Investigation of the effect of positive and negative crossfalls on road safety at roundabouts

Roundabouts are a common form of road intersection and can be used to allow self-regulating flow of traffic. As part of a roundabout there are generally three critical geometric considerations, namely, approach geometry, the circulating carriageway and the departure geometry. In many instances this geometry is compromised and designed to slow traffic. Horizontal geometry is a significant aspect of the introduced compromise. Road Design Guides rely on the use of horizontal radii and crossfall to develop safe curves in road design. Consequently, the crossfall of a roundaboutʼs circulating carriageway becomes an important design consideration. The crossfall must be designed to allow a smooth transition and adequate drainage performance of the circular carriageway. There are two common approaches to roundabout crossfall in Australia, positive and negative crossfalls. This dissertation investigates and analyses the road safety performance of existing roundabouts with these different crossfalls to evaluate the safety benefits that each arrangement may offer. Data was obtained for crashes at roundabouts in Queensland. The data spanned from 2001 to 2012 and was analysed to determine the trends, proportions and common causes of roundabout accidents. The data was limited to Queensland to highlight the differences in performance between conventional, negative crossfall roundabouts and positive crossfall roundabouts, which are sufficiently scattered throughout Queensland. Corresponding road design information such as horizontal radii, pavement crossfall and speed limits were obtained using aerial imagery, road design plans and crash reporting records. The data was used to contextualise the crashes and ultimately determine the role that the type of crossfall at the roundabout played in the crashes. Crash data and review of literature indicated five major crash types at roundabouts, with four types linked to the crossfall at the roundabout. Crash included approach rear end, entry/circulating, single vehicle and overturned crashes. Analysis of the data concluded that speed zoning played a significant role in all crash types, with crossfall a secondary affectation of speeds. It was found that positive crossfall sites were overrepresented in approach rear end crashes and overturned crashes. Negative crossfall sites were overrepresented in single vehicle and entry/circulating crashes. The results obtained were unevenly distributed against crossfall and statistically indicate that crossfall is a significant affectation of accidents at roundabouts. More in-depth research is required into specific case study sites and crashes. Consideration could include traffic volumes and crash severity at the different roundabout crossfall sites. This will aim to validate and extend on the literature and crash data analysis in this dissertation