Modern roundabouts are a relatively new intersection treatment in Canada, particularly in the highway context. However, their use is on the rise due to the increasingly recognized safety and operational benefits associated with them. Current roundabout design guidelines stipulate that the size of a modern roundabout should be minimized in order to reduce vehicle speeds within the circulatory roadway. However, in the highway context, a large-diameter roundabout may be required to safely transmit tractor-trailers. This paper will discuss the safety benefits and disadvantages of large diameter modern roundabouts, using as an example the proposed roundabout at the intersection of Highway 8 and Highway 22 in the Province of Alberta where the co-authors conducted a Road Safety Audit. While large diameter roundabouts may be required to meet the needs of the design vehicle, the size of the roundabout may encourage smaller vehicles to travel at high speeds within the roundabout, hence limiting its effectiveness. The width of the apron can be increased to allow for truck off-tracking, but a wide apron may introduce other safety issues. This paper will discuss the trade-off between accommodating larger vehicles and keeping vehicle speeds to a minimum. Finally, other issues related to large-diameter roundabouts, including the use of bypass ramps, driver comprehension and cyclist accommodation will be discussed, using the proposed Highway 8 and 22 installation as an example.
