Billboards advertising fast food may inspire people to eat more, and thus gain weight. Advertisers are careful not to place billboards advertising cigarettes or liquor too close to schools, but fat people are everywhere, so there's no way to regulate these ads.
A team of researchers walked around Los Angeles and New Orleans, noting every outdoor advertisement they saw. When they compared the height and weight of over 2,000 residents in these locations, after paying them to answer a telephone survey, they found that for every 10% increase in food ads, the odds of being obese increased by 5%.
However, in the March 10th edition of the New York Times, Christopher Chabris and Daniel Simons make the point that this could be canniness of the part of the advertisers: If food vendors believe obese people are more likely than non-obese people to buy their products, they may be placing more ads in areas where obese people are more likely to live.
If YOU'VE been looking at too many food billboards lately (and succumbing to temptation), then YOU need Anne Strieber's famous diet book, which has been REDUCED $2 so that you can too!