Weeks of checking with charter bus companies and other travel sources prompted officials to lower the original guesstimate by about half, The Washington Post said.

"It's more of an art than a science," City Administrator Dan Tangherlini said. "The fact is, earlier it was speculation. Now we're beginning to flesh it out and what the physical capacities of the system are."

Secret Service officials dismissed the 4 million mark, but they agree with inauguration planners that a huge number of people will try to catch a glimpse of the festivities and the swearing-in ceremony Jan. 20.

Turnout could reach 2 million, officials told the Post, easily surpassing the 400,000 who attended the 2005 inauguration for President George Bush.

Jawauna Greene, a Maryland Transit Administration spokeswoman, said inaugural planning committees initially considered up to 6 million attendees but recently scaled back their projections to about 2 million.

The District of Columbia government conducted telephone and Internet surveys of charter bus companies east of the Mississippi River, concluding about half of their 23,000 vehicles were booked for the inauguration. Washington's Metro transit service can ferry about 1.2 million to the inauguration, he said.

"I'm at 1.7 million already," Tangherlini told the Post. "And I haven't included walkers, drivers, railroaders, Greyhounders, people who already live here."

David Castelveter of the Air Transport Association said airlines are planning to add about 100 incoming and outgoing flights to the three airports servicing Washington Jan. 15-Jan 22, which could carry another 5,000 people.