This time next week, many travelers who use the region's freeways may be facing another significant "back-to-school" jump in congestion-related travel delays, following two months -- July and August -- that usually bring a temporary reduction in traffic back-ups in the Washington region.
Last year, the average daily delay per traveler on most of the major limited-access highways in the region dropped 18% at the beginning of summer -- between June and July -- from 25.1 minutes to 20.6 minutes, according to detailed traffic information collected by the Transportation Planning Board.
Average daily delay per traveler remained lower, around 20.4 minutes, in August, but jumped back up 26% in September to 25.8 minutes.
Travel delays, as measured by the TPB, can result from a variety of factors, including heavy traffic volume, traffic incidents (like accidents and stalled vehicles), construction, or adverse weather conditions.
The TPB measures delay by comparing actual travel times of vehicles on the region's freeways to the travel times that would be expected under free-flow conditions. The actual travel times are calculated using GPS-based speed information gathered by a private company from travelers who have agreed to share the information from their smartphones or in-vehicle navigation devices.
One common explanation for the "back-to-school" jump in delay between August and September is that the overall amount of driving is down during the summer months because more people are on vacation. When summer comes to a close and schools are back in session, the logic goes, the amount of freeway driving goes back up.
But the detailed information on travel delay, which the TPB started collecting in 2008, shows that the September jump in delay is caused not by changes in the overall amount of driving, but by changes in what times of day people travel, especially during the morning commute.
The 26% increase in average daily delay per traveler that occurred between August and September of last year happened at the same time that total driving -- measured in vehicle-miles traveled, or VMT -- increased by less than 1%.
In the previous two years, travel delay increased -- by 9% in 2009, and by 14% in 2010 -- while average daily VMT actually dropped by 2.5% from August to September in both years.
By looking at how travel delay varies over a 24-hour period representing a typical day in August and a typical day in September, traffic engineers and transportation planners can see shifts in travel patterns that contribute to the September jump in delay.
During the most congested part of the morning peak period on a typical weekday in August 2011, actual travel times were found to average 36% longer than the amount of time it should take to travel the same distance under free-flowing traffic conditions. In September, that number jumped to 60%.
Another change came in the duration of the morning peak period, which started earlier and ended later in September than in August.
Together, these shifts show that in September more people were using the region's freeways at the same times during the morning peak period than were in August, leading to more congestion and greater delay lasting for a longer period of time. Increases in afternoon travel times also occurred, but to a lesser degree.
Numerous factors could be responsible for the sharp increase in morning travel delays. 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.
The arrival of September also means that people who enjoy the warmer, longer days of summer by making trips to outdoor sporting or recreational events, barbeques, or to museums, for example -- trips which contribute to total VMT but typically occur mid-day or during extended daylight hours, outside of normal peak commute times -- are less likely to do so.
Other factors like the return of Congress and its staff and the seasonal rhythms of work schedules also lead to more drivers trying to hit the road at around the same time each morning in September as compared to August, contributing further to the jump in overall delay.
The relatively new, more detailed traffic information being collected by the TPB allows traffic engineers and transportation planners to develop a better understanding of complex travel patterns in the Washington region. When it comes to the "back-to-school" jump in travel delay that has occurred in Semptember of each of the last three years, the detailed traffic information shows the collective impact that changes in the time of day that people choose to travel have on freeway congestion and travel delay.