Modified logistic regression modelling was used to reanalyse free travelling speed case control data in an urban 60 km/h speed limit environment. An exponentiated second order polynomial function was used to model the relative risk of being involved in a casualty crash based on free travelling speed. The relative risk was found to approximately double for each 5 km/h increase in free travelling speed. This curve and the original data were then used to estimate the effects of various hypothetical speed reduction scenarios. Illegal speeding was found to be a major factor in casualty crashes and it was found that even very small reductions in the speeds of vehicles in general could be expected to result in a major reduction in the frequency of casualty crashes in an urban area.