We’ve been here before:
http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=82385
and
http://boards.straightdope.com/sdmb/showthread.php?threadid=63168
and
http://boards.straightdope.com/sdmb/showthread.php?threadid=21449
The best guess would seem to be the “hippie trail”, but the explanation seems unsatisfactory to me, though it would have been contemporary with the song. And an allegorical thing concerning the Beatles doesn’t seem likely, given that the rest of the references are so concretely historical.