An Assessment of the Highway Capacity Manual Edition 6 Roundabout Capacity Model

The latest TRB Highway Capacity Manual (HCM) Edition 6 (2016) introduced major changes to the roundabout capacity model parameters compared with those given in the HCM 2010 Edition. This paper presents an assessment of the HCM Edition 6 model compared with the HCM 2010 model and the SIDRA Standard capacity model for roundabouts employed in the SIDRA INTERSECTION software which is widely used in US practice. In particular, the Environment Factor parameter used in the SIDRA Standard capacity model is discussed in relation to calibrating the model to match the capacity estimates from the HCM Edition 6 model. Comments are also included on various extensions to the HCM Edition 6 model provided in the SIDRA INTERSECTION software. Using a multilane roundabout example given in the HCM, capacity and the resulting degree of saturation (v/c ratio), delay, level of service and queue length estimates from the HCM Edition 6, HCM 2010 and the SIDRA Standard capacity models are compared.