Approximately 30,000 alcohol ignition interlocks, which are designed to prevent the operation of a vehicle if the driver has been drinking, are in use in the US and Canada. Ignition interlock programs are also being initiated in Sweden and Australia. The best-controlled studies that are currently available suggest that ignition interlocks are effective in reducing impaired driving recidivism while on the vehicle. However, in the US, the practical effectiveness of these devices is limited because only a small number of offenders are willing to install them in order to drive legally. This paper reports on a study of a court policy that created a strong incentive for impaired driving offenders to install interlocks by making traditional penalties, such as jail or electronically monitored house arrest, the alternative to participation in an interlock program. Comparison of the recidivism rates of offenders subject to this policy with offenders in similar, nearby courts, not using interlocks, indicated that the policy was producing substantial reductions in DUI recidivism.