A vehicle safety system, comprising: an alcohol concentration determining arrangement, for making a determination as to the concentration of alcohol in the blood of a vehicle occupant; a control unit adapted to receive and analyse data from the alcohol concentration determining arrangement, and in dependence upon the results of the analysis, operable to output a blocking signal or a dissuasion signal; a blocking arrangement, adapted to receive the blocking signal and to prevent driving of the vehicle by the vehicle occupant; and an output device adapted to receive the dissuasion signal and to present one or more warnings to the vehicle occupant to dissuade the occupant from driving the vehicle; wherein: the control unit is operable to categorise the concentration of alcohol in the occupant's blood into one of three categories, comprising a low concentration category, in which the blocking signal will not be generated, a high concentration category, in which the blocking signal will be generated, and an intermediate category, in which the blocking signal will not be generated and the dissuasion signal will be generated; and the system is configured to execute a system self-test automatically in conjunction with each breath test, characterized in that in the system technical errors are distinguished from attempts at circumvention or manipulation, with technical errors automatically causing the control unit to categorise the concentration of alcohol in the intermediate category, and in which an attempt at circumvention or manipulation automatically causes the control unit to categorise the concentration of alcohol into the high concentration category. an alcohol concentration determining arrangement, for making a determination as to the concentration of alcohol in the blood of a vehicle occupant; a control unit adapted to receive and analyse data from the alcohol concentration determining arrangement, and in dependence upon the results of the analysis, operable to output a blocking signal or a dissuasion signal; a blocking arrangement, adapted to receive the blocking signal and to prevent driving of the vehicle by the vehicle occupant; and an output device adapted to receive the dissuasion signal and to present one or more warnings to the vehicle occupant to dissuade the occupant from driving the vehicle; wherein: the control unit is operable to categorise the concentration of alcohol in the occupant's blood into one of three categories, comprising a low concentration category, in which the blocking signal will not be generated, a high concentration category, in which the blocking signal will be generated, and an intermediate category, in which the blocking signal will not be generated and the dissuasion signal will be generated; and the system is configured to execute a system self-test automatically in conjunction with each breath test, characterized in that in the system technical errors are distinguished from attempts at circumvention or manipulation, with technical errors automatically causing the control unit to categorise the concentration of alcohol in the intermediate category, and in which an attempt at circumvention or manipulation automatically causes the control unit to categorise the concentration of alcohol into the high concentration category. the control unit is operable to categorise the concentration of alcohol in the occupant's blood into one of three categories, comprising a low concentration category, in which the blocking signal will not be generated, a high concentration category, in which the blocking signal will be generated, and an intermediate category, in which the blocking signal will not be generated and the dissuasion signal will be generated; and the system is configured to execute a system self-test automatically in conjunction with each breath test, characterized in that in the system technical errors are distinguished from attempts at circumvention or manipulation, with technical errors automatically causing the control unit to categorise the concentration of alcohol in the intermediate category, and in which an attempt at circumvention or manipulation automatically causes the control unit to categorise the concentration of alcohol into the high concentration category.