Susan Rudisill was approaching the west-side checkpoint at Washington Dulles when an airport official stopped her and directed her to a different security area, where she would receive new instructions. Among them: Do not remove your shoes, light jacket or belt.

“I planned my whole wardrobe around having to take off my shoes!” exclaimed Rudisill, an Oregon traveler who had been randomly selected for PreCheck.

The Transportation Security Administration program, which launched in October 2011, rewards so-called trusted travelers with relaxed procedures reminiscent of the pre-9/11 days. The agents flip the usual script: They remind passengers to keep on clothing items that they would shed at traditional security outposts and instruct them to leave laptops and compliant liquids in their carry-on bags. Travelers also walk through old-fashioned metal detectors, not the newfangled body-image scanners. Dust off the disco ball, because airport security is going retro.

“I was pleasantly surprised at how fast it was and that I didn’t have to dig through my bag for my 3-1-1 liquids,” said Nancy Strahl, an Oregon-bound traveler and first-timer in the special lane. “I have to recommend it to everyone.”

The TSA hopes that you will.

Since its introduction, more than 20 million passengers have used PreCheck nationwide, according to the TSA. At Washington Dulles, about 5,000 of the daily 25,000 passengers walk the line. The program is rapidly expanding, with more than 100 airports participating, up from 40 earlier this year, in 42 states and two territories (Puerto Rico and Guam). Nine U.S. airlines, including Southwest and JetBlue, two recent additions, are involved. (Currently, only JetBlue passengers with mobile app boarding passes are eligible; the airline plans to expand to include paper boarding passes by the start of next year.)

Earlier this year, TSA Administrator John Pistole set an end-of-2013 goal to funnel 25 percent of all travelers through speedier security arrangements, which includes PreCheck and gentler processing for those 75 and older and children 12 and younger (both populations can leave their shoes and light coats on). He hopes to goose the number to 50 percent by the close of 2014.

The program benefits both civilians and government officials. Streamlined measures can help alleviate the stress on normal lanes, unclogging bottlenecks and brightening passengers’ moods. For security purposes, preapproving travelers will hopefully help officials weed out non-threatening fliers and heap greater attention on riskier individuals.

To achieve Pistole’s goal, the agency recently announced a more direct route to PreCheckdom. Previously, passengers have qualified for the service if they’ve already enrolled in a trusted-traveler program, such as Customs and Border Protection’s Global Entry, or are active members of the military. (Men and women in the U.S. armed forces can currently take advantage of the PreCheck program at 10 airports; it will be available to them at all participating facilities starting Dec. 20.) Airlines can also opt-in their frequent fliers.

More serendipitously, TSA may grant individuals whom it deems low-risk a PreCheck stamp on their boarding pass. “If you show up clean, you can be randomly selected by TSA,” said a department spokeswoman. “The more information you give us about yourself, the more likely you will be selected.” She added that “25 percent of passengers will be selected one way or another” through the various channels.

To attract more participants, the TSA plans to set up an online application and establish two enrollment centers in the Washington area and the Indianapolis airport for fingerprinting and supplemental interviews. The cost: $85 for five years, $15 less than Global Entry. The target date: end of the year.

Denise Leschinski of Sarasota, Fla., had been through PreCheck twice in one day, first in Sarasota and now at Dulles. “I think it’s wonderful,” she said.

Would she pay for the privilege?

“Not if I can get it for free,” she joked, referring to her luck in getting randomly selected. Then she thought again. She might sign up. “We do travel a lot.”

As the program gains momentum, PreCheck converts need to remember who’s the boss (TSA) and how the boss behaves (unpredictably). The agency may, without warning, bump participants to the standard line, just as it may pull non-members into the special lane.

“We always have to have the random possibility,” a TSA representative said.

Matt Bradford of Pasadena, Calif., received the perk on his last four or five flights. “I’m not sure how I got it,” he said. “It’s been fantastic.” However, his winning streak went bust when Dulles bouncers turned him away at the cordoned area and sent him back to the commoners’ section.

The message: TSA wants you to embrace PreCheck, but don’t grow too cozy with it.

