Drivers endure the slow traffic on I-66 through Dunn Loring on the day before Thanksgiving. (Alex Wong/Getty Images)

By now, you probably have shaken off most of the bitter memories of the Thanksgiving getaway traffic. But don’t forget the things that will help you with your end-of-December adventures.

Let’s look at the similarities and the differences between the November trips and those in December. I’ll start with this question, which arrived during one of my recent online chats.

Q: While home for Thanksgiving, I somehow managed to promise to be in New York City over Christmas. Yikes. I usually try to avoid traveling on the holidays unless it is mandatory. I have two vacation days that I can use. If I leave on Saturday (Christmas Eve) will that be a horror show, or probably a bit after everyone else is on the road?

I would prefer to drive when it is light out, and I can’t imagine that Friday night (Dec. 23) would be anything other than just as horrible, if not worse. I probably would drive to a friend’s house on Long Island, take the train into Manhattan and then visit with the Long Island friends Tuesday before leaving to drive back here Wednesday.

DG: Take heart. If you did the Thanksgiving getaway, you’ve endured the worst holiday traffic of the year. There will be difficult travel times around Christmas and New Year’s, but for most drivers, they won’t match the depths of despair experienced over Thanksgiving.

Christmas travelers have more flexibility to spread out their trips, thanks to work and school schedules. Yet travelers working out their December plans can still benefit from the Thanksgiving traffic history — up to a point.

First, the experience of being delayed by highway congestion will be fresh in their minds. Even people who travel regularly at Thanksgiving can spend a year forgetting how bad the delays can be. That’s why my top tip in our Thanksgiving getaway guide — the most widely applicable suggestion — was “You’re probably not leaving enough time for the trip.”

The holiday travel trends identified by the staff at the Washington region’s Transportation Planning Board have proven useful to many drivers.

The data available for the past few years shows that the pre-Thanksgiving traffic on Wednesday afternoon is bad, but Tuesday afternoon is worse. That’s because the early-getaway crowd mixes with homeward-bound commuters. By Wednesday afternoon, a lot of the people driving out of the region already have reached their destination.

Most years, Tuesday and Wednesday mornings are good times to launch a trip. But the planning board staff did some quick research and found that this year’s Thanksgiving eve was different. Slow travel speeds prevailed all day, although they still were at their worst in the afternoon.

What happened to change the usual scenario on Thanksgiving eve is not entirely clear, but the researchers know — and many of you also know — that a truck overturned on the Capital Beltway’s outer loop just north of the American Legion Bridge during the morning rush. The backups on both loops lasted several hours, and that could have been enough to bring down the average traffic speeds across the region.

So you want to keep several things in mind as you develop your plans for the December holidays and beyond. One is to be wary of the weekday afternoons approaching the long holiday weekends, when many commuters will be mingling with the early-getaway traffic. The days before Christmas shouldn’t be as bad as pre-Thanksgiving, but the phenomenon will still be visible.

Also keep in mind that even though travel forecasting is better than ever, you should expect the unexpected, give yourself plenty of extra time and have a Plan B. The unexpectedly severe Beltway crash that closed all the outer loop lanes Nov. 23 illustrates this.

Our New York-bound traveler also should keep this in mind about a Saturday Christmas Eve getaway. The Black Friday shopping phenomenon showed up very strongly in historical traffic data supplied by the INRIX traffic information service. Traffic around suburban malls was really slow.

Remember that behavior pattern if your trips take you near major shopping areas on Christmas Eve, which is likely to be a big day for the last-minute shoppers. They’ll be on some of our local highways, along with the people who had no flexibility in their work schedules and had to wait until Saturday to launch their Christmas trips.

The Christmas Day traffic is likely to be even better than the Thanksgiving Day traffic. People can wait until the holiday morning before driving to a Thanksgiving dinner, but many people want to be at their destination by Christmas morning.

The federal holiday for Christmas is Monday, Dec. 26. So many people will stage their return drives for that afternoon and evening. A trip Tuesday — or Wednesday, as the Long Island-bound traveler plans — should be a happy return.