Of course, the answer will vary by jurisdiction, but my WAG is that most of the 50 states will have very similar laws. And perhaps it’s even a federal matter (hiking on the Interstate, anyway).
According to Wikipedia it’s not illegal but local laws may restrict it.
When I really came into this thread to ask is, why “Really Bad Idea”. There are countries in the hitchhiking is common and accepted, is it really not like that in the states? Sure, I can see problems but in reality the number of nutcases out there is vanishingly small.
SD
I am pretty sure it isn’t outlawed everywhere and Wikipedia hints at that. It may be illegal near some Interstate highways but that certainly doesn’t constitute all road, even many big ones. I have seen news stories that send out reporters to test the current state of hitchhiking in the U.S. and they seem to so fine even for long distances. It would be difficult to completely outlaw anyway. It isn’t illegal to walk beside the road in many places and some people will just stop and offer a ride. It happened to me in the U.S. Virgin Islands a month ago and I had a great time.
The U.S. has a rather large population and very aggressive news sensationalism. Hitchhiking was common up until the 1970’s. I don’t know what put the fear of God into people but it had to a mere handful of cases. The problem is circular. People are reluctant to pick up hitchhikers which makes hitchhiking more difficult so most people don’t bother. The very large size of the U.S. geographically also makes it hard to find people that are going near where you want to go and you may have to score many rides to get there especially if you want to go coast to coast. Long haul truck drivers are more likely to pick up hitchhikers than others because they get bored. A grown man is unlikely to have safety problems with professional drivers.
I had a cop in Tennessee tell me it was illegal. Don’t know how true it was though.
A classmate of my girlfriend disappeared while hitchhiking between home and college.
Just sayin’.
This site has some good summaries of specific provisions. USA Laws on Hitching
This looks pretty good too: http://www.digihitch.com/search---0--usa-7.html
In Virginia, it’s illegal to stand or stop in any roadway for the purpose of soliciting rides. It’s also illegal to enter, as a pedestrian, any highway marked with a sign prohibiting pedestrian traffic. Most every interstate in Virginia is so marked.
It is illegal, but only in the roadway.
Would that mean that trudging along the road with a sign on your back saying “give me a lift?” would be legal, as long as you keep walking?
Isn’t that just on the interstate highways? It’s been a long time since I last hitchhiked but it was mainly in VA and I never had a problem with police. I haven’t actually seen more than a handful of hitchhikers in the past 10 years though, so maybe that’s the reason.
It’s illegal to enter the interstates as a pedestrian for any reason. It’s illegal to stand in any roadway and solicit rides. You’re allowed to use the sidewalk or shoulder to solicit rides - but not on the interstates, where you can’t enter at all.
Good question. 
I can’t find any appellate cases on point in a quick search.