With the Fourth of July falling on Wednesday, this will be the busiest travel week of the summer -- almost five million Americans are expected to fly -- and after the events in London and the attack at Glasgow airport, they can expect tight security and the delays that go with it.
At Los Angeles International Airport, checkpoints are already up. SUVs and other vehicles are being searched and passengers are being interviewed.
Miami International Airport security director Lauren Stover said he has stepped up patrols.
"While we haven't had any specific info that any U.S. airport is a target, all the nation's airports have been mobilized," he said.
New security measures across the country on both air and other transit systems include a beefed-up police presence, random patrols with dogs and new parking restrictions.
Secretary of Homeland Security Michael Chertoff said in a statement that while there is no credible evidence of an increased threat against the United States, "DHS will be implementing plans to increase our security measures at U.S. airports, mass transit and other transportation facilities. ... Some of these measures will be visible, others will not."
While the nation's airports have increased security, the aviation threat level has not been raised. It remains at orange, where it has been since last summer.
Many airports have already taken steps to guard against attacks like the one in Glasgow. Reagan National Airport in Washington, D.C., has steel barriers in place to prevent cars from crashing through the terminal doors.
Passengers say increased security makes them more aware of what's going on, and at times it can be scary.
One passenger told ABC News, "Any time there is an incident like this it sort of makes you wary of getting on a plane to go anywhere."
And wary passengers could soon become weary ones. The extra security will likely cause new delays, compounded by the crush of holiday travel.
Officials said passengers should be prepared to arrive at airports with plenty of time and come prepared to wait.
Liz Marlantez contributed to this report.