With the sun coming up on Sept. 12, 2001, state Transportation Secretary John D. Porcari drove home for a change of clothes after a day of helping direct Maryland's response to the terrorist attacks that killed nearly 3,000 people the previous morning.

"I remember saying to myself that the world will never be the same, and that's certainly true of the transportation world," said Porcari, now deputy secretary in theU.S. Department of Transportation.

Sweeping changes that have affected virtually every mode of transportation in the United States began almost immediately after hijacked airliners slammed into the World Trade Center in New York, the Pentagon outside Washington and a farm field in Pennsylvania.

They are continuing to this day, and there is no end in sight — especially at airports.

To board flights at Baltimore-Washington International Thurgood Marshall Airport, Marylanders must take off their shoes and submit to a much more intrusive scan or pat-down than they ever would have imagined before the attacks. If they're carrying bottled water or an oversized tube of toothpaste, they can expect to be flagged by officers of the Transportation Security Administration — an agency born out of the airport security failures of Sept. 11.

"Taking the shoes off has a certain herding-the-cattle aspect to it. It's kind of undignified," said Baltimore architect Klaus Philipsen, a frequent traveler who had to surrender several Swiss Army knives at security gates when the restrictions were new. "In the beginning, I kept forgetting."

It's not just air travelers who have been affected. Across the nation, Sept. 11's legacy is a heightened awareness of security — and much more scrutiny from government.

When transit riders board MARC, Metro or light rail trains in Baltimore, or when motorists drive through tunnels under Baltimore's harbor, they are much more likely to come under video surveillance. Amtrak passengers are more likely to see officers with bomb-sniffing dogs at Penn Station.

And motorists crossing the Bay Bridge may not spot it from up high, but there's probably a police boat — financed with federal anti-terrorism funds — in the waters below, patrolling the Chesapeake Bay.

Meanwhile, almost every new transportation project undertaken by the state costs more, so tougher security features can be included. For example, a BWI expansion, already well along when the attacks occurred, had to be revised to add counterterrorism measures at a cost of $6 million to $8 million. Porcari said.

The changes didn't come all at once. Some evolved over several years, as officials worried that terrorists would exploit a new vulnerability in the nation's transportation system. It wasn't until years after the attacks, for instance, that first responders in Baltimore began receiving advance notice of hazardous railroad shipments moving through the Howard Street Tunnel.

Today, some of the shock of 9/11 has worn off, and it's easy to forget how scared people were. Porcari recalled that soon after the attacks, the state declared a toll-free day on the Bay Bridge to encourage travel.

"People were afraid to go to the Eastern Shore," he said. "We were just trying to stimulate getting back to some semblance of normalcy."

Flights across the U.S. were grounded after the attacks, and once airports reopened, fear lingered.

"I remember the trepidation with which people were coming into the airport to fly," said Beverley Swaim-Staley, the acting head of BWI on the day American normalcy ended. "People were very scared, and that included the people who worked for the airlines. There was a lot of tension in the air."

Airports haven't been the same since.

"It really changed the way an airport functions and the way people feel about a trip to the airport. You'd wait by the gate and watched the plane come in. You can't do that anymore. … Airline travel is much more of a business experience as opposed to a social event," said Swaim-Staley, who later became Maryland transportation chief.

Paul J. Wiedefeld, who became executive director of BWI in 2002, said that one of the first issues he faced was the federal government's decision to replace the companies that ran airport screening programs with an agency that would be called the Transportation Security Administration.

"The customers didn't know what to expect. … The airlines didn't know what to expect. Everybody was feeling their way through it," he said.