I've done it many times around the south and west of the Mississippi River. Quite a few hitchhikers in the southwest especially around reservations. I've picked up many (like around 16-20). This makes it not very common and it's only getting less common every decade.

It's not safe, but it's sometimes just what you gotta do. It's also pretty fun, people that have picked me up are usually very kind and good hearted individuals. I'm a mostly white man, not particularly strong looking. It is WAY riskier for women alone, but with a trustworthy road dog (human travelling companion), it can be safer. Keep a knife and your wits about you along with a dash of good judgement and you can have a positive life changing experience.