WHEN the negative effects of advertising are being considered, the villain most often discussed turns out to be the television ad. In recognition of the persuasive powers of the medium, tobacco and hard-liquor ads are excluded from the tube. Parents' groups keep a critical watch on the sugary cereals and violent or potentially dangerous toys hawked to children when the cartoons pause for a hard sell.
Meanwhile, advertising that appears on billboards tends to be overlooked. Some 30 years after Lady Bird Johnson launched her ``Beautify America'' campaign, billboards are enjoying renewed popularity, their blank expanses as available to cigarettes as to health plans. They provide equal space for whiskey ads and public-service campaigns against drinking and driving.
Do consumers underestimate the drive-by impact of a billboard ad, registering instant messages on car after car of adults and children? Advertisers don't. About the same time the Joe Camel billboard in Times Square was finally replaced after a five-year run on Broadway, a survey revealed that among six-year-olds, 91.3 percent identified the connection between Old Joe and cigarettes.
In Boston, a billboard company allotting 30 locations for an ad depicting a gun-wielding Eddie Murphy in ``Beverly Hills Cop III'' refused to mount the violent image in high-crime areas - a naive and perhaps patronizing response, but one acknowledging the influence of billboards, now a $1.56 billion business that is growing at an annual rate of 6 percent.
It would be as wrong as it would be futile to suggest banning all billboards. Billboards can be informative. Billboards can be witty. And the issue of freedom of speech comes into play.
But apart from the cluttered scenery or the risk to motorists who eyeball the advertisements at 65 miles an hour, billboards should be taken seriously precisely because they slip past consumers' warning systems, conditioned to defend mostly against electronic spiels.
A billboard salesman has observed that billboards are ``the last unavoidable medium,'' adding: ``People can't shut off a billboard.'' These words - fitting as they do within the maximum eight-word limit customary to billboard art - ought to be inscribed as a public service warning on the tallest billboards across America.