HomeMy WebLinkAboutMULBERRY LEMAY CROSSINGS, LOT ONE, FILING ONE - FINAL PUD - 36-96D - CORRESPONDENCE - CITY HALLMULBERRY
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.` ed Shepard - Ra vs_tc Page ?,_
Traffic Circles vs. Modern Roundabouts
Modern Roundabouts Nonconforming Traffic Circle
Entering Traffic Yields to
Circulating Traffic
♦ Circulating traffic always
keeps moving
Works well with heavy traffic
No weaving distance is necessary
Roundabouts are compact
Entering Traffic aims at the
center of the central island
and is deflected around it.
• Slows traffic on fast roads
and reduces accidents
Deflection promotes yielding
process
Upstream Roadway often
flares at entry, adding lanes.
♦ Provides high capacity in a
compact space
Permits reduced number of lanes
between roundabouts saving
pavement, land and bridge
area
Entering traffic cuts off circulating
traffic
♦ Circulating Traffic comes to a dead
stop when the circle fills with
entering traffic
Breaks down with heavy traffic
Long weaving distances for merging
entries cause the circles to be large
Entering traffic aims to the right of
the central island and proceeds
straight ahead at speed.
♦ Causes serious accidents if used on
fast roads
Fast entries deflect the yielding
process
Lanes are not added at entry
♦ Causes serious accidents if used on
fast roads
♦ For high capacity, requires multi-
lane roads between circles, wasting
pavement, land and bridge area
Ted epard - Ra vs tc _ Page 1
Memorandum — via email
DATE: December 27, 1999
TO: John Fischbach, City Manager
THRU: Ron Phillips, Transportation Services Director
Gary Diede, Transportation Operations Director
FROM: Eric L. Bracke, P.E.
Traffic Engineer
RE: Traffic Circles vs. Modern Roundabouts
As part of the Mulberry and Lemay intersection analysis project, questions have come up
regarding the differences between non -conforming traffic circles and modern roundabouts. Many
people rely on their past experience with the old type of traffic circles that are in the eastern part
of the United States. These experiences are not always positive and they bring that opinion to
bear in their reactions to the proposed roundabout at the intersection of Mulberry and Lemay.
There are major differences between a nonconforming traffic circle and a modern roundabout.
The two key differences are traffic control and geometric design. The traffic control for a non-
conforming traffic circle is a yield line within the actual traffic circle. With heavy traffic — OLD
volumes, the traffic within the circle yields to entering traffic, the circle fills with automobiles
and there is a total breakdown in traffic as vehicles are blocked from exiting. With a Modern
Roundabout, all entering traffic yields at the entry line. Therefore, the actual circulating lanes
can never fill without opportunities to exit and the circulating traffic keeps moving.
The primary geometric design difference between the traffic circle and the modern roundabout is
the entry angle. In the case of the traffic circle, the entry angle tends to be perpendicular to the - oc-o
center of the circle, causing a difficult entry turn. The modern roundabout design incorporates a
splitter island that slows down traffic as it approaches the yield line and makes for a safer entry. - n1EA
The first modern roundabout was constructed in the United States approximately nine years ago
in a suburb of Las Vegas. Since that time, numerous roundabouts have been constructed and
FHWA estimates that over 50,000 modern roundabouts will be constructed in the United States
in the next fifteen years. This large number of roundabouts is due to the fact that roundabouts are
safer, more efficient, and less costly to maintain than traditional intersections. /v a S/t. wea S
The following table displays the differences between the modern roundabout and the old style
traffic circle.
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