"A vehicle means great freedom but not when everyone's on the same road at the same time," he said.

The concept of the daytime population refers to the number of people, including workers, who are present in an area during normal business hours, in contrast to the resident population present during the evening and nighttime hours.

Some key findings:

• New York City has the largest estimated daytime population, at more than 8.5 million people. The daily increase of more than half a million people is larger than any other city, but that's only a 7 percent bump.

• About 250,000 people worked in New Orleans prior to Hurricane Katrina. Almost 150,000 of them were residents of New Orleans; the remaining 100,000 lived outside the city.

• One of the most extreme examples of daytime population increase is Lake Buena Vista, Fla., which has almost no permanent residents but is an employment center of more than 30,000 people during the day.

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E-mail: dbulkeley@desnews.com