the traffic capacity of roundabouts

a study has been made of the entry capacities of conventional and offside priority roundabouts at eighty-six public road sites, and a unified formula for capacity prediction developed. the traffic flow entering a roundabout from a saturated approach was found to be linearly dependent on the circulating flow crossing the entry. the most important factors influencing the capacity are the entry width and flare. the entry angle and radius have small but significant effects. the inscribed circle diameter, used as a simple measure of overall size, is more effective as a predictive variable for the capacity than the category distinction between conventional and offside priority roundabouts, and for capacity prediction there is no need to retain this distinction. in addition to normal capacity prediction, methods have been developed which allow: (i) the predictive equation to be corrected to take account of local operating conditions at overloaded existing sites; and (ii) the equation to be used specifically to predict the effects of changes in the entry geometry of existing sites.(a)