The Swedish road network contained around 120 roundabouts in the early 1980s, most of them with large-scale designs. That number has nearly tripled and is still growing owing to excellent safety records, traffic performance, and traffic calming properties of roundabouts. Roundabouts are classified as mini (R sub 1<2), small (2<R sub 1<10) and normal (R sub 1 >10) depending on inner radius, R sub 1 and their driving properties for buses and trucks. The basic design principle is to provide the smoothest car path through the facility restricted by the condition for turning path radii, requiring R<100 m (32.8 ft) for driving speed </=50 kph (31 mi/h) and 50 m (16.4 ft) for </=30 kph (18.6 mi/h) The design vehicle is a 16-meter-long (52 ft) semi-trailer with an R10 driving strategy, which results in 5- to 6.5-meter-wide (16 to 21 ft) entries and exits and weaving widths of up to 8 meters (104 ft) for single-lane normal roundabouts. Swedish accident statistics give vehicle injury accident levels for roundabouts in the range 0.04 to 0.08 injury accidents per million vehicles, depending on major road speed and environment, with very few severe accidents. Roundabouts have successfully been used in a full-scale central area traffic calming project with an estimated 44% improvement in traffic safety. New traffic performance research proposes critical gaps depending on weaving width, length, and approach lane with a bunched major flow model, without consideration of vehicle direction. The current CAPCAL model gives higher capacities at 50 kph (31 mi/h) and lower at 70 kph (43.5 mi/h) when compared with these new results. (A)
Abstract