The companion paper to this one reported on the predictors of (and predicting with) failed BAC tests logged on the interlock recorder during the period the interlock is installed. Interlock offenders in the site that made available a human service adjunctive motivational intervention had significantly fewer fail BAC tests. Overall those with more failed interlock BAC tests were more likely to have post-interlock repeat DUIs. During the 12 months after the interlock has been removed first-time offenders exposed to the intervention had approximately half the number of repeat DUI offences than those not exposed to the intervention. In the intervention site 1.2 per cent (vs. 2.4 per cent in the non-intervention site) of the first offenders had repeat DUI offences. This finding is based on an initial 1373 first offender but with such low reopens rates it is not yet a statistically significant difference. Preliminary data from 505 multiple offenders have shown no evidence of an intervention effect with this group. In both intervention conditions, multiple offenders had approximately 3 per cent re-offences within the first 12 months after the interlock was removed. The research model presumes that the services intervention will affect road safety by improving the motivation of these offenders to separate their drinking and driving. The lower rate of failed BACs and the fewer repeat offences among first offenders within the intervention site could not, with the measures available, be directly attributed to processes (dose measures) associated with the intervention itself. Those who had more visits during comparable intervals of time or provided higher general compliance with the expectations of the motivational intervention did not show fewer failed BACs than those who were less compliant or had fewer repeat offences. Within the intervention site, the best predictors of not recording failure BACs were lower self-reported drinking, less required time on the interlock (e.g., the best of the offenders), those who volunteered for the interlock and those who are married.