When Gary Smith was in trucking school, a classmate asked the instructor what to do about women who hung out in truck stop parking lots, knocking on the door of the truck to sell sex.

The instructor described a window decal saying “No Lot Lizards,” with a drawing of a woman with green skin and a tail wearing scanty clothing. “You can buy one in any truck stop store,” the instructor told them.

A few months after Mr. Smith started driving, he was parked at a truck stop in Indianapolis. It was October 2008, during a pouring rain. At 3 in the morning a knock on his cab door woke him.

It was a girl of 15 or 16, he said, soaked and shivering. “Do you want company?” she asked. Mr. Smith was furious at being awakened. “I’m not lonely — go away,” he yelled at her.

“There was absolutely no emotional response out of that young lady at all,” he said in a recent interview. “It was like looking into a doll’s eyes. She was completely lifeless.” She just turned and walked away.

“I probably thought: ‘She made her choice. This is what she wants to do.’ And I didn’t want to be bothered in the middle of the night.”

Mr. Smith went back to sleep and thought no more about it — until six years later, when he visited a trailer fitted out like a mobile museum about sex trafficking and labeled the Freedom Drivers Project. The trailer travels the country going to trucking conventions and events like county fairs. It’s a project of the 10-year-old group Truckers Against Trafficking, which teaches truckers, their companies and the government agencies and law enforcement that intersect with commercial drivers how to spot sex trafficking and how to respond. T.A.T. has trained more than 700,000 truckers.

Now Mr. Smith knows that it is unlikely for a high school-age girl to voluntarily spend her nights in a truck stop parking lot, having sex with strange men. And he knows that it’s unlikely that she keeps any of the money she makes.

“Most people say it’s the world’s oldest profession,” he said. “I correct them: It’s the world’s oldest oppression. These are slaves.”

He has thought often about the girl in the rain. “Whether through a lack of knowledge or just my own selfishness, I never gave her the opportunity to be rescued, and I have to live with that,” he said. “I failed her.”

Today Mr. Smith, 58, has a different decal on his truck, a T.A.T. sticker that says, “Do you need help?” with contact information for the National Human Trafficking Hotline. He has hauled the Freedom Drivers Project museum. He speaks to audiences of truckers and trucking company officials about sex trafficking. He is a leader of a new T.A.T. project called “Man to Man,” which seeks to train truckers to talk to other men about the harm that can come from buying commercial sex. Truckers, often seen only as potential customers, are now using their proximity to sex trafficking to help its victims.

Until recently, the biggest challenge in the fight against sex trafficking was convincing people that it was real — and a real problem in Everytown. Kendis Paris, a co-founder and executive director of Truckers Against Trafficking, said she found out about human trafficking only in 2007, when her mother asked the family to read a book called “Not for Sale.” That year Ms. Paris, three sisters, her mother and a family friend founded a ministry called Chapter 61 Ministries to do something about human exploitation. T.A.T. was their first project.

The Polaris Project, which runs a national human trafficking hotline, defines the crime as stealing freedom for profit. Most trafficking involves labor — often in the construction, agriculture, restaurant and domestic industries. A much smaller component is sex trafficking: compelling someone, through fraud, force or coercion, to provide commercial sex against her or his will. If victims are minors, employing someone in the sale of sex is always trafficking, no matter how they are recruited or kept. “There is no such thing as a child prostitute,” Ms. Paris said.

According to the International Labor Organization, at any given moment in 2016 (the last year for which it has data), 4.8 million people were being forcibly exploited sexually worldwide. But while there are fewer people in sex trafficking than in labor trafficking, there’s a lot more money. In 2014 it was a $99 billion industry worldwide, double the size of labor trafficking.

You can sell a gun or a bag of heroin only once; you can sell a human being over and over. And someone sold for sex multiple times per day brings in far more money than someone sold into other kinds of work. Unfortunately, it’s hard to imagine a more lucrative business.

When people think about sex trafficking, they think about a young woman from, say, Ukraine who was tricked into coming to America with the promise of modeling work. That happens, but sex trafficking need not involve international travel, or even travel at all. Many victims (perhaps most; we know very little about the dimensions of this crime) live in the United States. Many don’t go far from their hometowns.

Many are runaways or otherwise in dire straits, lured by someone who buys them a meal and shows them what seems like kindness — until that benefactor says, “Now you owe me.” They are kept from leaving by threats and beatings, but probably more often by mental and emotional coercion. Most pimps are not strangers but intimate partners.

