Utilisation of Turbo Roundabouts to Increase Safety at Existing Two-Lane Roundabouts in Queensland

The population in Brisbane, Queensland’s capital city is growing rapidly with extensive road networks being developed. With this population growth the rise in traffic volume puts increased pressure on the road networks causing capacity and safety problems. To manage higher traffic volumes, two lane roundabouts are commonly used, however, additional safety risks are introduced. This dissertation examines the use of turbo roundabouts to reduce safety risks at two-lane roundabouts. Whilst internationally turbo roundabouts have been utilised successfully, there is limited research on the use of turbo roundabouts in Australia. Therefore, the aim of this study was to analyse road crash data for single and double lane roundabouts and investigate the geometric design guidelines of turbo roundabouts to assess the suitability for use in Queensland’s road network. Preliminary analysis of the road crash data comparing frequencies and crash types between single and double lane roundabouts identified a clear increase in the frequency and severity of crashes at two lane roundabouts compared to single lane roundabouts. A combination of road safety measures was utilised to rank the top five most hazardous two-lane roundabouts in the northern suburbs of Brisbane and examine the suitability of turbo roundabouts. The combination of road safety measures used analysed factors including traffic volumes, regression to the mean and societal crash costs to rank the top five worst performing roundabouts. The measures utilised were Relative Severity Index method, The Crash Rate method, and Excess Predicted Average Crash Frequency using Method of Moments. An average ranking method combined with a weighting matrix was used to combine the 3 methods to get a final ranking list. The investigation into the top five worst performing two-lane roundabouts sites highlighted deficiencies such as; non-conforming central island shapes and radii, non-standard entry geometry due to maximum entry path radii too large, legs aligned at angles other than 90 degrees, undesirable leg separation and spiral line marking. A comparative analysis of European standards provided a basis for the initial concept designs which concluded that European turbo roundabout geometry was not suitable to cater for Australian design vehicles swept paths. However, the maximum entry path radii check was found to be conforming. Therefore, the conforming designs were constructed to allow enough space for the Australian design vehicles with 0.5m clearance to kerbs and lane lines considered. Despite the larger geometry, the maximum entry path radii check was found to conform at each leg of each site. It was concluded that suitable turbo roundabout geometry can be constructed to meet Australian design vehicles and still adhere to the safety performance checks