Over their 40-plus-year history, the Nitty Gritty Dirt Band have gone from reviving American traditional music to becoming a tradition themselves. Speed of Life captures their continued vitality as an acoustic folk-country unit by keeping things loose, rootsy and rambunctious. The links that unite generations and eras are repeatedly celebrated here — songs like “The Resurrection,” “Good to Be Alive” and the title tune embrace themes of community and endurance with a homespun poetic touch. Playful humor finds release in “Tulsa Sounds Like Trouble to Me” (spurred on by Jimmie Fadden’s chugging harmonica) and “Earthquake” (a Texas swing-style number lit up by high-gloss harmonies). With the nonchalance of seasoned pros, they serve up Stealers Wheel’s “Stuck In the Middle” and Canned Heat’s “Going Up the Country” as likeably ragged back-porch jams. Resident banjo virtuoso John McEuen steps out for a tasty solo track, “Lost In the Pines.” As with the best of the Dirt Band’s work, a shared spirit of friendship pervades these tracks, sprinkling old-fashioned sentimentality with hints of irreverence.