Lucinda Williams mixed fast and slow jams Saturday at Merriweather Post Pavilion. (Kyle Gustafson/For The Washington Post)

What if you put on a terrific concert and no one showed up? Drive-By Truckers and Lucinda Williams are both critically admired, road-tested stalwarts of the over-40 Americana scene, but their co-headlining bill at Merriweather Post Pavilion on Saturday night had such a slim turnout that it sometimes felt like a dress rehearsal. Not that either sounded like they needed the practice.

Undaunted by the empty seats, each offered a tight, energetic, 70-minute set — half of what they play at the 9:30 Club, where they’re steady draws. But even gorgeous weather couldn’t lure fans to the much larger outdoor venue in substantial numbers.

Maybe the problem was that the show had been promoted as the Route 29 Revue, which sounds like one of Maryland Gov. Martin O’Malley’s old bands. (Local singer-songwriter Justin Jones opened the show.)

The Truckers — a hard-charging, Georgia-based six-piece whose songs chronicle America’s white underclass in sympathetic detail ­­— were up first. Patterson Hood and Mike Cooley, the band’s frontmen, each sang a half-dozen song, taking turns. Family history is a deep creative vein for Hood. He introduced “Box of Spiders” with a yarn about his 10-year-old grandfather chauffeuring his great-grandfather around to bootleggers and whorehouses in a Buick during Prohibition. The story would’ve played like a Tom Waits parody coming from anyone else. Cooley’s selections were pithier: The rollicking “Get Downtown” had a sharp-tongued housewife haranguing her husband to get a job, while “Shut Up and Get on the Plane” closed the Truckers’ set in fiery fashion.

 Williams, dressed all in black, fronted a four-piece band that (she said) restored guitarist Doug Pettibone to the fold after a long absence.

“We don’t want to cause a riot or nothin’, but you’re welcome to stand up and shake your booties,” she drawled after the feral slow-burner “Essence,” perhaps having seen the staffer telling patrons in the front rows to sit. Perversely, she followed the invitation with the slowest jam of her 10-song set, the smoky “Born to Be Loved.”

After a cover of Gregg Allman’s “It’s Not My Cross to Bear,” it was up-tempo on to the finish line, a rave-up of “Get Right With God.” But a hoped-for collaboration between the headliners — like the bulk of the hoped-for audience — never came.

Klimek is a freelance writer.