Abstract
Monthly data on road casualties in Great Britain are analysed in order to assess the effects on casualty rates of the seat belt law introduced on January 31st 1983. Such analysis is known technically as intervention analysis. The form of intervention analysis used in this paper is based on structural time series modelling and differs in significant respects from standard intervention analysis based on ARIMA modelling. The relative merits of the two approaches are compared. Structural modelling intervention techniques are used to estimate the changes in casualty rates for various categories of road users following the introduction of the seat belt law.