1、,DESIGN OF MODERN ROUNDABOUTS IN URBAN TRAFFIC SYSTEMS,Antonio Pratelli Department of Civil Engineering “Vie e Trasporti” University of Pisa (Italy),Lectures at the IOWA STATE UNIVERSITY April 2009,Introduction Main features Traffic safety issues Public opinion Conclusions Sample cases,Outline,First

2、 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

3、is related to each entrance.,Introduction (1),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 in

4、troduces 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

5、) 2000: about 15.000 modern roundabouts are operating in France.,Introduction (2),Columbus Circle, New York, in a picture of 1915,Main features (1),Three categories: MINI roundabouts (D 40 mt),Main features (2),ENTRY CAPACITY has been investigated using two approaches: a) the “gap-acceptance” theory

6、; b) the empirical regression method.,Most design guidelines use statistically derived linear formulas, such as: Ce = f(geometry, Qc, Qu),Traffic safety issues (1),Basic aspects: 1) Lowered circulating speeds, this is achieved by entry path deflection (expecially in well-designed roundabouts). 2) Re

7、duced number of conflict points.,Influence of the average speed variations on accidents.,(European Project “Master”, Final Report, 1998),Without path deflection entering traffic aims to the right of the central island and proceeds straight ahead at speed, then: causes serious accidents, expecially i

8、f applied on fast roads; the yielding process is defeated by fast entries.,To avoid!,Vehicle path deflection requirements from Italian standard guidelines.,A four-legged roundabout shows 8 instead 32 conflict points of a four-way intersection.,At least a 7.4% average annual reduction is needed to ha

9、lve road deaths between 2001 and 2010,Roughly 43.000 people are still dying every year on Europes roads.,EU: Although there has been a 20% drop of fatal accidents in seven years, the EU statistic is stillsome way off from its 50% target. Italy: In 2007, 5313 people died on the Italian roads; 25% les

10、s than in 2001. Italy needs to do further efforts to progress in the way of improving raod safety.,Traffic safety issues (2),Crash Type: 1) Total crashes 2) Injuries 3) Moped/Bicycle injuries (based on a B b=26,6 m; b/a=0,87),1,2,3,4,1,B D2=19 m; X=7 m),1,2,X,Example n.1 Westbound aerial view of the

11、 “mini twins”.,Example n.1 Eastbound roadway view.,Example n.1 Westbound roadway view with bus.,Example n.2,Double-large roundabout. Loc. “Battifollo”, Sarzana, La Spezia (It.) 2005,Example n.2 Satellite view of the previous state,Example n.2 Layout (D1=50 m; D2=35 m; X= 50 m),1,2,X,Example n.2 Nigh

12、t lighting design simulation.,Example n.2 Aerial Southbound view during the construction stage.,Example n.2 Northbound roadway view.,Example n.2 Satellite view of the actual state.,Example n.3,Elliptical compact roundabout. Viale Carducci-Viale Europa, Lucca, (It.) 2006,Example n.3 Aerial Westbound

13、view of the previous experimental roundabout (dangerous due to its poor geometry).,Example n.3 Layout with weekday AM peak flows (A=42 m; B=37,5 m; ratio B/A=0,89).,Example n.3 Satellite view of the actual state.,Example n.3 Northbound view during construction (April 2006).,Example n.3 Southbound vi

14、ew during weekday PM peak (April 2009).,Example n.3 Observed exiting flows on each branch (after study).,Example n.3 Observed average exiting flow speeds (after study).,Example n.4,Roundabout modernization. Loc. “Querciola”, Sesto Fiorentino, (It.) 2005,Example n.4 Map of the previous state (D=55 m;

15、 single-lane entries; low circulatory lane width).,Example n.4 Satellite view of the previous state (D=55 m; single-lane entries; low circulatory lane width).,Example n.4 Design layout (D=54 m; double-lane entries; large circulatory lane width).,Example n.4 Expected results (simulated scenario: AM p

16、eak),Example n.4 Aerial Eastbound view of the actual state.,Example n.5,Improving C.B.D. accessibility. Porta S.Maria, Lucca, (It.) 2009 (work in progress),Example n.5 City map of Lucca; the CBD (i.e., historical city centre) and Porta S. Maria location are highlighted.,C.B.D.,Example n.5 Two satell

17、ite views of Porta S. Maria roundabout in its present state.,90 m,Example n.5 Aerial NB view of the Porta S. Maria roundabout in its present state.,Example n.5 The basic idea: a new roundabout (Dn=60 m) built inside the actual one (Da=90 m) and surrounded by an external lane with parking stalls.,YELLOW = actual RED = design,Example n.5 Design layout of the improved roundabout: reduced Dn=60 m and over 65 parking stal