Speed Zoning Based on the Road Environment

For over 40 years, speed limits have been calculated in New Zealand using a model based on the number and types of developments. Also included are some additional activity factors, such as pedestrian and parking movements, and some road environment factors, such as the number of lanes, visibility and traffic controls. However, these additional factors must not outweigh the development factor. This means undeveloped roads in rural areas where road environment factors create a low speed environment can not have a lower speed limit - the default 100 km/h rural speed limit applies. This assumes that drivers see that it is a low speed environment a select the appropriate driving speed. Unfortunately, some drivers still drive too fast, either intentionally or misreading the road environment and so crashes occur. To counter this problem, some road controlling authorities in New Zealand have requested the ability to set speed limits in these rural, lower speed environments. That is, a method of setting speed limits that takes into account road environment factors that influence the speed of drivers. In New Zealand, this type of speed limit setting is called speed zoning. This paper looks at the proposed policy and processes for speed zoning in New Zealand. The three important aspects are: determining whether speed zoning is appropriate for a road; determining the best speed limit based on the 85th percentile speeds; and calculating the best speed limit based on the actual risk of a speed related crash.