CHICAGO — Several police officers are waiting in a hotel room, handcuffs at the ready, when they get the signal. A female undercover officer posing as a prostitute is with a would-be customer in an adjacent room, and she has pushed a secret button indicating that they should charge in to make the arrest.

The officers shove at the door connecting the rooms, but somehow it has become locked. They can’t get in. The undercover officer is stuck with her customer. Tension soars. Curses reverberate. A million fears surge.

Then, suddenly, the door frees and the police officers rush in and arrest a graying 64-year-old man, Michael. His smugness shatters and turns to bewilderment and shock as police officers handcuff his hands behind his back.

Michael had reason to feel stunned. Police arrest women for prostitution all the time, but almost never their customers.

Yet that is beginning to change. There’s a growing awareness that sex trafficking is one of the most serious human rights abuses around, with some 100,000 juveniles estimated to be trafficked into the sex trade in the United States each year.

Some women sell sex on their own, but coercion, beatings and recruitment of underage girls are central to the business as well. Just a few weeks ago, New York City police officers rescued a 14-year-old girl in Queens who had run away from home and ended up locked up by pimps and sold for sex. According to court documents, she was told she would be killed if she tried to run away, but after three months she managed to call 911.

Police increasingly recognize that the simplest way to reduce the scale of human trafficking is to arrest men who buy sex. That isn’t prudishness or sanctimony but a strategy to dampen demand.

Polling suggests that about 15 percent of American men have bought sex, and back-of-envelope calculations suggest that a man has about a 1 in 100,000 chance of being arrested while doing so.

Yet stings to arrest johns are marvels of efficiency. Here in Chicago, the Cook County Sheriff’s Office places ads on prostitution websites. When men call, an undercover officer directs them to a hotel room. The officer negotiates a price for a sex act, and then other officers jump in and arrest the customer.

It’s an assembly line, almost creating traffic jams in the hotel. One time, a customer had just been handcuffed when the undercover officer’s phone rang: it was another john downstairs in the lobby.

“Just give me a few minutes to freshen up,” the undercover officer purred.

Donna M. Hughes, an expert on human trafficking at the University of Rhode Island, notes that police often are tougher on men who download child pornography than on johns who have sex with girls or women.

“I think there is still the old idea around that ‘bad woman’ lure men into bad behavior,” Professor Hughes said. “And the police don’t want to bring shame on the whole family by arresting the man.”

Thomas Dart, the sheriff here, says that a basic problem is that the public doesn’t much sympathize with victims of trafficking. He remembers his department once raiding a dog-fighting operation to free pit bulls, and soon afterward raiding a sex-trafficking operation to free girls and women sold for sex. There was an outpouring of sympathy for the pit pulls, he said, but some carping about why the department was in the morals business and worrying about sex.

Yet, slowly, understanding is growing that this isn’t about policing morals but about protecting human rights. In more and more states, pimps are prosecuted more often, and minors are not arrested in prostitution cases but are directed to social programs. Sometimes that’s true of adult women, too.

As appreciation grows that human trafficking is one of the most serious of human rights abuses, so is the recognition that a starting point in addressing it is to stop making excuses for the men who perpetuate it — and start arresting them.

That’s happening more often, although the punishments are typically minimal. Here in Chicago, the men arrested were taken to another hotel room and made to watch a video about the risks of prostitution — such as sexually transmitted diseases — and then given a $500 ticket. They are advised to pay the fine immediately or a registered letter will be sent to their home address. There is no criminal record, and the men are released in about 30 minutes.

The men’s cars are also towed, which costs them another $700 or so. Mike Anton, commander of the vice unit, says that he always tells the married men that they can avoid towing fees if they call their wives to have them pick up the car.

“None of them has ever taken me up on that,” he added.