Researchers at the D.C. region’s Transportation Planning Board have taken a quick look at the pre-Thanksgiving traffic on the major escape routes. They found a pattern that’s pretty consistent with the past few years, but there was a significant exception: Wednesday’s traffic amounted to an all-day rush hour.

What’s consistent is that midafternoon on Thanksgiving eve is a really bad time to launch a getaway, and late Tuesday afternoon is even worse. But in previous years, there’s been a window of opportunity for escaping during the mornings, when traffic still is comparatively light.

What happened to create the change on this Thanksgiving eve is not entirely clear, but the researchers know — as do many of you — that a truck overturned on the Capital Beltway’s outer loop just north of the American Legion Bridge in Maryland 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.

Chart shows average travel speeds on D.C. area freeways during the three days before Thanksgiving 2016. (Transportation Planning Board)

I’ve been looking at the results of the research for several years now, and during this Thanksgiving week, I was impressed to see that the real-time traffic maps and cameras backed up what the historical models showed. Traffic really was awful Tuesday in the afternoon and early evening. As the researchers theorize, the early getaway crowd mingled with the regular homeward-bound commuters, and average traffic speeds plunged. Wednesday afternoon drivers probably didn’t feel like they were getting a break, but the traffic maps and cameras during that time period didn’t look as bad as they had on Tuesday afternoon.

My observation, like the data analysis, is an overall assessment. The big slowdowns of Thanksgiving week’s Tuesday and Wednesday were not consistent along getaway highways. On a long route, drivers might do okay for several miles before coming to a standstill. The Transportation Planning Board researchers are looking at average freeway speeds for the entire region.

As I monitored the traffic impact of the Wednesday morning crash, I thought I noticed something that supported the researchers’ analysis about these extremely busy holiday weeks: The Beltway incident created a ghastly jam in that corridor, but during a normal rush hour, that type of situation could have been much worse. I saw the backups on both loops, and the effect on the northern end of the George Washington Parkway. But traffic still flowed freely across much of the region during the morning rush. The crash’s effect on traffic did not ripple out across numerous other highways the way it might during a normal Wednesday morning.

To me, that backs up the researchers’ theory that by Wednesday many getaway drivers already had gotten where they are going for Thanksgiving and were absent from the morning commute.

That early getaway phenomenon is something worth keeping in mind for your upcoming holiday travels. Watch out for the eve of the eve of a holiday weekend.

Another tip I think we can apply based on the chart for Thanksgiving eve 2016: Do your planning with a sense of history — the travel forecasting is better than ever — but expect the unexpected, give yourself plenty of time and have a Plan B. The Beltway crash that dragged down Wednesday’s traffic speeds illustrates that.