Carry in a shoulder holster and shoot through the seat - saw that done once by a Ft. Worth cabby. In Texas, you are also allowed to shoot the fleeing robber, so you can give him your money, let him start running, then send a lead messenger after him.
"Look, live game!"
Not worth your life for a few bucks....
However, once he's clear of the cab one should have the ability to light his ass up or even better, pancake the SOB.