It varies from state to state.
Some are legal for it to be done on the shoulder of the road. (green)
Others off the travelled part of the road (eg stand in grass). (yellow)
Others – completely illegal. (red)
Others – unclear. (grey)
There’s a map and guide on Hitchwiki indicating the status for each one. This comes from the main USA page on Hitchwiki.
Table of Contents
Source: DEL CODE § 4147
Source: 39:4-59
Source: NRS 484B.297
Source: 49-709
Source: 41_04117
Note: federal laws supersede state laws, and the federal law states that the legality of hitchhiking within national parks is decided by the superintendent of that park (Source: CFR Title 36 section 4.31).
Credit: All the hard work was done by Hitchwiki, I just compiled all the references in one place.
Credit:stackoverflow.com‘