"Time for a change," the billboard at 1st Street and Hennepin Avenue said. Diaper Dan was, to put it gently, full of it. And this was supposed to make you eager to switch to Ohleen milk, because … changing brands was like changing diapers?

Needless to say, Ohleen Dairy isn't around anymore.

This might not be the best example of the power of the once mighty billboard, but it's a memorable one.

For decades, we've been told that billboards are a form of visual pollution, an imposition on the landscape. If you wanted to show a blighted neighborhood, a peeling billboard selling liquor was the standard image.

But that assessment isn't quite fair. Billboards once were the largest form of public art that the citizenry could see.

People have pasted words and pictures on walls since Roman times. In fact, there are advertisements on the excavated walls of Pompeii. Handbills and wall signs were common for hundreds of years, but it took American technology — and the ballyhoo spirit — to make the big, colorful, temporary sign.

In the 1800s, advance men would put up signs on brick walls when the circus was coming to town, and of course left them to fade and tatter as a memory of more exciting days. The most influential development, however, was the introduction in 1900 of a national standard for billboard dimensions. Having a uniform size for billboards allowed companies to mass-produce signs.

You might think that was the beginning of mass blight, now that advertisers could blanket the country, blaring encouragements for smokes and soap.

It may not have been considered that way at the time. In our ad-saturated world, it's difficult to imagine the impact of a new billboard on the edge of town, painted by the greatest commercial artists of the day. Cream of Wheat had decided your town was important enough to know about, well, Cream of Wheat.

In the next few years, your town would get a movie theater, an embassy for the rest of the world. Then radio threaded all the towns together, and the nation could listen to a single song at the same time. Culture was coast-to-coast by the 1930s. And the billboard was the beginning.

But was it art? The early billboards were.

By the 1920s, billboards featured illustrations by some of the nation's best commercial artists, like J.C. Leyendecker. He may have lent his brush to cigarettes and Arrow shirts, but his work was the equal of his fine-arts peers. The main reason it's not hanging in museums is because of the subject matter.

Around the end of the 1950s, photography changed the billboard, for the worse, perhaps. The elegant illustrations were replaced by pictures whose realism leached away the romanticism of the images. Design became stark and modern, slogans were short and loud. The billboard became an eyesore in the eyes of those who wanted a driving experience unsullied by commercial appeal.

In 1965, the Highway Beautification Act, pom-pommed by Lady Bird Johnson, limited the location of billboards, banning the boards along interstate freeways.

Billboard companies were accused of targeting distressed communities with cigarette and liquor ads. In 1990, the Outdoor Advertising Association of America agreed to "limit" the location of signs advertising stuff that minors shouldn't have, which is why there are no whiskey billboards next to schools. In 1999, the association dropped tobacco ads, which is why kids graduating from high school this year have never seen Joe Camel grinning from a sign in the sky.

The future, of course, is digital. High-resolution LED billboards can change in a second, swapping one ad for another in an unending parade of advertisements. They're crisp and kinetic, eye-catching spectaculars that make you want to slow down and see what's next.

And that's the problem: Digital billboards move, which makes them more like TV.

The great billboards of Minneapolis' past weren't just ads. They weren't just paintings.

Before Diaper Dan made his pitch to switch milk brands, the corner had a huge Dr Pepper sign with a clock reminding you to knock one back at 10, 2 and 4. The famed Golden Guernsey signs for the Ewald Bros. Dairy had two enormous bovine heads in 3-D, sticking out of the sign, regarding you with contented indifference.

At the corner of 7th and Hennepin, above the Shinder's Bookstore on shabby Block E, there was a two-story billboard for a Vegas getaway: a big, happy blonde on vacation, sculpted in relief, promising the winter-weary citizen sun, sin, slots and Jimmy Durante in the showroom.

Today the picture would be replaced every minute with an ad for beer, but back then the happy blonde stood there, night and day, rain and shine, for months, inviting you to paradise.

And then one day she was gone. One day they scrape the paper off, and you're hard-pressed to remember what was there the day before. The canvas is empty, waiting for the next tableau.

Watch this space, it said, and you did.