Roundabouts: capacity - 2 |

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We can obtain a better estimate of the capacity of a "typical" 1-lane roundabout from the report "Roundabouts: An Informational Guide", NCHRP 672 ed. 2. |

The plot at left below shows the nominal capacity of a roundabout as a function of the cross-traffic fraction.The equation 4-3, from which this plot is derived, is an empirical fit to data from real roundabouts. With no cross traffic, the capacity is 1130 veh/sec. With cross traffic at 10% of the main flow, the main-flow capacity decreases to about 1000 veh/hr. |

Once the capacity is has been estimated, the plot on the right allows estimation of the delay experienced in passing through the roundabout. The least delay occurs when the roundabout is empty: about 3 seconds. If the traffic volume approaches the capacity of the roundabout, the delay time becomes large-- over 40 seconds. |

A more comprehensive design tool is available, a commercial computer program called SIDRA (easily located via a search engine). SIDRA is a more elaborate model for calculating the performance of roundabouts with various geometrical configurations. Until recently, SIDRA has been significantly too optimistic with respect to actual measured performance. Version 5.1 (the latest) has adopted the standard US model for roundabouts, the "Highway Capacity Manual" (Transportation Research Board, 2010 edition), HCM2010. |

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