I once was a cabbie: Yellow Cabs, Los Angeles. Not for long, though.
I know that when a rear-seat passenger pricks your neck with a knife, or points a pistol at your head, there is precious little that even an armed cab driver can do about it.
Think about pulling your own pistol out, turning around in your seat while steering a moving vehicle, and then overcoming a guy who "has the drop on you." The odds are seriously against you.
We were told that the best response was driving like a maniac to scare the heck outta the robber-doofus, while doing everything you can to attract the attention of the nearest cop.
Get pulled over. See what Mr. Robber-Doofus does then.
Seemed a good plan to me.