At this perilous moment for the advertising biz, the last thing Gotham would seem to need is another billboard in Times Square.

But the word “billboard” doesn’t capture the 17,000-square-foot, 17-story, three-sided diagonal stripe that will be lit up Thursday outside the city’s newest Walgreens drugstore. The video ad space is billed as the world’s largest — so vast and bright that its creators prefer to call it a “spectacular.”

It spans the south, east and west sides of 1 Times Square, a triangular structure on the north side of 42nd Street between Broadway and Seventh Avenue.

The block has always famously bustled, but Walgreens is helping to extend the “Crossroads of the World” in both directions down 42nd. The glitz now stretches from the green glass Bank of America tower on Sixth Avenue to the Renzo Piano-designed New York Times building on Eighth.

Ads outside Walgreens will generate about 1.6 million impressions a day thanks to heavy pedestrian traffic, according to the store and ABC New Media Sales, the Mouse House division selling the ad space. The east and west sides will be visible for major events such as the Macy’s Thanksgiving Day Parade, the ball drop on New Year’s Eve and daily broadcasts of ABC’s “Good Morning America” and “20/20.”

Early clients will have direct tie-ins to the stock inside Walgreens — brands such as L’Oreal, Kraft, Johnson & Johnson and Colgate are set to take turns. After that, the ultra-high visibility and unusual design of the sign will afford a range of opportunities, including live video or special interactive promos.

Of course, much depends on the economic climate in the months ahead and the willingness of blue-chip advertisers to put coin into one-off campaigns in Gotham.

The novelty is such that inventory is selling well, the ABC division maintains.

“The Times Square space has proven recession-proof to this point,” said Teresa Rix, VP at ABC New Media Sales, which also sells ads in taxis, gas stations and other “out of home” venues. “This space really gives us unique opportunities. If Nestle wants to sell water on a hot day, they have an incredible space to do that right at the point of purchase.”

Arthur Gilmore, brother of Sundance guru Geoff Gilmore and head of design and brand consulting firm the Gilmore Group, said the breakthrough is the sign’s height. “People told me it was crazy to put video up so high,” he said. “But my feeling was that if we’re going to do this, let’s do it big. Let’s make it work from as many sight lines as we can.”

The first structure at 1 Times Square was Times Tower, named for the 1904 newspaper building. Its northern point has long been the site of square-shaped billboards for Coca-Cola and other mainstays, stacked like brightly colored holiday presents.

Over the past decade, the general spiffing up of Times Square has been accompanied by a high-tech boom for advertisers. “Zippers,” the horizontal spools of continuous news updates, have proliferated with the advent of cable news crawls and text messaging.

Giant video walls for Nasdaq stock quotes and ABC fare in front of their respective buildings have also boosted candlepower. Under Peter Gelb, the Metropolitan Opera has taken advantage of the video capability by staging a popular simulcast in Times Square.

A prime example of the square’s evolution is the Bow Tie Building at 44th and Seventh, two blocks up from Walgreens. From 1936 to 2000, it was home to the Criterion Theater, a venerable 1,700-seater that hosted the preems of “My Fair Lady,” “The Ten Commandments” and many more.

Bow Tie Partners, run by descendents of the original Criterion owner, redeveloped the building once AMC and Loews built megaplexes around the corner. (It also owns an exhib chain with 128 screens in four states.)

A flashy Toys R Us, complete with Ferris wheel, is now the Bow Tie’s anchor tenant. The blocklong building also has an LCD screen programmed by MTV (whose base is across the street at Viacom HQ) and a rooftop billboard leased by Liz Claiborne.

Charles B. Moss, a partner in Bow Tie, said any brand able to pay for a bit of placement outside Walgreens is likely to see some automatic benefit just from the address.

“There is no measurable standard for it, but it is enormously helpful to a company’s image,” he said. “And it doesn’t hurt in terms of giving directions. Whenever people are looking for my office, I say, ‘Do you know where the Toys R Us is?’ And instantly, they know.”