Travel delays on freeways in the Washington region dropped by 18% between June and July of last year thanks in largest part to changes in daily travel patterns that accompany the arrival of summer.
According to traffic information collected by the Transportation Planning Board, the average daily delay per traveler on most of the major limited-access highways in the region dropped from 25.1 minutes in June 2011 to 20.6 minutes in July 2011.
Delay is caused by traffic back-ups, which can result from heavy traffic volume, traffic incidents (like accidents and stalled vehicles), construction, or adverse weather conditions. The TPB calculates total travel delay by comparing observed travel times to what would be expected under free-flow conditions. Per-person delays are an average; some travelers experience more delay while others experience less.
At the same time that delays dropped by 18% between June 2011 and July 2011, total driving -- measured in vehicle-miles traveled, or VMT -- decreased only 0.6%. Average daily VMT fell from 34.05 million in June to 33.85 million in July.
Because the decline in total driving is so slight, the much more significant decrease in delay between June and July is due mainly to shifts in travel patterns that accompany the arrival of summer, when schools are out, days are longer, and the weather is warmer.
With most schools out of session in July and August, parents in the region who aren't busy ferrying or sending their children off to school at the same time each morning are freer to choose other times to travel to and from work. Some might choose to leave for work earlier while others might choose to leave later. With school out, summer is also a popular vacation time, which means that on any given weekday there are fewer people than normal commuting to and from work.
The longer, warmer days of summer also bring greater opportunity for people to make trips to outdoor sporting and recreational events, barbeques, museums, or other summertime activities. These trips add to total VMT, but they typically occur mid-day or in the extended daylight hours of the evening rather than during peak morning and late-afternoon travel periods.
Together, the more even distribution of trips throughout the day and the slight overall decrease in total miles driven that occur during the summer months result in fewer and shorter traffic back-ups and less total delay.
The summertime trend of reduced travel delay comes to an abrupt end in September, however, when school is back in session and days again start growing shorter and cooler. In 2011, average daily delay per traveler shot back up nearly 27% between August and September, from 20.4 minutes in August to 25.8 minutes in September. At the same time, VMT bounced back only slightly, from 32.71 million vehicle-miles to 33.00 million vehicle-miles, a 0.9% increase.
The observations of travel delays on the region's freeways are part of the TPB's ongoing Congestion Management Process, which is an effort to monitor traffic conditions and identify appropriate response strategies to reduce congestion on the region's roadways and to improve operational efficiency of the road network by responding to incidents and providing more real-time information on travel conditions.
Amidst the drop-off in congestion that occurs in the Washington region each summer, travelers right now are enjoying far less travel delay than they do in the late spring or when the region's schools are back in session in early September. As the TPB's data show, these changing conditions are due not to a reduction in total driving, which has historically been very slight, but to the redistribution of travel away from the peak commute periods to a more even distribution throughout the day.