From: "Randy Presuhn" <randy_presuhn(_at_)mindspring(_dot_)com>
I had to google it as well. The word "roundabout" (in the
sense of "traffic circle") led me to mistakenly think it
had something to do with navigating British streets, but
this seems to be where the idiom comes from:
http://www.oldpoetry.com/Patrick_R_Chalmers/Roundabouts_and_Swings
Randy
I am pretty sure that the usage of "roundabout" to refer to a traffic
circle is derived from its usage as a carnival ride, which in the US would
be called a "merry-go-round".
Janet