In the aftermath of Saturday's tragic event, I've already heard it suggested that the cab driver (the infamous "person of interest) should have known that something was wrong with his passenger, Jared Loughner, and might have been able to prevent the shooting.
I have not spoken with the cab driver - I know of him, he's been a long time driver with Tucson Yellow Cab - but I know of him as a decent and hard working driver. The bottom line is that no one else was in the cab with the driver and Loughner, and we don't know at this point what, if any, conversation took place. Given the driver's reputation, I would have to say that if he thought or was suspicious that anything was amiss, he would have alerted someone immediately.
Cab drivers are not trained in the behavioral sciences, but can generally discern when something is amiss with a passenger. But it cannot be expected that we will or must ascertain a passenger's motives or plans when entering the vehicle, or put "two and two together" as we proceed to the destination. While what happened in Tucson is a horrific tragedy, don't blame the cab driver.
Arizona Daily Star, 1/9/11, "Authorities clear man called "person of interest" in Giffords shooting case