Traffic gathers on Leesburg Pike near the Silver Line’s Spring Hill station as drivers head into Tysons Corner. (Jahi Chikwendiu/The Washington Post)

This is the season when commuters park on a lower level at the Metro station garage and get through downtown intersections on just one light cycle. They spend less time stopped on the Capital Beltway ramps at Chain Bridge Road and Georgia Avenue. They get to see their outer suburban homes in the daylight, coming and going.

The summertime perks disappear with such notorious reliability that we have named the phenomenon “Terrible Tuesday,” for the morning after Labor Day, and “September Shock” for the cumulative effect of vacations’ end.

“Beginning the day after Labor Day, our normal commute returns to its abject state of abnormality for the remaining 78 workdays left in the year,” said John Townsend of AAA Mid-Atlantic.

Ben Hampton and Wenjing Pu, part of the transportation staff at the Metropolitan Washington Council of Governments, crunch numbers on the seasonal variations. Those numbers support our alliterative terminology, but it’s not true that everybody leaves for the summer while a few of us stick around to keep the lights on in the capital.

The data analyzed by the staff at COG’s Transportation Planning Board show that driving declines only slightly in the summer. A study of the seasonal shifts in 2011 revealed what turns out to be a typical pattern.

Vehicle miles traveled on the region’s highways declined less than 1 percent from June to July, yet average daily delays dropped by 18 percent.

People drive differently. Long summer days and more flexible schedules spread out some of the traffic.

Drivers make fewer of those trips between home and work at 8 a.m. and 5 p.m., and they aren’t locked into a pattern of dropping off, then picking up, children at schools. Instead, they might be taking the kids to summer camps or driving to Nationals Park, or visiting Smithsonian museums, Hampton and Pu said.

The travel pattern changes that begin in late August and continue into September stagger us. It isn’t that overall driving increases so much. But it becomes much more concentrated on commuter rush periods.

In the study of the 2011 pattern, travel times during the morning peaks of August were an average of 36 percent higher than they would have been if traffic moved freely.

Sound bad? Well, on September mornings, the average delay time was 60 percent worse than in free-flowing traffic. Delay time during the afternoon peak also tends to rise from August to September, but the jump isn’t as severe.

That’s probably the back-to-school effect, Pu said. The morning peak is close to school start times, but the afternoon peak occurs well after the end of the regular school day.

These summer-to-fall driving patterns show up consistently in travel data, Hampton and Pu said. But the seasonal patterns are more complicated than summer-and-everything else. Each year is a bit different, but the trend lines over recent years show an increase in fall travel delays on our major highways, usually cresting in late November and then going into a sharp decline through the end the year, before beginning to rise again each January.

Here are some things you need to know to survive this year’s installment of September Shock.

Departure times: Start your commute earlier or later. You might have an easier time applying this to the afternoon commute. In September, the morning’s peak periods tend to spread out over a longer time before and after 9 a.m. You might need to make a big adjustment. The afternoon peak, about 6 p.m., also spreads, but not as much.

Telecommute: If you can pick a day to telecommute, consider Tuesday, Wednesday or Thursday. Those tend to be the peak days for traffic.

Back to school: Public schools in the Washington region return on different dates. D.C. schools and many public schools in Maryland reopened this week. Schools in Montgomery, Loudoun and Prince William counties are scheduled to start Monday. Many other Virginia students, including those in Fairfax County, will go back to school Sept. 8.

Papal visit: Pope Francis is scheduled to arrive in Washington on Sept. 22 and depart Sept. 24. He has a busy schedule that includes trips to the National Shrine of the Immaculate Conception in Brookland, St. Matthew’s Cathedral downtown, the Capitol and the White House. But there will be no big event comparable to Pope Benedict’s 2008 Mass at Nationals Park.

Metro: Transit ridership data also show seasonal fluctuations on Metrorail and Metrobus. Declines in ridership are strongest in August, before beginning an annual climb through September and October. Riders will see more of the new eight-car trains in service, but they still will endure uncomfortable crowding during rush hours.

Transit center: The long-delayed opening of the Silver Spring Transit Center seems likely this fall. It will unsettle downtown traffic patterns, but the new transit hub will finally remove many bus stops from nearby streets.

Driving: “Driving in heavy traffic demands patience, perseverance and special skills,” Townsend said. Resist the temptation to glance down at an electronic device when stuck in a traffic backup, look far ahead to make sure you are alert to changing traffic conditions and avoid sudden moves.

Road work: This fall, a year-and-a-half-long resurfacing project is scheduled to begin on heavily traveled Route 50, between Lottsford Vista Road and the border of Anne Arundel and Prince George’s counties.

This month, the District Department of Transportation began an 18-month improvement project on Minnesota Avenue NE that will restrict the travel lanes — and affect Metrobus routes — to make room for the work zones.

The Virginia Department of Transportation is beginning a major makeover for the interchange at Interstate 66 and Route 15 in Haymarket.

The ramp between the Interstate 395 HOV lanes and Seminary Road by the Mark Center is scheduled to open next month. But the most interesting traffic development of the fall is likely to occur when VDOT lights up its Active Traffic Management system on I-66. It’s a network of electronic monitors, alerts and lane controls meant to help drivers make smarter travel decisions and thus alleviate congestion.