Washington's taxi riders will no longer have to share their cabs with strangers under a regulation adopted yesterday by the D.C. Public Service Commission.

The rule, which is expected to take effect in about two weeks, will not eliminate cab sharing but gives passengers the right to order cab drivers not to pick up additional passengers. It reverses an earlier PSC ruling, passed in 1973 during the gasoline crisis, that gave cab drivers the authority to decide whether to take additional fares.

Over the past 10 years, cab sharing has become commonplace in Washington, particularly on trips throughout downtown and to and from Capitol Hill.

Spokesmen for the city's taxi drivers blasted the change as unnecessary and said they are considering whether to challenge the ruling in court. The spokesmen also said the ruling will sharply reduce cab drivers' incomes because many people would rather not share cabs if given the choice.

However, Commissioner Patricia Clement said the drivers' claims of revenue loss were "mere supposition."

The three-member commission approved the change by a 2-to-1 vote with Clement and commission Chairman Ruth Hankins-Nesbitt voting for it. Commissioner Wesley H. Long voted against the change.

"We're terribly disappointed that the commission could arbitrarily make such a change," said Fred D. Matthews, executive secretary of the Taxi Industry Group, which he said represents more than 60 percent of the city's cab drivers. Matthews said that the District's system of charging passengers by zones, as opposed to the generally more expensive meter systems used in other major cities, is based in part on the assumption that drivers can boost fares by having passengers share rides.

"No evidence has been given by the commission to justify such a radical change in the system," added Fred Denby, executive director of the Professional Cab Drivers Association.

Both Denby and Matthews said no one filed a formal request with the commission to seek the change.

Hankins-Nesbitt said the new rule was designed to benefit the public. A commission staff member said in an interview that the commission decided to initiate the change in response to telephone calls from citizens complaining about drivers abusing the cab-sharing privilege.

Cab-sharing authority has been bandied back and forth for many years. During World War II, drivers were given the authority to decide whether to pick up additional passengers. In 1957, the ruling changed, and passengers were allowed to decide whether they wished to share their cab rides. And in 1973, during the gasoline crisis, drivers were again given the authority to carry multiple fares as an energy conservation measure.

Taxi industry spokesman Matthews said he believed that the vast majority of drivers were being punished because a small number of drivers have abused the cab-sharing privilege.

Matthews said that the first passenger in a cab is protected by existing rules that prevent a driver from deviating by more than five blocks from the first passenger's route to drop off another passenger.

He said he expects some drivers to refuse to pick up potential customers who are unwilling to share their cabs.