More than 41 million Americans will travel at least 50 miles (80.5 kilometers) from their homes this week despite heightened security because of failed car bombings and an airport attack in Britain.
Most travelers, 34.7 million, are expected to be driving. About 4.7 million will fly, with additional security in place after the weekend's bomb scare in London and the airport attack in Glasgow.
At U.S. airports, officials randomly searched vehicles while increasing the number of canine units and transportation security agents.
"Our advice for all passengers - mass transit or aviation - is to remain aware of their surroundings and immediately report any suspicious activity to TSA, law enforcement or other officials," Transportation Security Administration spokesman Christopher White said.
Airport officials said they expect less congestion than normal as travelers split their trips between two weekends. The holiday is July 4.
The TSA also deployed security inspectors - both undercover and in uniform - to Amtrak rail stations in the Northeast.
"We clearly have ratcheted up a little bit, focusing on those places where we think there might be a heightened risk. But nobody's really sure," said New York Mayor Michael Bloomberg. "My advice to everybody is to enjoy the weekend."