After six hours of lofty debate about constitutional principles at a historic challenge to the prostitution law, a Vancouver lawyer rose to recite a licence plate number to the Supreme Court of Canada bench.

The lawyer, Katrina Pacey, said the plate is on a "bad date list" which street prostitutes hurriedly check before they are talking to a prospective customer and are spotted and arrested by police.

"The driver of that vehicle is alleged to take prostitutes down to the river to try to drown them," said Ms. Pacey, a lawyer for Vancouver's Pivot Legal Society.

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"Screening clients can make the difference between life and death."

The vignette imparted a sobering moment of street-level reality to a landmark challenge that could result in three prostitution provisions being struck down.

Three sex workers who are spearheading the litigation – Terri Jean Bedford, Amy Lebovitch and Valerie Scott – argue that prohibitions against pimping, working in a brothel or communicating with customers increase the dangers faced by prostitutes.

So far, an Ontario trial judge and five Ontario Court of Appeal judges have largely agreed that prostitutes would be safer if they had the right to set up brothels and hire staff to protect them.

The appeal court split 3-2 in upholding the communicating provision – a result the applicants hope to reverse.

In another riveting moment Thursday, lawyer Jonathan Shime picked up on an earlier, chance reference to Brinks security guards to illustrate the folly of exposing workers to enhanced danger by eliminating safeguards.

Mr. Shime, a lawyer for the Canadian HIV/AIDS Legal Network, asked the judges to picture a law that prevents guards from driving in armour-plated vehicles; being accompanied by a second guard; or from communicating with the bank where they are transferring money.

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He continued the parallel with a reference to prostitutes who urgently need to elicit agreement from a prospective client that he will use a condom.

To a security guard, Mr. Shime said, a bulletproof vest is the equivalent of that condom: "It would be improper for the state to tell me, as a Brinks guard, that if I want a bulletproof vest … to protect myself, I can't," Mr. Shime said.

A lawyer for the litigants, Marlys Edwardh, likened the "screening" of potential clients to the use of a seatbelt. It doesn't guarantee safety, she said, but it greatly reduces the risk in an accident.

Much of Thursday's hearing was dominated by one word: choice.

Federal and Ontario prosecutors argued that prostitutes voluntarily choose a risky lifestyle that degrades themselves and the community as a whole, causing Justice Marshall Rothstein to take umbrage.

"They are victims of human trafficking or they come from native reserves where they haven't been given a choice," he said. "What happens to those people?"

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While federal prosecutor Michael Morris conceded that for some prostitutes, choices are "profoundly constrained," he said that, "the fact that some operate on a diminished moral culpability does not mean it will invalidate the criminal law from applying to them."

Justice Rosalie Abella then took issue with the notion of subsistence sex workers exercising choices.

"You're acting like this is a decision whether to go to law school," she said. "Before we start talking about choice, do we really know?"

At another point, Justice Michael Moldaver invoked the spectre of prostitutes who have been killed in large numbers by predators such as Robert Pickton.

"These are the people found to be the most vulnerable," Justice Moldaver said. "These are the type of victims the Picktons prey on. So we take away from them their one window for assessing whether to get into a car or not?"

Alan Young, a lawyer for the prostitutes, said the provisions that have been challenged date back to 1913 and are replete with irrelevant moral positions.

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Justice Abella agreed, observing to one prosecutor: "I feel I'm in a time warp when I listen to you using arguments from years ago rather than arguments about now."

The court is expected to reserve its decision for several months.

Editor's note: An earlier story on a court challenge to the prostitution law incorrectly quoted federal prosecutor Michael Morris saying..."the fact that some operate on a diminished moral capacity does not mean it will invalidate the criminal law from applying to them." In fact, he said "the fact that some operate on a diminished moral culpability ..."