Using newly available, minute-by-minute traffic information from one recent Thanksgiving, researchers at the Transportation Planning Board have identified the busiest times and roadways that Washington area drivers might expect during the upcoming Thanksgiving holiday. Knowing when and where the worst congestion might occur can help both travelers and transportation agencies avoid or manage the worst impacts.

In their analysis, researchers used commercially available speed information provided anonymously by drivers and other travelers via GPS-equipped smartphones and in-vehicle navigation devices. With the data, they calculated average travel speeds on the region's Interstate highways and a few key arterial roadways throughout each of the days of the holiday week, then compared those averages to typical non-holiday days.

The TPB analysis found measurable Thanksgiving-related traffic impacts as early as the Sunday before Thanksgiving and as late as the Monday after the holiday. Impacts varied by day, time, and location, but the worst travel conditions relative to a typical weekday generally occurred in the afternoons of each of the three days leading up to Thanksgiving, starting unusually early compared to a typical non-holiday week.

Of those three days, Tuesday was the worst, at least in terms of the average travel speed on the analyzed roadways. Between 5:00 and 6:00 p.m., the average speed dropped to its lowest point of the entire week, some 20 mph below the "free-flow" speed one would expect without any traffic. According to the analysis, that's about twice as bad as during a typical Tuesday afternoon commute.

Wednesday afternoon was somewhat better, with average speeds dropping about 15 mph below free-flow conditions at their lowest point. But the holiday-related slowdown started much earlier, with speeds beginning to deteriorate as early as 11:00 a.m., and the worst conditions coming between 3:00 and 4:00 p.m.

More: Average Travel Speeds Throughout Each Day of the Holiday Week

The TPB's recent analysis also mapped how travel speeds on individual routes and road segments changed throughout each of the impacted days, and how they compared to typical non-holiday days.

In fullscreen mode, use the control panel below the map to start, stop, and control map animation.

Travelers on many roadways saw travel speeds drop well below the regional average, like on I-66 West outside the Capital Beltway. Speeds on the entire 14-mile segment between the Beltway and US 29 near Centreville slowed to less than 20 mph around 4:00 p.m. on Wednesday afternoon. During a typical Wednesday afternoon commute, such slowdowns are usually confined to the four-mile stretch between the Beltway and Chain Bridge Road.

In Maryland, the Beltway between I-270 and I-95 was one of the worst roadways to travel on Wednesday afternoon, with speeds in both directions slowing to less than 20 mph. During a typical Wednesday afternoon commute, average speeds usually don't drop below about 40 mph.

More: Interactive Maps of Thanksgiving Traffic Impacts

The minute-by-minute speed information used in the recent TPB analysis is just one example of so-called "Big Data" -- large data sets that could not be collected or were too complex to process with previously available technology. The TPB's latest analysis is one way Big Data can aid travelers and help transportation agencies better understand and manage traffic back-ups, including those caused by holidays, special events, and severe weather.

Each quarter, the TPB publishes a report summarizing the latest hour-by-hour trends in congestion on area roadways and highlighting the effects of any special traffic events that occurred during the study period. The quarterly reports, each of which has a unique story to tell about regional travel patterns, are available at www.mwcog.org/congestion, as is an updated "Congestion Dashboard."

Related TPB Weekly Report:

• "'Big Data' Unlock Possibilities for Better Understanding Congestion on Area Roadways" (6/10/14)