But what's most interesting is the system's potential for customization through global positioning system technology, which allows clients to choose ads targeted to the demographics of a cab's home base, or even for ads to change as the cab moves from one area to another. The GPS feature enables advertisers to cater their pitch to particular sales territories, neighborhood demographics, even local dialect, says Ditoro. A client, for example, might decide to make one pitch to observers on Manhattan's Park Avenue and another to the locals in the Bronx. The system could also change the language of the ad copy to reflect whether the cab was in San Francisco's Japantown or in the predominantly Hispanic Mission district. "With the multicultural boom that's happening in the United States, this is really the way that ads should be going," says Ditoro.

One stumbling block that Clear Channel had to overcome was the issue of safety. "We can't do full-motion video because cities consider it a safety hazard," he says. "New York City asked us to do a safety study to make sure it wasn't a distraction to drivers." An outside research firm found that although the Smart Tops were three to seven times more noticeable than traditional static tops, they didn't present a significant distraction to drivers. The company at first was planning to put a third panel on the back of taxis, but city administrators balked. "They said no way, because it would probably cause a lot of fender benders," says Ditoro.

Program costs run up to $2000 per month, which is about five times the price of a standard taxi-top ad, but that hasn't stopped such clients as Bacardi, the NFL and Morgan Stanley from signing on.