This thesis reports on an investigation carried out to study the effect of flaring and turning movements on the performance of roundabout entries. A computer simulation program was developed to carry out the investigation. The model simulates an entry with two lanes at the approach section and four at the stop line. It can be modified easily to simulate straight entries by changing the input and one DATA statement. Data were collected at three public road sites at Sheffield to validate the model. A method of analysing the data was developed to obtain values of the gap-acceptance parameters. The values arrived at were used subsequently as input into the model to allow direct comparison of observed and simulated values. The comparison concluded that the model represents adequately the real conditions. The results produced showed that average delay for below-capacity operation is reduced by at least 40% when an entry is flared. Capacity improvement, measured as the effective number of lanes of a flared approach, is shown to be influenced by the circulating flow. There is an improvement of 50% for all studied casesfor circulating flow of 2300 veh/hr and more. Turning proportions do not affect capacity of straight entries but do affect that of flared entries. There is a difference of 25 - 30% between the extreme values depending on the proportion of left-turning vehicles. Turning proportions affect delays of both straight and flared entries. Minimum delay was obtained for combinations which include 30 - 40% left-turning proportion.