SAN FRANCISCO (CBS SF) — Bay Bridge commuters go through this every year. Traffic is heavy and horrible every morning for months. You set your alarm clock and know exactly how early you have to leave to make it into The City, barring some freak accident or a breakdown. It’s painful, but it’s predictable, and most days, it works.

Then, suddenly it’s June, and the traffic backup into the maze is almost non-existent. You whiz through the toll plaza, roll past the metering lights, and find yourself at work a half-hour early. Your hour and 45 minute commute has been cut in half. Where did everybody go?

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From water coolers to cocktail parties, everyone is talking about how wonderful their commute is and speculating why.

The most common explanation is school is out, and as if a giant bell rang out across the bay, traffic has been dismissed.

Metropolitan Transportation Commission spokesman John Goodwin says that’s true, but the traffic is not gone.

“Overall, traffic on the bridge does not decline,” he says. “What happens is a flattening of demand so you lose the bunny ears.”

That’s an adorably cute metaphor for the spike in traffic that occurs every morning between 6 a.m. and 8 a.m, and 4 p.m. and 6 p.m.

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“So it rises more gradually and plateaus,” says Goodwin. During the summer months, “travel patterns change for enough people which creates a ripple affect throughout the day.”

Basically, your Bay Bridge commute has morphed from a long-eared bunny, to an earless owl — smooth, with no spikes at all.

But Goodwin says there are still the same number of people crossing the bridge. Parents who drive their kids to school all year, make those trips during the summer months, but at other times of the day.

“Or, they are replaced by tourists coming in from parts unknown. So what happens is you have just as many ping pong balls, but they’re not so concentrated.”

Be warned, however, as soon as you get used to that easy commute, it will be gone.

“Come Tuesday after September, they will all be back,” says Goodwin. “And there’s the BART shutdown over Labor Day weekend, so take the joy while you can because it’s not going to last.”

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And so it goes: One morning, you leave the house at your usual time, and traffic is backed up through the maze, all the way to the 24. You inch through the toll plaza, past the metering lights and crawl across the bridge. Finally, you arrive at work more than a half-hour late. Grrrr…. the bunny ears are back!