Maybe. A lot of circumstances would come in to play in my assessing the risk. Location, apparent reason for hitchhiking (stranded car? walking down the street/highway? alone or friends/couple? ...etc etc), and how they look. I'm not too trusting of people I just meet, so giving a total stranger a lift is high on the list of untrustworthy. And how an individual looks and acts, including clothing, accessories, and how they talk. But since there is the range of thug threat, to high-class conman threat, that's even tough.
I'm a good samaritan when the need arises, but picking up a hitchhiker is probably something I won't do unless I'm carrying a firearm.

Unless I've had a chance to converse with the individual, or I see they are injured and that is their reason, such as being a victim of an attack.
edit: replaced 'hijacker' with the correct word 'hitchhiker'. Although it might turn out to be the former in a few instances.
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