Mayor Plans an All-Hybrid Taxi Fleet 
I guess they are trying to eliminate all but hybrid taxis.
While the plan does not specifically require that the new taxis be hybrids, it calls for all new vehicles entering the fleet beginning in October 2008 to get at least 25 miles to the gallon, rising to 30 miles to the gallon for cars entering the fleet the following year. City officials said the only vehicles that currently meet those fuel standards, as well as tougher emission standards that the mayor is proposing, are hybrids.