Security line race

So, can the much-ballyhooed PreCheck stand up to the test of two travelers? Ready, set, go . . . to your respective lines.

A little after 11 on a recent Tuesday morning at Dulles, Becky and Andrea split up and set off in different directions. Andrea heads for the black-and-yellow PreCheck sign; Becky descends the escalator that will take her to the normal security area. May the fastest lane win.

The queue for the regular lanes isn’t filling up the whole waiting area, but it’s not short either. Becky follows protocol, slipping off her fleece jacket and her sneakers and removing the hand sanitizer from her bag and placing all the items in a bin. She enters the full-body scanner and throws up her arms in a frozen-jumping-jack pose.

Upstairs, Andrea skates through the PreCheck line to the sound of “beep, beep, beep” as attendants scan the boarding passes of those behind her to determine whether they belong in the queue. (If rejected, a traveler can follow a parallel path that adheres to normal security procedures. Families with non-PreCheck members may also use this parallel line so that everyone can stay together.) About five people precede her, and she doesn’t have to decelerate until she reaches the lectern where the official checks her ID.

At the X-ray machine, Andrea places her carrier containing her computer on the belt (no people jam!). She walks through the metal detector in rubber boots and a black trench coat. An alarm sounds. She flashes her heavy brass bracelets at the agent, who nods and lets her through.

Total time for Becky: 13 minutes.

Total time for Andrea: four minutes.

PreCheck claims victory.

But could this be a fluke? To quash such doubts, the pair schedules a second run in the afternoon. This time, they switch positions. Becky achieves an impressive race time of 3.5 minutes and zero trays in the PreCheck line; Andrea clocks in at eight minutes and three bins. PreCheck does it again.

Jacket and shoe confusion

PreCheck reigns in speed and efficiency, but it stumbles, too, especially in the sartorial department. Take shoes, for example.

Agents inform passengers to keep their footwear on, but they don’t make distinctions, such as between sneakers, which remain silent through the metal detectors, and steel-toed boots, which can cause a ruckus.

“It’s usually pretty good, but this time I had to take them off,” said Alison Taylor of Washington, who had to remove her boots. “But the nice agent said it happens to good shoes.”

The culprit: a metal shank in the heel.

The instructions pertaining to overcoats are also open for interpretation. Travelers can wear “light” overcoats, but the adjective is somewhat vague. When asked for clarification, a high-ranking security official at Dulles said that a suit jacket is acceptable, but puffy, bulky or heavy wool coats are not. Certain hats are also allowed, such as baseball or knit caps. A man in a Stetson, however, didn’t care to test the rules, placing his topper in a bin.

Officials also remind passengers to empty their pockets of metal objects such as phones, keys and coins. Men coughed up enough items to fill a treasure chest.

“Place it in the dog bowl,” an official repeated in an endless loop.

Large belt buckles, chunky jewelry and medical devices can also agitate the metal detector. After several failures, a passenger may have to submit to a pat-down.

“I think educating the public is key to making this successful,” said Oregon-bound traveler Strahl, who doesn’t fly with her computer because of the usual hassle but added, “I would rethink that now.”

Some travelers were unfamiliar with the program until an agent rerouted them to the special area. “It’s brand-new for us,” said Mary Lou Werner of Willmar, Minn.,“but it’s wonderful.”

As awareness grows, passengers may notice more PreCheck signs popping up in terminals. Dulles is the rare facility with a stand-alone section featuring multiple lanes and bold signs overhead. At BWI Marshall and Reagan National, the special lanes are integrated into the main security areas. Small postings direct passengers to the designated lane.

“It’s inconsistent,” said a Virginia traveler who works with the military and has sampled the service around the country. “Dulles is in the lead. It’s the smoothest.”

Still in its infancy, PreCheck continues to outpace the standard lines. On a Tuesday afternoon, the wait for the regular checkpoints clocked in at 20 minutes, about twice as long as for the new kid. But as the program grows, the zippy hare lanes could turn into slow tortoise tracks. If that occurs, one traveler has already devised a backup plan.

“When more people take it this way,” he said, “we’ll go back to the other.”