Conventional and Unconventional Roundabouts: A Review of Geometric Features and Capacity Models

Conventional and Unconventional Roundabouts: A Review of Geometric Features and Capacity Models

Antonio Pratelli Simona Casella | Alessandro Farina Marino Lupi

DICI – Department of Civil and Industrial Engineering, University of Pisa, Italy.

Available online: 
30 September 2018
| Citation



Road intersections play a key role in traffic management. Modern roundabouts require entering vehicles to yield to the circulating flow, and have proven highly effective in granting high performance levels to both new and redesigned intersections; therefore, their use has widely spread around the world over the years. The choice of the correct shape, size and signage of a roundabout is essential to accomplish the desired results in terms of performance and safety. In order to achieve these goals, designers have moved away from conventional layouts such as single-lane and multilane roundabouts, conceiving more and more unconventional solutions (spiral, turbo and two-geometry roundabouts). Alongside this design evolution, research has been conducted on functional analysis of roundabouts: several authors and authorities have developed capacity assessment models that are suitable for the analysis of unconventional roundabouts, but nowadays no complete review of such models is available. The aim of this paper is to collect descriptions of the main types of conventional and unconventional roundabouts, with a focus on their geometric features and applicable capacity assessment models.


conventional and unconventional roundabouts, empirical and stochastic capacity models, roundabout capacity models, spiral and turbo roundabouts


[1] Transportation Research Board, Roundabouts: An informational guide. National Cooperative Highway Research Program Report 672, Transportation Research Board: Washington, DC, 2010.

[2] Italian Ministry of Infrastructures & Transport, Norme funzionali e geometriche perla costruzione delle intersezioni stradali. Decreto Ministeriale n. 1699 of 10/4/2006, Italian Ministry of Infrastructures & Transport: Rome, 2006.

[3] Association of Swiss Road and Traffic Engineers, Carrefours giratoires. Swiss Standard n° 640263, Association of Swiss Road and Traffic Engineers: Zurich, 1999.

[4] Association of Swiss Road and Traffic Engineers, Capacité, niveau de service, charges compatibles. Swiss Standard n° 640024a, Association of Swiss Road and Traffic Engineers: Zurich, 2006.

[5] Centre d'Etudes sur les Reseaux, les Transports, l’Urbanisme et les constructions publiques (CERTU), Carrefours Urbains, Guide, CERTU: Lyon, 1999.

[6] Transportation Research Board (TRB), Highway Capacity Manual, 6th ed., The National Academies of Sciences, Engineering, and Medicine: Washington DC, 2016.

[7] Amanamba, E.C., Spiral lane markings: A review of lane discipline. Civil and Environmental Research, 8(5), pp. 106–116, 2016. ISSN 2224-5790 (Paper) ISSN 2225-0514 (Online).

[8] Homola, D. & Chan, S., A comparative study on turbo-roundabout and spiral roundabouts. Proceedings of the Fifth International Symposium on Highway Geometric Design (ISHGD), Vancouver, pp. 1–20, 2015.

[9] Fortuijn, L.G.H., Turbo roundabouts: Design principle and safety performance. Transportation Research Record, 2130, pp. 83–92, 2009. DOI: 10.3141/2130-11

[10] Tollazzi, T., Alternative Type of Roundabouts: An Informational Guide. Springer International Publishing: New York, NY, 2015.

[11] Gazzarri, A., Pratelli, A., Souleyrette, R. & Russell, E., Unconventional roundabout geometries for large vehicles or space constraints. Proceedings of Fourth International Conference on Roundabouts, Seattle, pp. 1–17, 2014.

[12] Service d’Etudes Techniques des Routes et Autoroutes (SETRA), Capacité des Carrefour giratoires interurbains: prémiers résultats. Note d’information n. 44, SETRA: Bagneux, 1987.

[13] Al-Madani, H.M.N. & Pratelli, A., Modeling and calibrating capacity of large roundabouts. International Journal of Sustainable Development and Planning, 9, pp. 54–73, 2014. DOI: 10.2495/SDP-V9-N1-54-73

[14] Gazzarri, A., Martello, M.T., Pratelli, A. & Souleyrette, R.R., Gap acceptance parameters for HCM 2016 roundabout capacity model applications in Italy. Intersections Control and Safety, ed. A. Pratelli, WIT Press: Southampton, pp. 1–16, 2013.

[15] Akçelik, R. & Troutbeck, R., Implementation of the Australian roundabout analysis method in SIDRA, Highway Capacity and Level of Service. Proceedings of the International Symposium in Highway Capacity, Karlsruhe, pp. 17–34, 1991.

[16] Bared, J.G. & Afshar, A.M., Using simulation to plan capacity models by lane for two and three lane roundabouts. Transportation Research Record, 2096, pp. 8–15, 2009. DOI: 10.3141/2096-02

[17] Brilon, W., Roundabouts: A state of the art in Germany. Proceedings of the National Roundabout Conference 2005, Vail, pp. 1–16, 2005.

[18] Mauro, R. & Branco, F., Comparative traffic operational analysis between turboroundabouts and roundabouts. Proceedings of the 88th Annual Meeting of Transportation Research Board (TRB), Washington, DC, pp. 316–322, 2009.