SIDRA INTERSECTION offers three roundabout capacity model options for US conditions: the US HCM Edition 6 model, the US HCM 2010 model and the SIDRA Standard model. In the US HCM (Customary) and US HCM (Metric) Software Setups of SIDRA INTERSECTION used in the USA, the Environment Factor parameter in the SIDRA Standard model of roundabout capacity is calibrated to match the roundabout capacity conditions observed at US roundabouts. On the basis of the US HCM Edition 6 model, the Environment Factor values used are as follows:

• 1.05 for single-lane roundabouts (both approach road and circulating road have one lane),

• 1.05 for roundabouts with mixed single-lane and multi-lane approach and circulating road arrangements (either approach road or circulating road has one lane and the other road has two or more lanes), and

• 1.2 for multi-lane roundabouts (both approach road and circulating road have two or more lanes).

In previous versions of SIDRA INTERSECTION, an Environment Factor of 1.2 was used to match the HCM 2010 roundabout capacity model.

Some US practitioners who prefer to use the SIDRA Standard roundabout capacity model recommend the use of Environment Factor values of (1.0 and 1.1) instead of (1.05 and 1.2), respectively. These values produce higher capacity estimates in future conditions assuming more aggressive driver behaviour due to increased driver familiarity with roundabouts and various other reasons as discussed in a paper by Akçelik (2011a).

Using the Capacity Model parameter in the Roundabouts dialog, Options tab, you can change between the US HCM Edition 6, US HCM 2010 and SIDRA Standard roundabout capacity models. There are various key parameters involved in changing between the two models. Refer to the user guide for detailed information.

The HCM Edition 6 and HCM 2010 roundabout capacity models based on research on US roundabouts are fully integrated into SIDRA INTERSECTION with various extensions offered. Refer to the following papers available for download from the SIDRA SOLUTIONS website - Publications page:

AKÇELIK, R. An Assessment of the Highway Capacity Manual 2010 Roundabout Capacity Model. Paper presented at the International Roundabout Conference, Transportation Research Board, Carmel, Indiana, USA, May 2011.

AKÇELIK, R. An Assessment of the Highway Capacity Manual Edition 6 Roundabout Capacity Model. Paper presented at the 5th International Roundabout Conference, Transportation Research Board, Green Bay, Wisconsin, USA, May 2017.