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.