Commuters' Vacations to End With a Screeching of Brakes

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."