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Antonio Pratelli Department of Civil Engineering
Vie e Trasporti University of Pisa (Italy)
DESIGN OF MODERN ROUNDABOUTS IN URBAN TRAFFIC
SYSTEMS
Lectures at the IOWA STATE UNIVERSITY April 2009
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Outline

• Introduction
• Main features
• Traffic safety issues
• Public opinion
• Conclusions
• Sample cases

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Introduction (1)

• First generation (Traffic Circles)
• - priority to entering traffic flows
• - design considers weaving movements as basic
  goal
• capacity is related to each track between
  successive branches.
• Second generation (Modern Roundabouts)
• - priority-to-circle
• - design is conceived for lowering speeds
• - capacity is related to each entrance.

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Introduction (2)
1903 (New York) Columbus Circle project 1907
(Paris) Place de lEtoile layout (today Place
Charles De Gualle) 1926 roundabouts became
traffic management tools for intersections with
priority-to-branches rule (first
generation). 1966 United Kingdom introduces the
priority-to-the-circle rule is the birthday of
modern roundabouts (or II generation). 1984
France introduces the priority-to-the-circle
rule and publishes technical guidelines
(1988) Other EU countries follow, as
Switzerland (1991) Netherland (1992) Germany
(1993) Italy (1994) 2000 about 15.000 modern
roundabouts are operating in France.
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Columbus Circle, New York, in a picture of 1915
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Main features (1)

• Three categories
• MINI roundabouts
• (D lt 22 mt)
• COMPACT roundabouts (22 lt D lt 35-40 mt)
• LARGE roundabouts (D gt 40 mt)

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Main features (2)
ENTRY CAPACITY has been investigated using two
approaches a) the gap-acceptance theory b)
the empirical regression method.
Most design guidelines use statistically derived
linear formulas, such as Ce f(geometry, Qc, Qu)
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Traffic safety issues (1)
Basic aspects 1) Lowered circulating speeds,
this is achieved by entry path deflection
(expecially in well-designed roundabouts). 2)
Reduced number of conflict points.
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(European Project Master, Final Report, 1998)
Influence of the average speed variations on
accidents.
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• Without path deflection entering traffic aims to
  the right of the central island and proceeds
  straight ahead at speed, then
• causes serious accidents, expecially if applied
  on fast roads
• the yielding process is defeated by fast
  entries.

To avoid!
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Vehicle path deflection requirements from Italian
standard guidelines.
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A four-legged roundabout shows 8 instead 32
conflict points of a four-way intersection.
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At least a 7.4 average annual reduction is
needed to halve road deaths between 2001 and 2010
Roughly 43.000 people are still dying every year
on Europe's roads.
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• EU 
• Although there has been a 20 drop of fatal
  accidents in seven years, the EU statistic is
  still some way off from its 50 target.
• Italy
• In 2007, 5313 people died on the Italian roads
  25 less than in 2001.
• Italy needs to do further efforts to progress in
  the way of improving raod safety.

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Traffic safety issues (2)

• Crash Type
• 1) Total crashes
• 2) Injuries
• 3) Moped/Bicycle injuries
• (based on a BA study in the Netherlands, 1992)

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Traffic safety issues (3)
Annual Crash Frequencies and Size of Inscribed
Diameter (Statistics for 83 roundabouts in
France, 1986)
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Public Opinion
Percent of respondents in favour of/opposed to a
roundabout construction (BA surveys in USA,
2000-2002).
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Ponte a Ema project example
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1
3
1
4
AFTER Compact roundabout (a30,7 m b26,6 m
b/a0,87)
BEFORE 4-arms signalized intersection
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Ponte a Ema project example
BA entry average delays during AM peak
calculated for Ponte a Ema roundabout project,
Florence (It.)
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Pollutant emission functions for buses with hot
engine (J.Hickman et. al., Methodology for
calculating emissions and energy consumption
MEET Project Final Report for the European
Commission DGVII by Transport Research
Laboratory, 1999).
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Conclusions
Modern roundabouts are one of the most important
traffic engineering development, both on the side
of road safety and operations. A brief overview
of modern roundabouts design and practice is
made. We wish conclude showing some sample cases
of roundabouts in Italy (all my own projects).
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Example n.1
Double-mini roundabout. Via Tonfano, Marina di
Pietrasanta, Lucca, (It.) 2003
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Example n.1 Satellite view of the previous
state.
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1
X
2
Example n.1 Layout (D120 m D219 m X7 m)
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Example n.1 Westbound aerial view of the mini
twins.
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Example n.1 Eastbound roadway view.
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Example n.1 Westbound roadway view with bus.
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Example n.2
Double-large roundabout. Loc. Battifollo,
Sarzana, La Spezia (It.) 2005
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Example n.2 Satellite view of the previous state
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2
1
X
Example n.2 Layout (D150 m D235 m X 50 m)
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Example n.2 Night lighting design simulation.
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Example n.2 Aerial Southbound view during the
construction stage.
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Example n.2 Northbound roadway view.
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Example n.2 Satellite view of the actual state.
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Example n.3
Elliptical compact roundabout. Viale
Carducci-Viale Europa, Lucca, (It.) 2006
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Example n.3 Aerial Westbound view of the
previous experimental roundabout (dangerous due
to its poor geometry).
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Example n.3 Layout with weekday AM peak flows
(A42 m B37,5 m ratio B/A0,89).
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Example n.3 Satellite view of the actual state.
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Example n.3 Northbound view during construction
(April 2006).
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Example n.3 Southbound view during weekday PM
peak (April 2009).
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Example n.3 Observed exiting flows on each
branch (after study).
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Example n.3 Observed average exiting flow
speeds (after study).
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Example n.4
Roundabout modernization. Loc. Querciola, Sesto
Fiorentino, (It.) 2005
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Example n.4 Map of the previous state (D55 m
single-lane entries low circulatory lane width).
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Example n.4 Satellite view of the previous
state (D55 m single-lane entries low
circulatory lane width).
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Example n.4 Design layout (D54 m double-lane
entries large circulatory lane width).
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Example n.4 Expected results (simulated
scenario AM peak)
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Example n.4 Aerial Eastbound view of the actual
state.
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Example n.5
Improving C.B.D. accessibility. Porta S.Maria,
Lucca, (It.) 2009 (work in progress)
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C.B.D.
Example n.5 City map of Lucca the CBD (i.e.,
historical city centre) and Porta S. Maria
location are highlighted.
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90 m
Example n.5 Two satellite views of Porta S.
Maria roundabout in its present state.
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Example n.5 Aerial NB view of the Porta S.
Maria roundabout in its present state.
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YELLOW actual RED design
Example n.5 The basic idea a new roundabout
(Dn60 m) built inside the actual one (Da90 m)
and surrounded by an external lane with parking
stalls.
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Example n.5 Design layout of the improved
roundabout reduced Dn60 m and over 65 parking
stalls on its external gained space (accepted by
the Major).
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Antonio Pratelli Department of Civil Engineering
Vie e Trasporti University of Pisa (Italy)
DESIGN OF MODERN ROUNDABOUTS IN URBAN TRAFFIC
SYSTEMS
Thank you very much
Lectures at the IOWA STATE UNIVERSITY April 2009