Civil Engineering Reference
In-Depth Information
In each example the diagonal roadway entering the intersection is re-aligned to
create a simple right angle intersection with the main arterial. The distance between
the existing and newly created intersection should be set to avoid vehicle spillback
and should allow independent and coordinated operations with the main
intersection.
17.2.2.4 Roundabouts
Roundabouts emerged in the United States in the 1990s after several decades of
successful implementation in Great Britain, France, and Australia [
11
].
They differ from the earlier traf
c circles and rotaries in several respects: (1) the
diameter of the turning circle is much smaller, (2) vehicles entering the roundabout
yield to circulating vehicles, and (3) the entering traf
c is sometimes slightly
de
fl
ected.
Design Principles of Roundabouts
NCRP 672 [
11
] sets forth the following basic design principles for roundabouts:
Provide slow entry speeds and consistent speeds throughout the roundabout by
using de
fl
ection
Provide appropriate number of lanes and lane assignments to achieve adequate
capacity, lane volume balance, and lane continuity
Provide smooth channelization that is intuitive to drivers and results in vehicles
naturally using intended lanes
Provide adequate accommodation for the design vehicles
Design to meet needs of pedestrian and cyclists
Provide appropriate sight distance and visibility for driver recognition of the
intersection and con
fl
icting users.
Features, Types, and Dimensions
The key features, types, and dimensions of modern roundabouts are shown in
Fig.
17.7
.
The types include: (1) mini-roundabouts, (2) urban compact roundabouts,
(3) urban single-lane roundabouts, (4) urban double-lane roundabouts, (5) rural
single-lane roundabouts, and (6) rural double-lane roundabouts.
The roadway dimensions vary with the types of roundabouts and the design
vehicles. Some key roundabouts entry speeds and design vehicle-related dimen-
sions are shown in Table
17.2
.
