Monitoring performance of road safety programmes in New Zealand

While the number of crashes during a certain time period such as a week, month or a year indicates the level of risk on the road, it may fluctuate without any change in the actual underlying risk. If the number of crashes increases or decreases during a period of time, it indicates one of two possibilities: (1) the probability of a crash during the period has changed, or (2) the difference is due to the stochastic nature of the event, i.e. it is due to a random fluctuation. A control chart system is developed in the paper to identify the occurrence of actual risk changes or deviation from the expected level and likelihood of meeting the target. The paper discusses the development of control charts to monitor fatalities at the national level, which can also be used at regional levels, with respect to the pre-set targets. In a traditional control chart, the production process is composed of identifiable and controllable inputs. The levels of the inputs can be designed before production. In the case of road safety, it is not feasible to do so. Here the nature and quality of the input 'controls' are such that they may influence the safety outcomes indirectly. Estimates of variability used in the analysis must take account of this peculiarity. The attainability of a target, to be achieved within a given period, depends not only on future safety outcomes but also on outcomes in the past. A cumulative control chart has been developed in this paper to particularly facilitate the monitoring of targets.