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This report details research carried out in Wellington, New Zealand, over the period 2012–13. The broad aim was to develop relationships between rut depths and crashes on New Zealand's state highway network.

A literature review suggested that deep ruts could either:

• increase crash rate because of reduced vehicle control, or
• reduce crash rate as drivers reduced speed in order to keep their vehicle under control.

A method of predicting pond depth on New Zealand's state highway network using New Zealand databases was developed. Comparisons of predicted flow path length with measured data were encouraging.

Key findings of statistical studies of the relationship between crash rates and rutting on New Zealand's state highways were:

• very little of the network has 10–30mm rut depths
• crash rates decrease slightly as rut depth increases over the normal range of rut depths - particularly for dry crashes
• water accumulating on the road surface may have an effect on crash rates because of poor run-off.

Due in part to the paucity of ruts in the 10–30mm range, statistically robust benefit-cost ratio estimates could not be calculated. However, for shallow ruts, the statistical modelling indicated that filling could not generally be justified.

Keywords: crash rate, flow path length, pond depth, ruts, water film depth