I like Shirley Knight as an actress but her character is a bloomin' idiot. She actually thought picking up a stranger for a one night stand is a good idea as a way to learn about herself? And leaving an upright husband to do it? Sounds like navel gazing to me.
I didn't like her at all... Some devious characters have charm, or a reason you like their performance, but i can't say the same about her. The actual character was a degenerate bitch, too.
1969 was a very different time, and that part of the film is very true to that time. People in general were much more open to picking up strangers. There was a far more trusting zeitgeist. As a young man back then, I [picked up hitchhikers & hitchhiked myself without a worry. Wouldn't do it today, though. Times have changed for the worse.
And finding oneself through sexuality was much more acceptable back then, as well.
Also, she wasn't 100% sure about picking him up. After stopping, she changed her mind and started to ride away, but saw him in the rearview mirror wherein she (evidently) felt sorry for him. Only then did she stop.