The increasing construction and implementation of roundabouts in the last 20 years is a result of a need for capacity increment, as well as the safety level increment on road intersections at-grade. Designing and shaping roundabouts, especially in urban areas, represents a complicated problem with a number of different conditioned elements that need to be satisfied. Geometrical elements such as the dimension of the outer roundabout diameter and number and width of the lanes considerably affect the trajectory of the vehicle's path through the intersection, respectively the vehicle speed that has an immediate effect on the safety and the capacity of the roundabout. Through a depiction of four existing roundabouts in the City of Zagreb, this paper will analyze the speed as an important roundabout designing factor. The research results will provide guidelines for roundabout designers, considering that the design speed is in correlation with the measured actual vehicle speed on a roundabout.

roundabouts in urban areas, modelling and designing, vehicle movement trajectory vehicle speed