And many sex trafficking victims are male. L.G.B.T.Q. boys and men are particularly vulnerable, according to the Polaris Project. One study of child sex trafficking in New York City found that boy victims were far easier to find than girls.

It’s harder to find every victim these days. Instead of street encounters, appointments are increasingly made through texts, apps and social media.

But the truck stop is an exception. “In certain hot spot areas you pretty much see it every day,” said Arian Taylor, a trucker who won T.A.T.’s 2018 Harriet Tubman Award for helping a young woman who knocked on his door get to a shelter and then back to her family. Mr. Taylor mentioned two neighboring truck stops in Ontario, Calif., that between them can park 1,000 trucks. “If a trafficker has 10 girls, they can make a lot of money between midnight and five o’clock,” he said.

We don’t know the proportion of trafficking victims who are children — it is probably a minority. But they attract the vast majority of citizen concern. “Horror and outrage fall off dramatically when the victim is an adult,” Kylla Lanier, a co-founder of T.A.T., wrote in an editorial on the organization’s website: Don’t see the number 18 as the cutoff to your compassion, she pleaded.

The compassion imbalance is an issue for trafficking in general. The plight of people kidnapped or defrauded into working construction or picking crops, their pay turned over to the labor version of a pimp, should inspire just as much horror and outrage, said R.J. Thompson, managing director of the Sex Workers Project at the Urban Justice Center. It doesn’t, he said, because of patriarchal attitudes about sex and the stigma attached to sex work. (Opposing prostitution is one of the few things the religious right and the feminist left can agree on.)

“Sex trafficking is a grave violation of human rights,” Mr. Thompson said. “But it’s become a sexy, popular, trendy issue. It shouldn’t be trendy. It’s a form of labor trafficking, and you don’t see that for farm workers or domestic workers and other workers.”

T.A.T. fights trafficking by working with and changing practices in the legitimate businesses that traffickers rely on, such as trucking. It’s now spreading that approach to other industries as well. One T.A.T. program, Busing on the Lookout, trains drivers of long-haul, city and even school buses; alongside it, T.A.T. is beginning programs for oil and gas workers, who, like truck drivers, are often sought as customers.

A report by the Polaris Project also looks at how other businesses that encounter trafficking might work to discourage it; most important is the banking and finance industry, said Bradley Myles, the chief executive of Polaris.

Mr. Myles said that the travel company Carlson began training employees in the Radisson and other hotel chains it owned to spot possible trafficking about 15 years ago. Carlson was one of the first businesses to sign a code of conduct from ECPAT, a global organization that fights child trafficking. (Mr. Myles also singled out Delta Air Lines.) “Most companies weren’t interested in touching the issue at that time — Carlson got the conversation started,” he said. “But businesses are really joining forces to fight against this in much greater numbers over the past two or three years.”

Truckers matter to the fight, Mr. Myles said. “They are important eyes and ears to be trained to look out for trafficking.” At any moment there are 3.5 million commercial drivers on the road, dwarfing the number of law enforcement officers. T.A.T. asks truckers to look out for labor trafficking. “But we focus primarily on sex trafficking,” Ms. Paris said, “because it literally comes knocking on their door.”

In addition, once trained, truckers can see the women they call “Lot Lizards” very differently. “You can usually tell when something is wrong,” Mr. Taylor said. “The women are cowering, there’s no confidence in them. So you ask: Where are you from? What are you doing here? Do you need help?” T.A.T. asks drivers to call the national hotline if they suspect trafficking; the hotline will then call the local police.

Ms. Paris said the hotline had received only three calls from truckers before T.A.T. was founded. As of last July, that number was 2,250. From then, the hotline turned 612 cases, involving 1,133 victims, over to law enforcement.

T.A.T. also works with government agencies that deal with commercial drivers; eight states now require T.A.T. training to get a commercial driver’s license. T.A.T. helped start a Mexican counterpart and this year will start an organization in Canada.

Mr. Myles said that truckers can also see the fight against trafficking through “the lens of having them not buying sex.” But reducing the demand is often difficult work. The “Man to Man” program has banners with tough-looking men standing, arms folded, in front of their rigs, explaining why they don’t buy commercial sex. It seems awkward. And just how do you bring up the subject in truck stop conversation? Mr. Myles said he believed efforts to reduce buying were not successful yet.

But there are new strategies to discourage buyers that are more promising. When men text the number in an ad offering sex, they assume they’re talking to a person. But what if that “Hi, baby,” is actually coming from a machine? And that machine works for the police?