I haven't been paying that close attention to the list recently, so perhaps
somebody already mentioned this.  Although not on the same scale as the
Watts Towers or concrete castles, the many scrap sculptures in front of
muffler shops seem to be an example of a tradition that began with a few
individuals and eventually became the norm for muffler culture.  In some
cases, these sculptures may have been considered whimsy, folk art, or good
advertising by their makers.  Regardless of the individual reasons, the
making of scrap sculptures came to be a distinctly muffler shop thing to do.
Students of folk art and culture might want to consider the spread and
variation of this tradition.  I don't have the references, but I believe
some interesting surveys have been conducted.

Chris Espenshade