WWW- THE moment a phenomenon becomes a full-blown trend may be when it acquires a nickname. The sightings of sexually explicit videos playing on screens in cars have achieved that distinction with at least two terms: dirty driving and drive-by porn.

On-board video has become widely popular with drivers, especially those with children. But while many motorists entertain back-seat passengers with DVD’s like “Willy Wonka and the Chocolate Factory,” a surprising number choose racier fare.

Andre Gainey, for example, was playing a pornographic video on three screens in his Mercedes last February as he drove past a police station in Schenectady, N.Y. Mr. Gainey, 35, was arrested and later convicted of a misdemeanor charge of public display of sexual material and other violations. Last August he was sentenced to three weekends in jail.

Few cars are equipped with as many screens as Mr. Gainey’s. But complaints from people who have spotted explicit images on screens inside cars are growing nearly as fast as on-board DVD sales.

According to Wardsauto.com, an online trade publication, in the first six months of 2004, 401,000 domestic vans, S.U.V.’s and light trucks were produced with DVD video systems compared with 136,000 vehicles in the same class for the entire year of 2002. This year, the Consumer Electronics Association estimates that more than 110,000 systems will be installed after a vehicle is bought.

Unscrupulous installers mount screens within view of the driver’s seat, even though 38 states have restrictions about placement of screens.

Passengers, however, are a different story. They can watch whatever they want, can’t they?

Not in some places. Sightings of adult videos by passengers – including children – in other cars “is a vexing new phenomenon,” said Jan LaRue, chief counsel for Concerned Women for America, a conservative advocacy group in Washington. Ms. LaRue, who wrote an essay titled “Four-Wheel Indecency” for the group’s Web site (cwfa.org), said new laws were necessary as monitors inside cars had gotten bigger (up to 17 inches, measured diagonally) and more visible from outside.

“It’s pretty incredible to have to write laws for people who lack the common sense and civility to think it’s O.K. to play such DVD’s in full view of other people, and especially children,” Ms. LaRue said.

The police and judge in the Schenectady case did not need a special law to prosecute Mr. Gainey. Nonetheless, measures to outlaw sexually explicit videos in cars are growing.

Tennessee was the first state to pass such a law. A vehicular obscenity bill, introduced in 2003 by State Representative W. C. Pleasant and State Senator Mark Norris, both Republicans, became law this summer. Under it, the display of sexually explicit material that can be viewed from other cars is deemed not only morally unacceptable, but also a driving hazard. “To avoid distracting other drivers and reduce the likelihood of accidents arising from lack of concentration,” the bill states, “no obscene and patently offensive material, motion picture, film, movie, videotape, DVD or other pictorial representation shall be exhibited on a television, monitor or other viewing screen visible to other drivers.”

Fines for violating the law range from $2 to $50. “We’re trying to take a measured response to what is becoming a growing problem, as ridiculous as it seems,” said Mr. Norris, who received his first complaint from a constituent last year.

“I wasn’t aware of the extent to which it was becoming a problem,” he said. “But I found a number of folks who were in their cars with children, being held hostage to this kind of display at traffic lights. Even pedestrians reported the experience.”

Since then, several more reports of so-called drive-by porn have come to his attention, Mr. Norris said. One Nashville radio host found herself trapped between two cars, each displaying explicit videos. “She was wedged in,” he said. “She couldn’t avoid it.”

Who decides what is morally unacceptable? That, experts say, depends on where you live. “Laws vary state by state and even by local ordinances,” said Barry Steinhardt, the director of the technology and liberty program at the American Civil Liberties Union in New York.

Over the summer, Louisiana passed a law with decidedly sharper teeth. Motorists convicted of playing pornographic videos on a screen that is visible from outside their vehicle face fines up to $500 or six months in jail or both. Repeat offenders face fines up to $1,000 or a year’s imprisonment or both.

State Representative Mickey James Guillory, a Republican, first heard about the issue in early April, when a constituent complained about being stuck at a stoplight next to a car playing an explicit video on a visible screen.

Another bill, proposed in Oklahoma, died last May in conference committee, but “might be reintroduced next year,” said State Representative Joe Dorman, a Democrat, who sponsored the measure.

“I witnessed it myself,” Mr. Dorman said. “I was at a stoplight and the car next to me had two DVD players. The same clip from a porn movie kept playing over and over.”

The three bills faced little opposition in their respective statehouses, though First Amendment concerns were raised by local newspapers, civil libertarians and the movie industry.

“Law enforcement officials have enough things to worry about,” read an editorial last April in The Leaf-Chronicle in Clarksville, Tenn. “Maybe one day a child will catch a quick and blurry glimpse of a nudie movie being played in someone else’s car. It won’t be a big deal if his parents don’t make it into one.”

In Tennessee, state representatives of the A.C.L.U. and the Motion Picture Association of America successfully pushed to modify that state’s bill from banning obscene “or” offensive material to banning obscene “and” offensive material. The term “offensive,” the association argued, was too vague.

In-car videos also raise the question of safety. According to the Louisiana State Police, at least two fatal accidents reported this year were caused by drivers who were watching pornographic movies.

Regardless of the legal status of video in vehicles, there seems little question that an explicit movie can be more distracting than a sighting of, say, “Shrek 2.”

“It really makes you do a double take,” Mr. Norris said. “It gives the term tailgating a whole new meaning.”