roundabout ~' ~ design guidelines - Institute of Transportation ...

Intersection Treatment
Mean Casualty
Accident Rate
Typical Range of
Casualty Accident
Rates
T-Intersections -Unsignalized 1.5 1.3 - 1.7
-Signalized 1.4 1.2 - 1.6
Cross-intersections -Unsignalized 2.4 2.1 - 2.7
-Signalized 1.7 1.6 - 1.8
Multi-leg Intersections -Signalized 3.2 2.8 - 3.6
Roundabouts (high volumes) 0.8 0.6 - 1.1
Roundabouts (low volumes) 0.4 0.1 - 1.0
TABLE 3.8 Typical Casualty Accident Rates for Diflerent Urban Intersections
with Moderate to High Volumes in Victoria, Australia
The previous tabulation (Table 3.8) illustrates the result of comparative studies carried
out in Australia. Similar results have been obtained in the UK.
The good safety record of properly designed roundabouts can be attributed to the
following factors:
The general reduction in conflicting traffic speeds (limited to less than 30 MPH)
passing through the intersection on all legs.
Elimination
conflicting
of high angles of conflict thereby ensuring low relative speeds between
vehicles.
Relative simplicity
of decision making at the point of entry.
On undivided roads, in high speed areas, long splitter islands provide good “advance
warning” of the presence of the intersection.
Splitter islands provide refuge for pedestrians and permit them to cross one direction
of traffic at a time.
Roundabouts always require a “conscious action” on the part of all drivers passing
through the intersection, regardless of whether other vehicles are present or not.
An understanding of the above factors and their implications, in respect to the geometric
design of roundabouts is essential to the full achievement of the safety benefits of
roundabouts.
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