The Labor Day weekend is upon us. We hear authoritative pronouncements from AAA Mid-Atlantic confidently predicting the number of persons in the metropolitan area who will be traveling over 50 miles outside the area during the holiday weekend. We hear similar predictions for Memorial Day and Thanksgiving. How do they know that? What methodology do they use?

Vincent McDonald, Fairfax Station

Answer Man has often thought that it would be useful to possess a time machine. He would use it to go into the past to rectify certain bad decisions -- an unfortunate quip at a social event, a poor choice of haircut, the more ill-advised portions of his senior prom -- and, occasionally, to peer into the future.

He also would lend it to AAA so it could leap forward, count some cars, then come back and tell us exactly what's in store for us, traffic-wise.

Sadly, such technology does not exist. And so AAA has to rely on a different method for its predictions of holiday traffic volume. Here's what it does: It asks people if they're going to travel.

Actually, the Travel Industry Association of America asks that question. To arrive at this year's Labor Day forecasts, TIA pollsters called at random 1,300 U.S. adults between July 15 and 25 and surveyed them about their travel intentions, said AAA spokesman Justin McNaull. The results of that survey were then massaged, with the massage oil being a proprietary forecast model developed by TIA's economist. Its ingredients are the results of the surveys the association is doing continually, along with such indicators as unemployment rates, consumer spending and gross domestic product. The final figure was then broken down for various geographic locations.

Last week the results were announced: According to AAA, 34.5 million Americans, including 856,000 Virginians and 550,000 Marylanders, were expected to travel 50 miles or more this Labor Day holiday.

How accurate are surveys such as this one? One problem with any survey that asks a question about a personal habit or decision is the possibility of a social bias. If a researcher asks how many alcoholic beverages you consume in a week, you might be tempted to lowball your answer, for fear that you'll look like a lush. If a researcher asks whether you intend to vote, you might say yes, just to seem as if you're civically engaged.

That sort of bias shouldn't apply here. Travel is not very controversial. Also, it's usually an activity that is knowable. It's not as if survey respondents were asked to predict what was going to happen in Iraq. People are able to control their own travel decisions.

Except when they change their minds, which might happen over this holiday. When TIA conducted its survey in mid-July, gas was a lot cheaper than it is now. Drivers, unwilling or unable to pay higher prices, might change plans.

There's another way to predict traffic flow, and that's basically to count vehicles. Governments do that in different ways. Sensors embedded in the pavement can count the cars and trucks that roll along the highway. There are also tubes that stretch across the pavement.

"It's a series of two hoses that are connected back to an automated traffic reporter," said David Buck of the Maryland State Highway Administration. There are about 100 permanent stations across Maryland. Virginia has its own network.

Extremely accurate figures can be obtained when motorists basically count themselves -- by paying a toll to cross a bridge or travel a stretch of asphalt. That's how the Maryland Transportation Authority came up with its Labor Day predictions: It expected that between Friday and today, more than 433,000 vehicles would use the Fort McHenry Tunnel, 245,000 would use the Baltimore Harbor Tunnel, 451,000 would travel the Kennedy Highway (Interstate 95) and 348,000 would cross the Chesapeake Bay Bridge.

The highway administration arrived at those figures by counting vehicles last year and then multiplying those numbers by 1.03, since traffic increases about 3 percent every year.

What use is any of this information? The real story would be if traffic went down over a holiday weekend. And if gas gets near five bucks a gallon, maybe that will happen.

I have been told there is a statue of Grover Cleveland in the Capitol Hill area. However, extensive online research has not led me to any information about it. Do you have any knowledge of such a statue?

Marcia Tiersky, Alexandria

Unless it's inside somebody's house, or buried deep underground in a lead-lined sarcophagus, there is no statue of our 22nd and 24th president on Capitol Hill.

(Our 22nd and our 24th? Yes, he was elected in 1884, then won the popular vote but lost the electoral vote to Benjamin Harrison in 1888. He returned to the White House after the 1892 election.)

Though there is no statue of Grover Cleveland in town, his legacy remains in the name of a Washington neighborhood: Cleveland Park. He purchased a summer house there in 1886 after his marriage to the 21-year-old Frances Folsom.

Mail, Man?

Would the reader who wrote in asking a question about pickup times from blue mailboxes in the District please send in the question again? Answer Man has misplaced it.

Julia Feldmeier helped research this column. If you have a question about something in the area, send it to