Only about 40% of all drivers restricted to interlock by the courts or DMV after DUI conviction actually get a device installed. The rest drive illegally or use public transportation, bikes, shoe leather etc. statistics on how many are in service is fluid and fluctuates. Most are restricted for 6 months to a year (first time offenders with low BAC) so they fall out of the program rapidly only to be replaced with new offenders. Like we've talked about before, many states are passing laws that mandate every DUI gets interlock... While I don't agree that's reasonable it will dramatically increase revenues.