Roundabout capacity in adverse weather and light conditions

Roundabouts are a widespread form of junction globally, and are particularly common in the UK. This study focuses on the entry capacity at roundabouts under adverse conditions of weather and light in order to determine the extent to which entry capacity is reduced, and the impact that this has on current practical design. So far no specific studies have attempted to quantify this effect. The study was limited to conventional at-grade, normal-size, non-signalised roundabouts in urban areas. Data were collected in 1998 using video capture techniques at four different sites with a total of 13 approaches, comprising 37 entry lanes. The data were transcribed and classified to exclude those minutes of data which did not satisfy the preset traffic criteria, resulting in the extraction and utilisation of 7721 minutes of detailed entry-lane based data. The entry capacities were classified according to whether the conditions were: dry and light; dry and dark; wet and light; and wet and dark. The results identified that wet and dark conditions had a substantial effect on the entry capacity of urban normal-size roundabouts. Entry capacity was found to be reduced by 14% under dry–dark, 17% under wet–light, and 25% under wet–dark conditions. The paper sets out the main practical implications of these findings for design engineers.