By Michael Laris
Washington Post Staff Writer
Tuesday, September 4, 2007

In the best days of summer, Colin Reed makes it from his Columbia Heights home to Metro Center in about 20 minutes. Today, his drive will take roughly twice that.

Welcome back, Washington. Your traffic vacation is over, and your transportation experts are in metaphorical overdrive.

"It's like the chickens coming home to roost," said John B. Townsend II of AAA Mid-Atlantic.

No, wait. "It will be the sparrows of San Juan Capistrano coming home," he said.

Still, not quite.

"It will be the return of frayed nerves and the anger at our inability to solve the problem," Townsend said. "It is a hydra-headed monster!"

Or, as Reed called it yesterday, "system shock."

AAA estimates that commuter congestion drops 10 to 15 percent in August, when Congress and schools are out of session, many commuters are out of town and people take days off at home that shake up their rush-hour routines.

"The day after Labor Day is known by many as Terrible Tuesday," said Joan Morris, a spokeswoman for the Virginia Department of Transportation. "By that time, all of the schools are back in, and the majority of people are back from their vacations. It just sort of hits all at once."

Students in Northern Virginia head back to class today, joining those in Maryland and the District, who have already returned. In Fairfax County, 1,570 buses will shuttle an estimated 110,000 students to and from school. Loudoun County school officials said 721 buses will be on the road, beginning the first of the more than 8 million miles they are expected to travel this school year. And thousands of parents will converge on schools in their cars.

Add that to the 1.73 million people who drive to work alone, population growth that has outpaced spending on roads and transit, and general post-vacation doldrums, and the region's traffic snarls will resume where they left off before the start of summer. According to census figures for last year, the Washington region's average commute was the second-longest in the United States, at more than 33 minutes each way.

With kids back in school, this might be a good time for some traffic-flow theory.

The summer break for commuters, and the slap of today's return, are subsets of the same basic point made by transportation engineers and self-described traffic geeks: Small changes can make a big difference in how large numbers of cars move, especially when roads are butting up against their capacity.

"Think of it this way: Can a few degrees in temperature make a big difference when Atlantic moisture is blowing in from the East? If it is 34F, then all you have is a cold rain. But it drops to 31F, and you have a huge, white mess," Shawn Turner, a researcher at the Texas Transportation Institute, explained in an e-mail. He has tracked summer driving respites in Washington and Phoenix, where some residents flee to cooler climates.

"It's the same with traffic. A certain number of cars can comfortably drive on the road (typically 2000-2200 cars per hour per lane, depending on the road), but add another 100 cars, and things freeze, so to speak. Just like slightly falling temps will change rain to sleet, then snow, slightly increasing traffic will change smooth flow to slowdowns and eventually stop-and-go," Turner wrote.

In some parts of the Washington region, the average number of cars on the road actually increases in the summer compared with other months. But transportation experts said those cars don't necessarily travel at peak commuting times.

There is some disagreement on the half-life of end-of-summer angst.

With vacations over and "everyone getting back into their school routines the Tuesday after Labor Day, there is a natural increase in traffic," said David Buck, a spokesman for the Maryland State Highway Administration.

"It's the same traffic as it was in May, but people get used to the calmer rush hours in July and August and assume things are drastically worse. . . . Two weeks into September, people are accustomed to the school traffic patterns."

But that return to form has Townsend irked, too.

"It's something we endure for almost 11 months out of the year," he said. "That stays with us until the next summer."