The effect of left turn slip lanes on safety and operational performance of signalised intersections

There are a number of arguments about the safety of using left turn treatments, particularly left turn slip lanes, at signalised intersections. Some suggest that left turn slip lanes might be a safer facility for all road users especially pedestrians. On the other hand, other arguments are not in favour of using them. There is limited research available in that area around the world, especially in New Zealand. Thus, the main aim of this thesis was to evaluate the effect of left turn slip lanes on the safety performance and the operational performance at signalised intersections in New Zealand. The safety performance of 625 signalised intersections in Auckland, including 1818 approaches, were evaluated using three key analyses: overall crash analysis, detailed crash analysis, and additional detailed pedestrian crash analysis. The results were examined and were statistically verified using the Chi-square test. It was found that the frequency of left turn crashes was minimal, especially pedestrian crashes. Furthermore, the greatest proportion of the left turn crashes was non-injury. In terms of safety performance, both left turn slip lanes and left turn conventional lanes have similar safety performance. The largest proportion of pedestrian crashes and injuries occurred predominantly at the shared conventional lanes and at the zebra crossing slip lanes, thus making them the least options to be chosen when designing signalising intersections. The operational performance was carried out by modelling 24 scenarios to assess the intersection performance for left turn slip lanes versus conventional lanes. The key parameters used were delay, queue lengths, and level of service. Further analysis was conducted testing a range of left turn flows, intersection flows and applying different pedestrian protection times. This was to determine the wider implications on the intersection operational performance as a result of those factors. The results showed that the left turn slip lanes contribute to the resilience of the intersection performance, even with increasing the traffic flow of the left turn movements and/or of the whole intersection. The use of left turn slip lanes can significantly reduce delays experienced by left turning traffic movements, to their relevant approaches and the overall intersection delay. On the contrary, the left turn conventional lanes, especially shared lanes, contributed immensely to the increase of the delay to these movements, to their approaches and to the delay of the intersection as a whole. The thesis was concluded by a set of recommendations for the safety performance and the operational performance of left turn slip lanes in comparison with left turn conventional lanes, at signalised intersections