"We've been circling each other like a couple of planes at O'Hare," Amy Rigby sings on "O'Hare," using the nation's busiest airport as a stand-in for the suspended-in-midair states of mind that have become the signature of her songwriting. Now four albums into her second-act career, Rigby has become an unrivaled explicator of the ambiguous condition Jerry Lee Lewis diagnosed as "middle-age crazy." Old enough to know better but too restless to settle down, Rigby lays it all out with the album-opening "Why Do I," lamenting a bad decision, presumably just before making it again. The forceful "Shopping Around" captures a displaced single's complaint, while the self-explanatory, country-inflected pop of "Are We Ever Gonna Have Sex Again?" serves as a reminder of how singles wind up displaced in the first place. Since the release of 2000's The Sugar Tree, Rigby has switched labels and found a new town. Til The Wheels Fall Off features traces of Nashville, alongside '60s garage rock and other influences, but it's mostly of a piece with Rigby's past few efforts, with spare instrumentation and surefooted melodies serving her distinctive, frail vocals. Which, as anyone who's heard them knows, is just fine. If anything, Til The Wheels Come Off offers more highlights than usual. The heartbreaking ballad "Even The Weak Survive" reverses the sentiment of a classic Jerry Butler song, finding a truism in its wordplay. But on the whole, Rigby's music has less to say about the struggle to survive than it does about the impossible search for satisfaction, and the paradox of that impossibility creating such satisfying music.