The following is from powerpoints for workshops I did on Roundabouts and Channelized Right-Turning Lanes for the SOMA/COMA and CAOMS conferences in 2012.
The drawing of the roundabout comes from Roundabouts: An Informational Guide (NCHRP Report 672)
Roundabouts (RABs) are NOT mini-roundabouts or large traffic circles!
They have the following features, as shown in the illustration to the right:
FEATURES:
Circular vehicular pathway (counterclockwise)
Splitter islands separating entry lanes from exit lanes
Crosswalks set back from “intersecting street”
Drivers entering RAB yield to traffic in the circle
Roundabouts usually also have no stop sign or traffic signal.
Important considerations of roundabout features are:
How many lanes does it have?
Multiple lanes present considerably more risk than one lane,
not only because it takes twice as long to cross,
but because drivers in the second lane may not see pedestrians
crossing in front of yielding vehicles in the first lane,
as illustrated to the right.
What is the turning radius?
A tight turning radius will make the drivers slow down and be more likely to yield, whereas a larger turning radius enables drivers to speed more.