I have often wondered why a definitive explanation was never available. Well it is, for Sumter County anyway.
Here it is:
http://www.districtgov.org/images/Wh...INAL070908.pdf
I believe this is much the same as most localities.
Caveat!!! Don't count on everyone observing these instructions. Most Villagers have their own interpretation of "Roundabout Etiquette" or the lack thereof.
DC