Perhaps it was a lack of good offers—heaven knows his earlier movies have been pretty terrible—that induced Dax Shepard to undertake what amounts to a vanity project that he wrote, stars in and co-directed. “Hit and Run” isn’t an embarrassment—it’s actually notches above a lot of today’s studio comedies—and in some respects might even be called enlightened in an era when gross, anti-PC humor not only runs rampant in Hollywood comedies but goes unchallenged. (It’s challenged here, but only after the joke’s been made.) Still, it seems a throwback to the days of drive-in movies, with Shepard as the sort of devil-may-care character racing around in a muscle car that Burt Reynolds might have played in his “Smokey and the Bandit,” “Cannonball Run” phase. Whether 2012 indoor audiences will take to it quite so readily is another matter.

Shepard plays Charlie Bronson—the name itself a telling bit of nostalgia—a scruffy, good-natured layabout living in a small California town with girlfriend Annie Bean (Kristen Bell), a psych teacher at the local community college who specializes in peaceful conflict resolution. When Annie’s chairperson Debbie Kreager (Kristen Chenoweth) virtually blackmails her into interviewing for her dream job—heading up a program in precisely her field at UCLA—the helpful push proves the catalyst for a eventful road trip to the big city.

Charlie’s initially irked by the idea of Annie’s departure, insisting he can’t return to L.A. for unexplained reasons. But being an essentially good guy, he dusts off the souped-up 1967 Lincoln Continental he’s been keeping in the garage and offers to drive here there. It’s soon revealed that he’s actually a fellow named Yul Perrkins, an erstwhile getaway driver who went into witness protection after testifying against a gang of bank robbers led by Alex Dmitri (Bradley Cooper), who’s now informed by Perrkins’ whereabouts by Annie’s obnoxious ex Gil (Michael Rosenbaum), who takes off after them. Also involved in what becomes a chaotic chase are Randy Anderson (Tom Arnold), the bumbling federal marshal who’s supposed to keep Charlie safe, and Terry (Jess Rowland), Gil’s cop brother.

Shepard has written for himself a loopy, grungy character very different from the sort of abrasive, pushy ones he’s often played in the past, and it suits him, and his banter with perky Bell’s Annie is ingratiating, as it should be since they’re an off-screen couple. Especially noteworthy are the instances when he offhandedly makes some offensive remark and she upbraids him for it, at which point he’ll apologize. (That’s called having your cake and eating it too—something that can also be said for a gag involving a gay-dating app that two characters use.) Chenoweth makes the most of her scenes as Annie’s no-nonsense, straight talking boss, and Beau Bridges has an amusing last-act cameo as Charlie/Yul’s tough-love dad. (A cameo by David Koechner, however, is a bust, and another by Jason Bateman, doesn’t fare well either, seeming phoned-in.)

Otherwise, though, “Hit and Run” spins its wheels and goes off track a lot. Arnold’s klutziness is way overdone, yokel slapstick that should have been toned down a few decibels. The same goes for Rosenbaum’s Gil, a real career decline for the erstwhile Lex Luthor and one he plays on a single frantic note. But the worst offender is certainly Cooper. Apparently trying to shed his customary blandness, he spouts dreadlocks and delivers his lines with a Satanic smirk throughout, like a high-school kid acting tough. It’s not so much a performance as a weird cadenza that goes on much too long.

“Hit and Run” also revs up too often, and too harshly. The car-chase sequences are fine, and actually act as a nice counterpoint to the conversational stuff between Shepard and Bell. But Gil’s gold club attack on Charlie is unpleasant rather than funny, and Alex’s assault on a shopper he thinks mistreats his do is even more so. (The only one of these bits that works is Bridges’.) And what’s with the totally extraneous scenes of a bunch of old debauchees in a motel room? Will audiences really find the sight of exposed, wrinkled flesh all that amusing?

Photographed by Bradley Stonesifer, the movie is technically fairly polished, though some of the on-road action scenes have a slightly ragged look, and the music by Robert Mervak and Julian Wass provides a jaunty backdrop.

“Hit and Run” doesn’t crash and burn, but it doesn’t exactly sail across the finish line a winner, either.