Although it is a big challenge (though not as big as an all electric mandate), I think a better long term solution to this problem would be to create a trolley system. The trollies could easily be electric with planned charging stations, it would generate money for the city, it would be infinately easier on the environment, and it would eliminate many of the other traffic problems that we sometimes experience (noise, crowding, etc).
The biggest obstacle to doing something of this nature is likely to be resistance from the taxi drivers associations. There are a great number of people who depend on their taxis for their livelyhood and this must not be ignored.