It’s occurred to most of us at one time or another—is it worth buying metro tickets in cities where public transportation ticketing is based on the honor system? Travelers on a budget might consider saving money by “riding black,” and in Prague, at least, they’d have ample company.

A recent study published by Czech newspaper Mlada Fronta Dnes found that one in six riders goes without a valid ticket. If you’re caught by one of the city’s plainclothes inspectors and you pay up right away, the penalty is CZK500 (€18; $23).

Of course, riding black goes against the spirit of the system and we don’t recommend it. But knowing the odds is a valuable piece of information, especially in cities with more expensive forms of public transportation. In Prague, you’re better off buying the CZK14 ticket (€.50; $.65) than risking it.