The traffic congestion in Tysons is something that many drivers never get used to, in any season. (Bill O’Leary — The Washington Post)

Some traffic observers refer to the first workday after Labor Day as “Terrible Tuesday,” but for many commuters, the phenomenon more generally known as “September Shock” is less of a sudden, one-day blow and more of a gradual realization stretched across several weeks in late August and early September.

Drivers notice it takes longer to get past traffic signals, highway trouble spots are more troublesome, Metrorail commuters find they must park on a higher level in the garage, bus riders find the schedules less reliable and seats more difficult to come by.

The traffic generators are spread over several weeks: Vacation season draws to a close. Public school begins Tuesday in Virginia. Classes began last week in Montgomery, Howard and Charles counties and in the District. They began two weeks ago in Prince George’s and Frederick counties. Many colleges and universities began their fall semesters last week. Congress will return on Monday.

It’s not just a matter of more people being in town. Where they’re going and when are important factors.

Some people are traveling to new schools or new jobs, perhaps from a new residence. They may be a little unsure about the directions, or about whether that last turn is a left or a right. Be patient in those situations, and give them a little extra room to maneuver.

The real difference-maker between deep summer and September is that so many people now will be trying to make the same trips at about the same time. A study by the D.C. region’s Transportation Planning Board showed the significance of trip times in highway congestion.

The decline in total driving that occurs in July and August actually is slight, the study said. Yes, many people take summer vacations, but others are getting out more because of the warm weather and extended daylight. They extra summer travel tends to be at midday or in the evening, rather than at rush hour.

Many people find that summer rush hours are easier because, with schools out of session, parents have more flexibility in timing their own work trips. If they don’t have to pack the children off to school and pick them up at set times, they can avoid the peak of the commute. That can lessen overall commuting delays, the study said.

That break ends in September as the students return and the days grow shorter and cooler. The study found that average daily delay per traveler rose nearly 27 percent between August and September of 2011. The average delay was 20.4 minutes in August and 25.8 minutes in September.

Meanwhile, total driving rose only slightly. Vehicle miles traveled went from 32.71 million miles in August to 33 million miles in September 2011.

Metrorail ridership, which drops in August, will rebound this month, though historically, the average weekday ridership in September doesn’t match the totals for July, which, with all the visitors and activities, tends to be an above-average month on the rail. Metrobus ridership also rebounds in September from August lows. See a pdf showing Metro ridership history.

Overall, Tuesday morning’s commute was unremarkable. The highway trouble spots included the year-round ones: The Capital Beltway through Silver Spring, Interstate 95 South approaching the Beltway, I-66 East through Manassas, Route 50 inside the Beltway in Maryland, I-395 near the 14th Street Bridge and I-295/D.C. 295/Suitland Parkway near the 11th Street Bridge.

Meanwhile, Metro reported few problems with train and bus service. An Orange Line train bound for New Carrollton was taken out of service at Capitol South because of a problem with its brakes.

In upcoming postings, I’ll have some tips on this month’s traffic and transit issues.