Etiquette doyenne Emily Post wrote the book “Motor Manners” in 1949. (Ogilvy Public Relations)

The winter holidays are just around the corner, a joyful season of hearth and home — the most wonderful time of the year! Unless “hearth and home” requires a road trip. And the trip involves miserable traffic, and the traffic is on one of those highways (*cough* the New Jersey Turnpike) where the exits are inexplicably sub-categorized — “8,” “8A,” “8A(ii)(d)” — obliterating even the faintest illusion of progress, and suddenly everyone in the car is identifying strongly with the Grinch.

The stress of holiday travel can make even the most courteous, Zenlike, yoga-practicing driver come slightly unhinged. This was true in 1949, when cultural etiquette doyenne Emily Post first wrote “Motor Manners,” a charmingly no-nonsense guide to proper behavior for the “ladies and gentlemen of the highway.” Back then, there were about 40 million cars on the nation’s roads.

Now there are 240 million, so a refresheron Emily’s guidelines seemed like a good idea. Her great-great-grandson, Daniel Post Senning, author and spokesman at the Emily Post Institute, worked with the Ford Motor Co. to release some modern holiday-driving etiquette tips for the roughly 91 million people AAA estimates will hit the road this month.

When he decided to tackle the project, Senning says, his Aunt Peggy (a.k.a. Peggy Post, Emily Post’s great-granddaughter-in-law and a director of the Emily Post Institute) dug her copy of “Motor Manners” out of her archive and sent it to him for inspiration. The booklet is filled with cheeky instructions, but it opens with a stern caution about the potential life-and-death consequences of driving behavior: “Bad motor manners can all too often result in MURDER.” (It’s not every etiquette guide that warns of possible homicide in the first paragraph.)

As Senning prepared a new set of rules for the 21st century, he was transported back to a kinder, gentler time, a time when honking was never intended to “threaten or scold” and when proper protocol was to assist anyone who signaled for help on the road. (“That’s not advice that we’re as comfortable giving today,” Senning notes dryly.)

But as he read over Post’s tips, “the big themes that jumped out at me were so striking in their similarities,” Senning says. “Right in her introduction, Emily talks about the car itself as new technology and the stresses that it puts on human relationships and us as a society.”

Which brings us to the top tips:

Don’t be a tech boor

Despite the vast array of sophisticated technology available to drivers and passengers — the voice-controlled phones, the endless selection of satellite stations, the built-in DVD players — Senning channels his great-great-grandmother and encourages people to, you know, actually talk to each other.

“Some of the rudest behavior you encounter in the car is someone who gets into the car, then disappears into their cellphone — the passenger who isn’t really present with you,” he says. (On the flip side, we’ve all been on those road trips with the talkative weirdos you wish would disappear into their phones.)

It’s not that you have to ignore your phone entirely, but you should use it appropriately.

“Identify whether what you’re doing is part of a shared experience for the people in the car or whether it’s taking your attention away from them,” Senning says.

So thumbs-up if you’re looking up the next rest stop or downloading a podcast. “But if you pick up the phone and call your boyfriend and spend 45 minutes talking about how you miss each other so much, while you’re driving home to see your family with your sister . . .” well, “that’s not a good use of technology,” he finishes politely. (Translation: You’re being a jerk.)

Be a good host or guest

As Post put it: “Well-bred people, whether drivers or passengers, are just as considerate of each other as are hosts and guests in a drawing room.”

Okay, nobody calls it a “drawing room” anymore. But you and your passengers should still consider the car an extension of your home, Senning says.

If you’re a driver, “take your passengers on a little tour of your car, especially the technology,” he says. “If there’s a port where they can charge their phones, let them know.”

If you’re a passenger, “offer to help before you’re asked — to fill a gas tank, manage a navigation system, bring snacks.”

Post insisted that drivers open car doors for their passengers; in our new keyless-entry world, this means unlocking the doors before your passenger reaches for the handle, Senning says.

And finally, remember that compromise is everyone’s best friend.

Most people “think that the driver gets the final say about things like the radio, the temperature, whether the windows are up or down,” Senning says, citing an institute study. “But the same study shows that most people think the driver should show some consideration to the passengers when making those decisions.”

Be merciful, drivers: Don’t give your passengers heat stroke and keep in mind that other people may not share your peculiar fondness for the Barking Dogs’ version of “Jingle Bells.”

Don’t wing it — be road-ready

Nowadays, we’re spoiled by a multitude of navigation tools — Google Maps and Waze, to name a few — and they’re wonderful. But not infallible.

“One of the biggest mistakes people make is depending on those systems, and when they fail, you’re suddenly in deep trouble,” Senning says. This “can prompt rude behavior.” (Yelling, cursing, compulsive stress-snacking, the usual.)

Keep it old-school: Bring an actual map — you know, the unwieldy paper kind that refuses to fold back into its original shape. Or at least check one out in advance.

“Know where you’re going and how to get there,” Senning says. “And ask your passenger if you’re going to need help spotting an exit sign.”

It’s also good to think ahead about keeping everyone entertained as the miles stretch before you. “An ounce of preparation is worth a pound of cure,” he says. “Think about content that everyone in the car will appreciate — podcasts, audiobooks, playlists — and download it ahead of time in case you wind up somewhere where streaming isn’t available.”

(Fortunately, the creators of “This American Life” released the second season of “Serial” just in time — a true Christmas gift.)

Senning is sure that his great-great-grandmother would approve of the new list. “There’s so much that’s stayed the same in terms of common sense, safety and courtesy,” he says. Not to mention human nature.

“Behind the wheel of a car, men and women both whose behavior in all other circumstances is beyond reproach, become suddenly transformed into bad mannered autocrats,” Emily Post wrote. “This inconsistency is certainly one of the great unsolved mysteries of our time.”

Sixty-six years later, we still haven’t solved that one.