A woman tries to hail a yellow cab displaying off-duty lights last month. Agence France-Presse/Getty Images

For years, travelers in New York City have peered at passing taxi lights as if parsing the uncertain messages in tea leaves.

The light atop a yellow cab could indicate opposite meanings, like a kind of urban Zen koan. It could mean the vehicle was available or full, done with a shift or in search of a fare (or both).

"The off-duty lights were just confusing," said Taxi and Limousine Commissioner David Yassky.

No longer. After more than a year of review, the TLC voted 7-1 Thursday for a simpler system. The city will adopt a binary method used in other cities with large taxi fleets, including London: A light means the taxi is available. No light means it's not. Period.

"I think to many New Yorkers it will just seem like plain, common sense," Mr. Yassky said.

In large part, confusion under the current system stemmed from two distinct sets of lights that separately indicated availability and off-duty status. The situation was made even murkier by the fact that off-duty cabs could still pick up passengers, albeit with the right to refuse inconvenient destinations (sorry, Brooklyn).

Adapting cabs to the new system—changes that officials called "minimal"—will be completed at vehicles' next scheduled inspections, between Jan. 1 and April 30.

Some drivers initially resisted the changes, fearing that they would hamper their ability to pick up additional fares, officials said. Under the new rule, drivers will still be able to take as many breaks as they want and will still be allowed to pick up passengers when they are off-duty.

Iris Weinshall, former transportation commissioner, cast the lone dissenting vote. She didn't return requests for comment.

City officials began considering changes to the taxi lights amid talks over designs for the Taxi of Tomorrow, which is set to hit the streets next year.

A survey of 7,353 taxicab passengers conducted by the TLC earlier this year found widespread confusion. A third of riders said that a roof light didn't necessarily signal availability; others mistakenly believed it always indicated availability.

From a distance, people found it difficult to distinguish between the off-duty light and the so-called medallion light, which means the cab is available for passengers, according to the survey. The result: needless arm flapping and the mistaken conclusion that the would-be rider had been deliberately—and illegally—ignored.

Other times, officials admit, those suspicions were true. Drivers would sometimes switch on the off-duty light so they could cherry-pick passengers. Drivers on duty are required to take passengers to any destination in the five boroughs, Nassau County or Westchester County.

Now riders will be able to hail darkened cabs with clearer expectations, officials said.