I have to disagree. For the exact reason that you state, that people who stop have a reason to do so. I have only found over the years that people who stop to give rides to hitchhikers are people who have hitchhiked themselves, or are people who are looking out for your best interests, i.e. people who think that by picking you up they are "saving" you from other people who might have a less than altruistic plan in mind.

As a hitchhiker I have never had problems with people who have offered me rides. Instead I have had problems with people in towns. People who do not travel and have never had to ask strangers for help.