A vehicle ignition interlock system includes a breath analyzer and a controller operably connected to the breath analyzer and to an ignition system of the vehicle. The controller compares detected breath alcohol levels of the vehicle operator with a threshold value, and is configured to prevent vehicle ignition if a breath alcohol level detected by the breath analyzer is greater than or equal to a threshold value. The controller also requires the vehicle operator to periodically take breath analyzer retests after vehicle ignition in order to allow vehicle operation to continue. The controller can reduce the frequency of periodic retests in response to one or more retests when the breath alcohol level of the vehicle operator is below a threshold value. The system may include a transdermal alcohol sensor, a mouth contamination sensor, a mouth contamination sensor, and redundant alcohol sensors.