CNN

How many people will show up for the swearing-in?

WASHINGTON (CNN) – Seven days to Inauguration Day and counting.

And counting. And counting.

- 20+ jumbotrons.
- 5,000 port-a-potties.
- 10,000 National Guard troops.

But the statistic most people want to know is currently unknowable: How many people will show up?

Logistics fever is gripping Washington as officials tweak plans to accommodate the throngs of people expected to flood Washington for the inauguration of Barack Obama.

At a press conference Tuesday, local officials struggled mightily to achieve an almost impossible balance, encouraging people to participate, on the one hand, while warning potential participants that the crush of people could be overwhelming.

People should not venture into Washington on Inauguration Day without a travel plan, said Maryland Gov. Martin O'Malley. Indeed, they should think twice before traveling on the Capital Beltway, far from the big event.

"This is not a typical day in our country's history. This is not a typical crowd," O'Malley said. "This is not like throwing the family in the van and heading down for a visit to the Air and Space Museum."

"If you wish to attend the inauguration, you really do have to have a plan. You need to have a plan for how you get into and out of the District that day. Do your research. Do your preparation beforehand."

Officials have rolled back early estimates of four million visitors, but say even half that number will result in traffic jams and disappointed people being turned away from the parade route.

D.C. Mayor Adrian Fenty said it will be impossible for people who are attending the 11:30 a.m. swearing in to also witness the 2:30 p.m. inaugural parade, even though the events are just blocks away. That is because parade route is expected to fill up even before the swearing in starts. Security checkpoints for the parade route open 7 a.m. Tuesday.

Secret Service Director Mark Sullivan said the parade route can accommodate 300,000 to 350,000 people. After the route is filled, people will be turned away, he said.

The relief valve will be the huge National Mall, where hundreds of thousands can congregate and witness both the swearing-in and parade on more than 20 jumbotrons.

"You can actually get to the mall with a lot less hassle and see both the swearing in and the parade on the jumbotrons," said Metropolitan Police Department Chief Cathy Lanier.

Lanier said she expects the crowd will be understanding.

"I think for the most part this crowd understands (restrictions)," Lanier said. "There is an excitement we saw on election night. People are going to feel they are part of this event. If they can't get on the parade route, but they can sit on the mall, on the mall in Washington, DC, during these ceremonies and watch it with a million other people, you are still part of it and I think that will help take away some of that frustration now."

For all their words of caution, officials were decidedly upbeat about the event.

"There are not a lot of days in a life where you know, when you wake up, you'll remember it for the rest of your life. This is going to be one of those days," said Virginia Gov. Tim Kaine.