A TERRIBLE threat stalks the streets of Washington, DC: unlicensed tour guides. These brazen lawbreakers imperil the public by showing them around the nation’s capital without a permit. Your correspondent went undercover to observe at first hand the dangers tourists face in their clutches. It was harrowing. First, your correspondent had to balance on a Segway, a two-wheeled vehicle from which she could have fallen several inches to the cold, hard pavement. “Just try to relax,” purred Bill Main, the outlaw guide, “It’s easy.” With white knuckles and a pink helmet, the tour began.

Mr Main never took the exam to become a tour guide, so your correspondent braced herself to hear a torrent of errors. Would he claim that the White House was once destroyed by aliens, as in the film “Independence Day”? No. Actually, he was pretty good. Yet he could be jailed for 90 days if caught. Washington requires all guides to pay $200 and take an exam. That adds up: Segs in the City, the firm Mr Main runs with his wife, Tonia Edwards, employs a dozen guides.

The permit system protects incumbents, raises prices and kills jobs. Mr Main also believes that it violates his right to free speech. Robert McNamara of the Institute for Justice, a libertarian law firm, agrees. “The government cannot restrict speech unless there is evidence the speech is causing harm,” he explains. Tour guide patter hardly qualifies. Mr McNamara helped Segs in the City file a lawsuit against the city government in 2010. The city won; Mr Main’s appeal reached a DC federal court on May 5th. The city defends the licence as an essential safeguard for consumers.

In the 1950s only one American worker in 20 needed a permit from the government; today that figure is around one in three. Some jobs, such as doctors, clearly need strict controls. But some states require licences for florists and interior designers. Such permits tend to cost hundreds of dollars and months of extra training, yet offer little benefit to consumers, says Morris Kleiner, an economist at the University of Minnesota. Sometimes customers, like undercover tourists, can look after themselves.