Upgrading Traffic Circles to Modern Roundabouts to Improve Safety and Efficiency – Case Studies from Italy

  • Antonio Pratelli Department of Civil and Industrial Engineering, University of Pisa (Italy)
  • Paolo Sechi Department of Civil and Industrial Engineering, University of Pisa (Italy)
  • Reginald Roy Souleyrette College of Engineering, University of Kentucky, Lexington, KY 40506-0281 USA
Keywords: traffic circles, modern multilane roundabout capacity models, multilane roundabouts design, operational efficiency analysis, traffic flow simulation,


This paper presents a procedure for analysing safety and operational improvements made possible by converting traffic circles to modern roundabouts. An Italian case study is presented for alternative layouts under various traffic demand scenarios. In the application of the procedure, the average waiting times and queue lengths at entries are computed with an analytical capacity model, using default values for gap parameters. Then, the roundabout is dynamically simulated. The simulation results in a revised set of gap parameters that are in turn used as inputs to a second trial of the capacity model, and in turn fed back into the simulation. The two steps are repeated until the parameters reach a pre-selected convergence criterion, so that gap parameter values for both the static capacity and dynamic microsimulation models are in equilibrium. Therefore, the applied procedure can conduct both static and dynamic roundabout design, usually applied separately. One can start with default values in guidelines and couple them with limited field data, improving both the expected results and cost-effectiveness of solutions. Next, safety is estimated using dynamic simulation software and a compatible conflict counting model to acquire surrogate measures of safety. Level-of-service
and surrogate safety indicators for the existing and redesigned roundabouts are then compared. The procedure is first demonstrated on an old “ultra-large” roundabout. The procedure is tested on this roundabout using the Highway Capacity Manual 2016 (HCM2016), AimsunTM, and Surrogate Safety Assessment Model (SSAM) software. A redesign is shown to be far superior in efficiency and safety. Finally, two cases are described where large first generation roundabouts
were upgraded to modern standards.


[1] Turner D. Roundabouts: a literature review. weblog, 2011. Available from: http://www.danielrturner.com/home/wp-content/uploads/2012/12/Roundabouts.pdf [Accessed 29th November2017].

[2] K. Todd. A History of Roundabouts in the United States and France. Transportation Quarterly. 1988;42(4): 599-623.

[3] Taekratok T. Modern roundabouts for Oregon. Oregon Department of Transportation. Rep. n.98-SRS-522, Salem; 1998.

[4] Tollazzi T. Alternative Types of Roundabouts- An Informational Guide. New York: Springer; 2015.

[5] Gates TJ, Maki RE. Converting Old Circles to Modern Roundabouts: Michigan State University Case Study. In: Proceedings of ITE 2000 Annual Meeting and Exhibit.
Nashville, Tennessee; 2000.

[6] Legac I, Pilko H, Brcic D. Analysis of traffic capacity and design for the reconstruction of large roundabout in the city of Zagreb. In: Pratelli A. (ed.) Intersection Control and Safety. Ashurst, U.K.: WIT Press; 2013. p. 17-28.

[7] Pilko H, Mandzuka S, Baric D. Urban single-lane roundabouts: A new analytical approach using multi-criteria and simultaneous multi-objective optimization of geometry design, efficiency and safety, Transportation Research Part C. 2017;80: 257-271.

[8] Vasconcelos A, Seco A, Silva A. Comparison of Procedures to Estimate Critical Headways at Roundabouts. Promet – Traffic & Transportation. 2013;25(1): 43-53.

[9] Al-Madani HMN, Pratelli A. Modeling and calibrating capacity of large roundabouts. International Journal of Sustainable Development and Planning. 2014;9(1): 54-73.

[10] Gazzarri A, Martello MT, Pratelli A, Souleyrette RR. Estimation of gap acceptance parameters for HCM 2010 for roundabout capacity model applications. WIT Transactions on the Built Environment: Urban Transport XVIII. 2012;128: 309-320.

[11] Transportation Research Board. Highway Capacity Manual. 6th edition. Washington DC: National Research Council; 2016.

[12] Gagnon C, Sadek A, Touchette A. Calibration potential of common analytical and microsimulation roundabout models: New England case study. Transportation Research Record. 2008;2071: 77-86.

[13] Bared JG, Afshar AM. Using Simulation to Plan Capacity Models by Lane for Two- and Three-lane roundabouts. Transportation Research Record: Journal of the Transportation Research Board. 2009;2096: 8-15.

[14] Federal Highway Administration. Surrogate Safety Assessment Model (SSAM) - User Manual. Federal Highway Administration. Rep. FHWA-HRT-08-050, Washington DC, 2008.

[15] Pratelli A, Margheri G. Studio di Fattibilità per Interventi sulla Rotatoria Querciola-Ariosto-Togliatti. Technical Report, Pisa, 2003.

[16] Suffredini E. Ristrutturazione della Rotatoria di Porta Santa Maria con Aumento dell’Accessibilità al Centro Storico di Lucca. BS thesis. Department of Civil Engineering of University of Pisa; 2008.

[17] Mauro R, Cattani M. Functional and economic evaluations for choosing road intersection layout. Promet – Traffic & Transportation. 2012;24(5): 441-448.

[18] Baric D, Pilko H, Strujic J. An analytic hierarchy process model to evaluate road section design. Transport 31. 2016;(3): 312-321.

[19] Tollazzi T, Sram M, Lerher T. Roundabout arm capacity determined by microsimulation and discrete functions technique. Promet – Traffic & Transportation. 2008;20(5): 291-300.
How to Cite
Pratelli, A., Sechi, P., & Souleyrette, R. (2018). Upgrading Traffic Circles to Modern Roundabouts to Improve Safety and Efficiency – Case Studies from Italy. PROMET - Traffic & Transportation, 30(2), 217-229. https://doi.org/https://doi.org/10.7307/ptt.v30i2.2571