THERE PROBABLY WON'T be many cheers from the back seat, but Washington's cab drivers did deserve the fare increases that were granted them yesterday. With jumps in gasoline prices of as much as 40 percent since January, they have been taking quite a financial beating -- for until now, the only fare increase they had received since January was an interim addition of a dime to the basic ride, no matter how many miles long that ride was.

For a four-zone ride, that amounted to a tiny increase -- about 3 percent. Then the D.C. Public Service Commission, which has authority over fares, took an unnecessarily slow look at drivers' expenses before approving the latest fare schedule. Now begins the next unnecessarily slow study by the commission, one that should have been made long ago, with all the others: to determine what more should be done 1) to improve the quality of cab service and 2) to change the zone system by which fares are computed.

The quality and fare schedule are related, too. There is, for example, the group-ride fare arrangement that encourages drivers to pick up passengers for different destinations. But how many times had this resulted in a 12-block, 15-minute side trip to drop off that second passenger when you were first and in a hurry? And meters or no meters, it would help drivers and passengers if there were a better factoring into the fares of time spent and miles driven during each ride; maybe then drivers would stop vanishing during rush hours or in bad weather.

Having granted fare increases without tackling these questions, the commission has created still another subject for study: will the new fares, which take effect at midnight, bring any relief for people traveling to and from the outlying areas such as Anacostia, or will the phone calls and waves for these trips be ignored? In the meantime, how the public reacts to the higher fares could depend considerably on how conscientious and law-abiding the cab drivers are in return for the financial relief they have won.