Conversation so far:
How can the lyrics be interperted?
Johnson describes his grief at being unable to catch a ride at an intersection before the sun sets.
Why would he grieve over that?
A crossroads is also where cars are more likely to slow down or stop, thus presenting the best opportunity for a hitchiker.
Did he at some point want to be picked up?
he expresses apprehension at being stranded as darkness approaches and asks that his friend Willie Brown be advised that "I'm sinkin' down".
How else can the song be interpreted?
The song has been used to perpetuate the myth of Johnson selling his soul to the Devil for his musical ability.

Question: Why is it speculated he sold his soul?
Self-contained rewrite: Is it speculated Johnson that sold a soul to Devil for his musical ability?