You wanted to fly USAir home for Thanksgiving. But your roommate was trying to accumulate frequent flyer miles on Delta.

So the two of you hop in a cab. Forty minutes later, you're both out $18.

If it seems like an injustice to pay twice for the same ride, the Cambridge License Commission agrees.

The commission yesterday voted to exclude trips to different airport terminals made through the "Share A Cab" program, under which two or more passengers are each charged a flat rate for trips to the airport.

In the past, any two passengers bound for different airport terminals were always charged a flat rate which was sometimes more expensive then if they had traveled without the "Share A Cab" condition and were charged by the meter and had split the fare.

With the change, individuals making a prearranged trip to the airport from the same location will not always constitute a "Share A Cab" situation, said Tom R. Cromwell, President of Ambassador Brattle Cab.

There is no additional charge for baggage, tolls or gratuities, Cromwell said.

"During peak periods when there is a scarcity of taxis, this allows the driver to get more than one fare," said Mike P. Holland, Director of Operations of the Brattle Transportation Services.

But the definition of the program created problems by charging the flat rate to all groups of two or more passengers traveling to two separate airport terminals automatically as part of "Share A Cab," Cromwell said.

The classic problem, Cromwell said, was when a husband and wife traveled to the airport together but went to two separate terminal. By definition, they previously would have constituted a "Share A Cab" situation.

"Everything is more efficient," Holland said. "The customer saves money, the driver makes out and conserves fueland reduces auto emissions."

The commission postponed the voting of somemore controversial recommendations until December16. These recommendations include the 'vintagerule,' which requires taxis to be less than eightyears old, and improvement in the printingcapabilities of the meters.

Some taxi drivers and committee members havesaid the new requirements would be unnecessary andcostly. Others have said that the changes areneeded to ensure efficient service and goodcondition of cabs.

Other proposals aimed at increasing thecompetitiveness of the taxis and protecting theindustry will be voted on in January.

These proposals include the enforcement ofhotel parking and private property regulationswhich currently favor limousines and discriminateagainst taxis, Commission Executive OfficerRichard V. Scali said yesterday