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The Transit New Zealand Project evaluation manual (1996) outlined the procedures to be adopted when evaluating roading projects in New Zealand. In 1996, as part of the periodic updating of project evaluation methods, two aspects of the accident analysis procedures were reviewed. The issues addressed were the increased severity of accidents that occur at higher speeds; and the prediction of reduction in accident rates that result from improving curves.

The review first considered the rationale behind the present procedures, the basis from which these had been derived, together with other supporting studies. Both procedures were soundly based, but they were found to have been constructed using different accident and speed data. It was therefore doubtful that the two procedures should be applied together.

An analysis of accident data has confirmed that:

• the cost of rural accidents, ie accidents occurring at 100 km/h regulatory speed, increases at a rate of NZ(1996)$5,828 per accident per 1 km/h increase in mean traffic speed
• the accident rate for curves is related to the difference (ΔSD) between the approach speed or approach speed environment and the mean curve speed, and is such that the accident rate decreases by 1.025 to the power of ΔSD; and
• because the accident severity is related to the speed environment, a curve improvement that does not alter the overall speed environment should not be subject to an adjustment for accident severity increase.

Keywords: accident, crash, New Zealand, project evaluation, road geometry, severity, speed