See the rules for... 
What is a roundabout?
A modern roundabout is a circular intersection without traffic signals, engineered to keep traffic moving smoothly and safely. Over the last few decades, thousands of roundabouts have been installed in Europe, Australia, the United States, and other parts of the world.
In Oregon,
the cities of Portland, Bend, and Springfield have installed several
roundabouts and plan for more. In the cities and towns where roundabouts
are used, drivers praise them for the increased capacity and safety
they provide, along with traffic calming and aesthetic benefits. Studies
done in the U.S. show that after converting more traditional intersections
to roundabouts, total crashes declined by 39%. Injury crashes declined
by 76%, and serious injury or fatal crashes dropped by 89%.
Other useful sites:
How
to drive in a roundabout (ODOT)
How
to drive in a roundabout (DOT of New York)