The term 'secondary safety' refers to the protection that a vehicle provides its occupants when involved in an accident. This paper studies information from the British database of road accident reports between 1980 and 1998, to estimate the reduction in the number of occupant casualties over these years which may be attributed to improvements to secondary safety in cars. The paper shows that the proportion of driver casualties who are killed or seriously injured (KSI) is lower for modern cars than for older cars. The reduction of this proportion is used to assess the improvement in secondary safety. Statistical models are developed to represent the proportion with 'year of first registration' as one of the independent variables, although only an incomplete assessment of the benefits of improved secondary safety can be made with the available data. The assessment compares the number of casualties that would have been expected if secondary safety had remained at the level found in cars first registered in 1980 with the actual casualty numbers. It is estimated that improved secondary safety reduced the number of drivers KSI by at least 19.7% in 1998, in comparison with what might have occurred if all cars had had that lower level of secondary safety. This figure relates to all cars on the road in 1998, and rises to 33%, when confined to the most modern cars (those which were first registered in 1998).