As the Louisiana coast prepared for Hurricane Gustav’s landfall, it was widely reported that 1.9 million people had evacuated from the area. Wondering how government officials could count the mass of fleeing humanity, I contacted the Louisiana State Police, which was the source of the number. “This is an estimate,” Lt. Doug Cain, spokesman for the state police, told me. “It is by no means an exact science.”
There are two parts to the estimate: approximately two million people living in areas that were evacuated, and the police’s estimate that 95% of those people had evacuated.
The affected areas were south of I-10, the interstate that crosses the southern part of the state from east to west, generally within 30 to 60 miles of the Gulf Coast. Lt. Cain wasn’t sure who counted the two million people below I-10, a figure that was repeated in many government agencies’ briefings ahead of the storm.
Nine Louisiana parishes are, in their entirety, south of I-10. These had a combined population of about 450,000 last year, according to the Census Bureau. The highway passes through another 15 parishes in the state, with a combined population of 2.1 million last year, but not all of those people live south of I-10. (Most of New Orleans is south of I-10; much of Baton Rouge isn’t.)
Those Census figures are based on estimates produced by the bureau between decennial censuses, the next one due in 2010. These are derived from records of population change, such as births, deaths and internal migration. That last number normally is measured using only tax records. But because Hurricane Katrina, in 2005, had a cataclysmic effect on some parishes’ population, the Census Bureau also incorporated change-of-address files from the U.S. Postal Service. The result: The 24 counties all or partially south of I-10 had about 235,000 fewer people in 2007 than in 2005, with the exodus from the Orleans, St. Bernard and Jefferson parishes accounting for the drop.
“They’re the best that we can do,” Census demographer Greg Harper said about these estimates. “We’ll have to wait until the census to get an exact number.”
To get the second piece of the evacuation number — the estimate that 95% of the two million people fled — the state police used “a combination of traffic counts, traffic volume and boots on the ground talking to local officials,” Lt. Cain said. The conversion Sunday of four major roads into one-way exits from the region — called “contraflow” — accounted for such a high rate of evacuation, he said. “It seemed the public waited for that magic word — contraflow,” Lt. Cain said, adding, “People left. They heeded the warnings.”