The aim of this study was to examine the changes in the trend and seasonal patterns of fatal crashes in New Zealand in relation to changes in economic conditions between 1970 and 1994. The Harvey (Harvey and Durbin, Journal of the Royal Statistical Society , 149 (3), 187-227, 1986) Structural Time Series Model (STSM), an 'unobserved components' class of model, was used to estimate the quarterly number of fatal traffic crashes. Independent variables included distance travelled, the unemployment rate (UER), real gross domestic product per capita (RGDP), the proportion of motorcycles, the proportion of young males in the population, alcohol consumption per capita, the open road speed limit, and dummy variables for the 1973 and 1979 oil crises and seatbelt wearing laws. Distance travelled, RGDP, UER, and alcohol consumption per capita were significant factors in explaining the short-run dynamics of fatal crashes with the effect of RGDP greater than UER. Increases in either RGDP or UER were related with decreases in fatal crashes. The STSM is a feasible approach to modelling the effect of economic factors on traffic crashes whilst accounting for unobserved components.