All segments of the District of Columbia's taxicab industry joined yesterday in an agreement that is expected to result in an additional 10 to 12 percent increase in cab fares in the near future.

If granted by the city's Public Service Commission, which took the proposal under advisement, the increase would be the third this year. A 9 1/2 percent stopgap rise went into effect in January, followed by a 10-cent-a-ride surcharge in June.

Under yesterday's agreement, the cost of a one-zone ride -- including a trip within downtown -- would rise by only 5 cents, to $1.35. The cost of a four-zone trip from downtown to the outer edges of the city would rise by 35 cents, to $3.45.

The agreement was signed by leaders of five organizations representing factions within the city's splintered taxicab industry. It was endorsed by the D.C. Transportation Department and the public service Commission staff, as well as by the Office of the People's Counsel, which represents the public interest before the commission.

After a two-hour hearing on the issues, commission chairman Elizabeth Hayes Patterson indicated that a decision would be reached soon. She would not speculate whether the agreement would be accepted in full, but added: "We certainly will give it great weight, because it came about as a result of our (commission) directives."

Militant cab drivers, complaining that current fares have failed to keep pace with the rising cost of gasoline and other expenses, have conducted several demonstrations and partially successful strikes in recent months in an attempt to bring pressure for the further increase.

The application for an increase, originally of an unspecified amount, was filed in September 1978 by the Alliance of Taxicab Businessmen. A rival organization, the Taxicab Industry Group, later asked for a 10 percent rise.

Their applications led to the stop-gap increases that were authorized in January and June of this year.

On paper, the agreement now up for action calls for fares that are expected to produce 27 percent more revenue for drivers than the fares that were in effect a year ago. Taken together, the January and June increases added up to about a 15 percent rise, so the pending one would produce an estimated 10 to 12 percent more.

Because statistics on cab riding in Washington are too sketchy, Caroline M. Smith, economic consultant to the people's counsel, said it is impossible to make a precise estimate.

In August, one industry source said, there were 6,570 cabs insured and operating in the city, a decline from a peak of more than 10,000 in the 1950s.

After the commission decides on the new fares, it has agreed to study and hold hearings that could lead to redrawing the zone map used for calculating cab fares.

The signers of yesterday's agreement asked the commission to expand this to include consideration of the quality of cab service, including its availability in the far Southeast and other outlying neighborhoods, as well as a number of other economic issues. Among them would be improving the mechanism by which cab fares are raised as costs increase.

Following is a summary of the pending fare proposal: (Text omitted)