A computer simulation study of the effects of flaring and turning movements on roundabout entry performance.

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.