I’ve hitchhiked once in the U.S. (with my husband). We were picked up by an old couple who had never picked up hitch hikers before, so it worked out for all of us.
In March / April of 2011 we hitch hiked throughout Europe.
I’ve never hitch hiked alone, and wouldn’t recommend it. Go with at least 1 other person, and listen to your instincts! They will keep you safe.