Left turning vehicles can block the sight lines for vehicles trying to exit the side road. This is called ‘Dynamic Visual Obstruction’ or ‘Masking’. The addition of a left turn lane can sometimes exacerbate this issue as the masked vehicle is given greater opportunity to maintain speed or speed up through the intersection, and the driver exiting the side road is more confident to exit when the left turner distinctly moves into the dedicated left turn lane.
The highest alignment with Safe System principles at high-speed rural intersections is a roundabout with separated cycling facilities, however the cost of this treatment is sometimes prohibitive. Some alternate ‘supporting’ treatments include; offsetting the left turn lane to open sight lines (see example below where a bike lane was used to open sight lines based on Road Safety Audit advice from Safe System Solutions) or adjusting the hold line position (if it can be done safely). Even when sight lines are open, there is still the opportunity for high-speed right-angle crashes, so speed management should be considered.