Get ready for traffic to pick back up as part of “September Shock”


Next week marks the beginning of what area traffic experts have dubbed “September Shock,” the month-long phenomenon in which traffic delays on the region’s roadways bounce back in a big way from their annual summertime lull. This jump is one of the most predictable and pronounced traffic events of the year, according to a recent analysis by researchers at the TPB.

One of the most predictable and pronounced traffic events of the year

The days leading up to two major holidays—Memorial Day and Thanksgiving Day—usually grab headlines for having some of the worst traffic of the year. As holiday travelers spill onto area roadways on their way out of town, traffic picks up and travel speeds drop. That’s what happens every September, too—sort of. This time, it’s commuter traffic picking up after summer comes to a close.

In its recent analysis, the TPB compared average weekday travel times in August to those in September across the entire road network for the past six years. The analysis found that the average regional increase in morning travel delay between the two months ranged from as little as 15% in 2010 to as much as 45% in 2015. That means that an extra hour spent sitting in traffic during your morning commute in August could have grown to 90 minutes or more in September.


For everyday travelers, the extra delay can add up to hours of extra time spent in the car each week. Regionally, added up across all travelers, it amounts to a drain on productivity and economic competitiveness.

The annual jump is so consistent from year to year that the TPB analysts say it’s one of the most predictable and pronounced traffic events the region sees. Their analysis also revealed that September is consistently one of the region’s top two or three worst traffic months.

It’s mainly a morning phenomenon—and here’s why that’s important

The effects of September Shock are not spread evenly throughout the day, and that gives the TPB’s traffic analysts clues about the exact cause of the increases.

According to the analysis, afternoon travel delays see a much less discernable pattern between August and September. Most of the past six years saw some increase in afternoon delays, but they were far smaller than the ones observed during the morning commute. Two years even saw a decrease in congestion.


This difference points to changes in morning travel behaviors as the main culprit of September Shock—and is a caution to area drivers as they plan their morning commutes.

The TPB’s analysts surmise that the morning jump results mainly from changes in what times of day people travel. With schools back in session, parents who must ferry or send their children off to school again at the same time each morning are probably less free to choose earlier or later times to travel to work. People are less likely to take time off from work for summertime sporting events or recreational activities—days when they might have still driven, just not at peak commuting times. And the return of Congress and the normal seasonal rhythms of work all lead to more drivers trying to hit the road at around the same time each morning.

The TPB’s traffic analysts advise commuters to be mindful of when they choose to leave for work in the morning. If they can adjust their departure time earlier or later to avoid the worst back-ups, they should. September might also be a good time to try teleworking to avoid being on the roads altogether.

Related: Detailed Traffic Information Helps Explain “Back-to-School” Jump in Travel Delays

“Terrible Traffic Tuesday” isn’t always so terrible

In recent years, the Tuesday after Labor Day has received the nickname of “Terrible Traffic Tuesday.” With the end of summer and schools and Congress back in session, it seems to many that there is a sudden jump in traffic headaches. In its recent analysis, however, the TPB found that Terrible Traffic Tuesday isn’t always the traffic nightmare it’s made out to be.


More: See how “Terrible Traffic Tuesday” stacked up from 2010 to 2015 in this animation

In 2015, the Tuesday after Labor Day indeed saw worse traffic than any day in August—making it a memorable kick-off to the longer September Shock phenomenon. But there were worse days to come, both that week and the next.

Four of the other five years the TPB studied saw no clear “Terrible Traffic Tuesday” phenomenon. Instead, in those years, the Tuesday after Labor Day blended in with the broader ramp-up of traffic that occurred between August and September.

Here’s where last year’s “September Shock” was felt most

Regional analyses and averages can gloss over the local variations in traffic patterns that give different travelers different individual experiences. Some routes and times see increases in traffic that far exceed the regional average, while others might even see slight improvements.

The animated maps below show what routes and times saw the greatest increases in traffic between August and September last year. Click the map image below to open and play the hour-by-hour animation.


The animated map has been updated since its original posting. The 7:00-8:00 A.M. frame was replaced with the correct map for that hour, and the map legend was changed to note that what is shown on the map are changes in travel times, not travel delay.

Cover photo by thisisbossi on Flickr.


This Car Free Day, try out travel modes that might help you avoid traffic back-ups
Change things up. Go car free or car-lite Sept. 22!