The current speed zoning project in New Zealand is considering two methods of determining speed limits for rural roads: using an 85th percentile speed profile; or using a risk based calculation. This paper considers the issues for both methods and discusses which method produces the best results. Comments made in this report are based on the experiences from the speed zoning project. The paper reaches the conclusion that combining both the 85th percentile speed profile and the risk based calculation methods means a speed limit based on the 85th percentile speed can be set conservatively, targeting only the most antisocial drivers, while the risk based calculation can be used to determine whether the speed limit should be lowered if the road is deemed to have a high crash risk. Some subjective judgements will be required by experts for the risk based calculation. However, the end result should be a method that will meet the expectations of most drivers, except where there is a definite safety risk and a lower speed limit is required. (a) For the covering entry of this conference, please see ITRD abstract no. 0612AR242E.
Abstract