BANGKOK — Bangkok’s legendary traffic jams may be getting a bit lighter. Or at least less of a joke.

A handbook distributed to police and other authorities Friday updates guidelines for motorcades and other public appearances by Thailand’s royal family. It overturns practices that irritated the public in a nation where open criticism of the royal family is illegal — punishable by up to 15 years in prison.

The palace hopes the changes will end a long-running joke about traffic congestion in the capital of more than 12 million people.

‘‘Every time there’s a traffic jam, everyone wonders if there’s a royal motorcade passing by,’’ palace official Chantanee Thanarak told a police training session in Bangkok.

About a dozen members of Thailand’s extended royal family travel by motorcade, including 84-year-old King Bhumibol Adulyadej, the queen, their children, and grandchildren.

Bhumibol initiated the changes himself, officials said.