Watch the Road Weary online guitar lesson by David Blacker from Lone Wolf Blues
In Road Weary, the concept of dropping the tuning of the E string down to D opens up a bunch of choices for a rootsy style blues in D major. The drop D allows us to mostly use open position chords which is what we want when playing in this style. The effect of the bass notes alternating between the low D of the open 6th string and the D octave on the open 4th string creates a hypnotic drone effect while the melody is picked out around a standard D major shape.
In the first four measures you have a melody that alternates between a Dsus2 and D major. Part of the haunting quality of this piece is the ambiguity between minor and major tonality created by the Dsus2 and the drone of the Travis picked bass notes.
Over the IV chord (G major) the bass notes alternate between the major 3rd and the 5th which is slightly more dissonant than the standard root to 5th or root to root Travis picking pattern. This helps create a nice tension leading back to the I chord.
Recommended listening: check out the song "Green Green Rocky Road" by Dave Van Ronk.