The Relevant Direction of Vehicle Movement for Checking the Operational Speed at the Roundabout

The main function of roundabouts is to assure traffic safety and capacity. Operational speed in roundabouts, important for both scopes, is regularly checked in the process of planning and design of roundabouts according to procedures (or models) defined in national technical regulation. There are two mainly used models, one developed in USA and other developed in Netherlands literature. The analyzed regulation for roundabouts from all over the world, except the USA, defines straight direction as a critical movement in the roundabout even there is no available research and comparison with speeds in other directions. In this research, the goal was to experimentally establish critical movement in the roundabout. The operational speed, in this case, was established through measurements done with precise GNSS equipment at single-lane, four-leg roundabout in Croatia. The results showed, with statistical relevance, that there are differences between speeds for different directions and that straight direction is the critical movement regarding the speed at roundabouts.