“Green Book” is a tonic for our toxic times,  a “Driving Miss Daisy” road dramedy through America’s increasingly retrograde racial history.

It never shies away from an easy laugh and may have all the makings of holiday cinema season comfort food. But the terrific lead performances and potent and timely social justice message make this film from one of the guys who gave us “Dumb and Dumber” the movie America needs to see — right now.

It’s based on a true story. In 1962, a classically-trained piano virtuoso is about to tour the racist American Midwest, upper South and Deep South. He needs a driver, one with particular skills and mass.

Because Dr. Donald Shirley, PhD, is black, African American — “colored.” And whatever level of tolerance he can expect from the performing arts halls, country clubs and Antebellum mansion “house concerts” on his itinerary, there are miles and miles of potential trouble in between.

He interviews potential drivers in his New York home. It’s an art-packaged apartment upstairs from Carnegie Hall. And Shirley (regal Oscar winner Mahershala Ali), multi-lingual, with impeccable manners, accent and carriage, sees fit to carry out these interviews from a throne.

Literally.

We’ve already met the burly, two-fisted Tony Vallelonga (Viggo Mortensen). At the Copacabana, where he works as a jacketed waiter/bouncer, and among “the boys,” he’s “Tony Lip.”

He’s 40something, a family man with a comically voracious appetite, big enough to have been a bully, brutish enough to still have those tendencies. He’s always looking for an angle, “misplacing” the right mobster’s hat to create a scene and “come to the rescue” and thus inviting an obligation from the boss. But the Copa is closing down for renovations, and he can only score so much spending money by winning hot dog eating contests.

Also, he’s a racist. His wife gives drinks of water to two black plumbers working in their crowded flat and Tony tosses the glasses in the trash afterwards.

He and his relatives switch to Italian when they’re talking about non-Italians, and the slurs come quick and easy. Interviewed by his social superior and informed he’ll need to launder shirts and serve as valet on Shirley’s tour as well, he suggests a previous candidate, “the little (Asian slur)” might be better suited for the gig.

But he’s got the right throw-weight for the job and they come to terms. Just keep this “Green Book,” a “guide” for “The Negro Motorist” handy. Shirley, who fronts a trio with two white musicians backing him, will have to stay in “colored” motels and hotels and eat in “colored” dining establishments from Kentucky to N.C., Atlanta to their final stop, right before Christmas in that oasis of tolerance, Birmingham, Alabama.

Tony is crude, boorish and badly misinformed about much in this world — including black people. He smokes constantly and offends almost as often. Forcing Kentucky Fried Chicken — “ALL you people love fried chicken!” — on Shirley, pricking his new boss’s pretentiousness at every turn, Tony is not the sort of vulgarian you introduce in the circles Shirley travels in. Might a makeover be in order?

“I can help you. You can do BETTER Mr. Vallalonga.”

Tony ain’t having it. And Shirley’s sarcastic comebacks fly right over his head most of the time.

Then he hears the man play, “Like LIBERACE, but BETTER — a genius, I think,” he writes to his wife. “VIURTUOSO,” he says, echoing the boss’s introduction to a fellow driver. “That’s Italian. It means he’s really good!”

But those letters home to Dolores (Jennifer Cardellini, warm and wonderful) start to soften up Tony Lip. Shirley dictates literate, evocative paeans to true love and devotion. Tony goes to school.

And in seeing Shirley’s first class behavior when repeatedly faced with second class treatment is a lesson, too — in “maintaining your dignity.” Not that this mug is swallowing that. Not right away.

Director Peter Farrelly may seem outside of his comfort zone, here. But he’s had a sentimental streak, and he’s done road comedies (“Kingpin”) before. And while he never ever leaves an obvious joke untold, while we just KNOW these two are going to wind up A) in jail and B) in a “juke joint,” and we know precisely what will go down in each locale, he stays out of the way of his dazzling leads.

The little confessional scenes between the two men in the car have a familiar snap to them as Tony learns he’s underestimating his prissy employer and that employer gets an earful about how “black” he isn’t.

“That’s Little Richard! You never heard of Chubby Checker? Aretha?”

Ali gives Shirley the fey demeanor of the pampered artist with barely a hint of the hurt and rage bubbling beneath. Mortensen makes Tony so lived-in that you forget how much of his career he spent on horseback, herding hobbits.

The film tilts towards patronizing at times. White viewers and black ones are almost sure to react to Shirley’s giving in on the fried chicken thing differently Don’t overthink the fact that a guy Tony’s age and generation and ethnicity would be more of a Louis Prima fan, or that the wonderful Mortensen is a little long in the tooth to still be “the muscle.” Accept the jokes you see coming and delight in the ones you don’t.

“Green Book” (opening Nov. 21) invites you to come along for the ride, the comfort food, the socio-political sparks and the laughs.

MPAA Rating: PG-13 for thematic content, language including racial epithets, smoking, some violence and suggestive material

Cast: Viggo Mortensen, Mahershala Ali, Linda Cardellini

Credits:Directed by Peter Farrelly, script by Nick Vallelonga, Brian Hayes Currie and Peter Farrelly. A Universal release.

Running time: 2:10