(CNN) -- Travelers setting off for the Fourth of July holiday can expect tighter security at U.S. airports in response to an attack Saturday on the Glasgow airport in Scotland.
The Transportation Security Administration said there would be more police and bomb-sniffing dogs at airports and that authorities would be conducting random vehicle checks.
Homeland Security Chief Michael Chertoff told CNN that the threat level for U.S. aviation had been at orange, or high, since last August.
"And that is basically the highest level, other than what happens when you have an actual attack. We have also taken some measures, people will see, some they will not see, in increased personnel, some of them obvious, some of them plain clothes, pushing out the security perimeter, not only with respect to airports, but also mass transit and transitions as well," said Chertoff.
The TSA said there was no evidence linking the United Kingdom attacks to a potential threat against the United States, but encouraged people to report anything that seems suspicious.
A survey conducted by AAA predicted that about 4.7 million people planned to fly over the holiday, which stretches over two weekends, since the Fourth of July falls on a Wednesday. About 34.7 million people said they planned to drive.
Almost 38 percent of the respondents said they planned to leave on Friday, before the airport attack.
The FAA reported no major flight delays on its Web site and most U.S. airports were experiencing delays of 15 minutes or less.
Atlanta's Hartsfield-Jackson International Airport, the nation's busiest, said on Monday that the wait times at its security checkpoints were less than 10 minutes.
Police at Phoenix's Sky Harbor Airport started conducting random vehicle searches on Monday, CNN affiliate KPHO reported.
Airport officials said on their Web site that the checks should take less than two minutes.
CNN's Jim Acosta said travelers at New York's La Guardia airport were not having any major problems, as long as they gave themselves plenty of time.
Tighter security there did lead to weekend delays and long lines of frustrated passengers. Watch how U.S. authorities are reacting »
"I'm a big fan of security," Rebecca Blake said. "This is not security though. This is poor management."
Another traveler compared it to Disneyworld, because the line was longer than the ride.
"We'll be on line for two hours to take a one-hour flight," Marcell Pevsner said.
An abandoned bottle of cologne sparked a security scare Sunday at New York's John F. Kennedy airport.
Authorities closed one terminal for about an hour until police determined the suspicious package was harmless. E-mail to a friend