Once upon a time drive-in movie theaters were everywhere on the American landscape. Not anymore.

Writer-director April Wright’s “Going Attractions: The Definitive Story of the American Drive-In Movie” chronicles the humble launch of the drive-in — a sheet hung between two trees — through its boom-time in the late 1940s and early ’50s and up to the present day.

The film is both a history of an American invention and an affectionate homage to a business that survived decades of sociological and technological change.

Miraculously, drive-ins still exist, although they’ve been reduced from 5,000 locations in 1958 to just hundreds of venues today.

Wright tells the drive-in story through reels of kitschy film footage, through additional, context-setting footage from the 1930s through the ’70s and through interviews with those who dare to operate drive-ins in the 21st century.

For the latter folks, the profession can be a labor of love and act of historical preservation.

From a sociological angle, the University of California’s Dr. Karen Sternheimer offers her expertise. An expert of another kind, Roger Corman — the indie film legend responsible for “Little Shop of Horrors,” “Boxcar Bertha,” “Wild Angels” and hundreds more — also weaves in and out of the narrative.

Wright visited more than 400 drive-ins, some still operating. Former drive-ins are, in many cases, the sites of Wal-Mart stores.

Only two Louisiana sites are on her list — a pair of former Showtown drive-ins in north Louisiana. Shreveport’s Showtown theater is now a storage facility. Monroe’s Showtown is abandoned, its screen covered in vines, she reports on the film’s website.

Depending upon one’s connection to drive-ins, the film’s images of dozens of abandoned theaters can be sad. Drive-ins, after all, were places where families went for a night of relatively cheap entertainment. Before liability became an issue, many of them had playgrounds.

“It was big event to go to the drive-in,” explains Barb Frankhouser, co-owner of Pennsylvania’s Super 322 Drive-In. “It was an important night.”

Of course, outdoor theaters were also dating destinations that afforded a degree of privacy that hard-top theaters did not provide.

Wright’s vintage footage includes excerpts from movies played at drive-ins. Those films tended to be lowly B-movies, monster, teen and exploitation flicks that respectable indoor theaters didn’t show.

Wright succumbs to the temptation many writers and filmmakers face. She loves her subject too much to know when to quit. Casual viewers get more detail than they need. But for those who love drive-ins, who have nostalgic affection for these places that, for most communities, are gone, the exhaustive “Going Attractions” must be the ultimate documentary.