Old taxi joke: The guy's so fat that when he wears a yellow raincoat, people yell, "Taxi!"

New taxi joke: What New Yorkers need, when they're in a cab fighting through Midtown traffic at three miles an hour, is a video screen flashing commercials for Hummers, banks, gyms and auto insurance.

But that's what they may get, if an experiment in rear-seat video viewing wins enthusiasts, advertisers and the all-important approval of the Taxi & Limousine Commission. So far about 400 of the city's 12,187 yellow cabs have been outfitted. The companies that market and install the systems are convinced that the taxi-riding public will love them. And just in case they're wrong, the screens are bulletproof.

"We're experimenting with some new technology," says Matthew Daus, head of the TLC, which is asking for riders' feedback via an online poll. He makes the process sound like an adventure in democracy: "It's great when you can get the public weighing in, helping government make decisions. It's almost like casting a vote." The TLC, whose most significant decision to date has been to mandate a "mute" button for each screen, plans to say yea or nay by fall.

How riders vote may depend on which model of in-taxi TV they encounter; several companies have waded into the nascent market. Hop into a cab that features I Love Taxi TV, which shows a 14-minute video loop courtesy of a DVD player in the trunk, and along with seven minutes of ads you'll see snippets of History Channel segments on olde New York and bits of A&E's "Biography" on well-known locals (now playing: Susan Sarandon and Jerry Seinfeld). Plus, a perky hostess pops up in front of various tourist attractions to tell you how neat the Statue of Liberty and Central Park are. "I call it short-attention-span theater," says spokesman Bruno Lucarelli.

If you hail a taxi with TaxiVu, the screen is smaller, the video loop longer and the perky hostess focuses on particular neighborhoods. The TaxiVu hostess is a ponytailed blonde from Pennsylvania, a would-be actress. I Love Taxi TV's dark-haired variant is actually a contestant on that icky Fox arranged-marriage show, "Married by America."

Then there's the alternative -- and hostessless -- approach, currently represented by Interactive Taxi, in which passengers touch the screen to call up news headlines and weather reports, directories of restaurants by location and cuisine, night life guides, movie times and museum hours. The ads keep running, but they slide over to make room for the listings. Interactive Taxi is also the only system a rider can escape, sort of: a switch replaces the video with a screen saver.

The most interactive prospect, and possibly the most useful, is eTaxiNY, whose high-speed wireless service will allow you to check your flight's status at LaGuardia and pay the cabby with a credit card. No ads in this model -- the company expects to make money on credit card transactions. But eTaxis are not yet on the streets.

As the experiment proceeds, a tastes great/less filling tussle is shaping up between those companies touting their systems' informational benefits and those arguing for entertainment value. Do taxi riders really want to watch TV (though it's not broadcast TV, so you can't change channels or catch "Touched by an Angel")? No way, say the interactive types. "I've already got 500 channels on my TV," sniffs Corey Gottlieb, CEO of Interactive Taxi. "The taxi of the future is not a DVD player." Besides, he notes, even if a feature on Murray Hill is fascinating the first time, with repeated viewings as in-taxi TV metastasizes, the drama may fade.

Oh yeah? Lucarelli of I Love Taxi TV cites a consultant's survey showing that 70 percent of taxi riders are Manhattan residents and most of the rest live in the other boroughs. "They know where they're going when they get in a cab," he argues. "They don't say, 'Hey, cruise around the block for a few minutes while I look for a nice restaurant.' "

Riders may, of course, declare a pox on the entire notion. Driver Mario Nunez, piloting an Interactive Taxi across town recently, said about a quarter of his passengers complained: "They tell me it's annoying."

"It's an extended infomercial," griped one rider who grabbed a video-equipped taxi at Port Authority. "Another advertising intrusion in a place you don't need it."

Ads have already invaded such precincts as physicians' offices and restroom stalls. Now they are morphing into product placement on television entertainment shows, just as they crept into Hollywood films a while back. A North Carolina company is contracting with municipalities in many states to put ads on firetrucks, ambulances and police cars. Maybe New Yorkers will say basta, or at least persuade the TLC to require that taxi videos come with "off" switches.

They've proved resistant, after all, to other in-taxi campaigns. Six years ago, the TLC instituted its talking-taxi program, intended to persuade riders to use seat belts. An entire cast of second-tier celebrities lent their recorded voices to the effort. But whether the appeals came from Joan Rivers, Elmo or Dr. Ruth, two recent studies have shown, New Yorkers paid absolutely no attention. They were no more likely to buckle up than before; in fact, a TLC spokesman admits with chagrin, about 12 percent ("the protest vote") actually avoided using belts because they loathed the recordings. Drivers were not fond of hearing Eartha Kitt growl every time they picked up a fare, either.

But perhaps taxi TV will win more fans, and if it does, other cities are in the crosshairs. Interactive Taxi, for example, has an agreement to start installing systems in Chicago this spring and has its eye on Boston, Miami and other taxi-dependent towns. Washington, this means you.

Taxis in New York are trying out video systems that display ads and information.On Interactive Taxi's video screens, installed in some New York cabs, ads make room for a touchscreen guide to restaurants, news and weather.