The impacts of a reduced speed enforcement tolerance threshold on road safety outcomes

Internationally, speeding behaviour remains a crucial concern for road safety agencies and researchers. In 2013, 1,193 people in Australia and 254 in New Zealand died as a result of crashes. Speed is estimated to have been a factor in over a third of these deaths. New Zealand sought to reduce these preventable deaths by introducing a speed enforcement campaign that focussed on lowering the speed enforcement threshold from 10km/h to 4km/h over a 2-month period. The campaign was implemented to reduce the incidence of speeding behaviour in an effort to reduce the number and the severity of crashes during the historically high-risk summer holiday period. Speed survey data collected by semi-overt mobile speed cameras was used to analyse the effects of the intervention on speeding. A quasi-experimental study design incorporating generalised linear models was used to evaluate the effects of the intervention on speeding and crashes. The results showed significant reductions in speeding by 10km/h or less (36%), and for speeding in excess of 10km/h (45%). Analysis of crash data revealed possible decreases of fatal (22%), serious (8%), and minor crashes (16%), but these reductions were not statistically significant. Although the campaign had a significant and substantial effect on reducing speeding behaviour, the duration of intervention period was likely not sufficient to reflect the full effects of this reduction on road trauma. While this initiative shows considerable promise as a road safety intervention, research over an extended period is recommended to more fully test the effects on reducing crash risk.