A ROUNDABOUT CASE STUDY COMPARING CAPACITY ESTIMATES FROM ALTERNATIVE ANALYTICAL MODELS

There has been some controversy about capacity estimates from the gap-acceptance based Australian and Highway Capacity Manual methods and the linear-regression based UK (empirical) method. This paper presents a single-lane roundabout case study from the United States to compare capacity estimates from these analytical models. Some contradictory results that can be obtained from these models are highlighted and reasons for differences are discussed. Such systematic differences have important design implications. The paper discusses the UK roundabout research, and explains why the UK Linear Regression model will underestimate capacity for low circulating flows and overestimate capacity for high circulating flows. The UK model appears to have been derived with a relatively small number of data points with low circulating flows, and it reflects peculiar effects of the geometric designs of UK roundabouts included in the database used for its development. These highly-flared roundabouts possibly encouraged merging and caused priority reversal at high circulating flows. The aaSIDRA model reflects the more uniform style of modern roundabout designs used in Australia and the USA. Another factor is lack of sensitivity to demand flow patterns in the UK Linear Regression and other models. The case study displays an unbalanced flow pattern which contributes to significant differences between the aaSIDRA and other models. Capacity is increased when heavy approach traffic enters against low circulating flow. Dominant circulating flows, originating mostly from a single approach, reduce the entry capacity as evident from the use of metering signals in Australia and the UK to help low-capacity roundabout approaches.